This is a 1.2-volt single transistor flyback (Joule Thief) circuit that features a third coil. With it, flash duration and brightness is much enhanced, even with just a 10uF capacitor, as can be seen in the schematic.

Description

The parts to the right of T1 form a simple Joule-thief (‘Blocking’ oscillator) circuit which boosts the 1.2v supply to 3.5v to operate the LED. C1 is used to extend the charge and discharge cycles to increase brightness and efficiency. Components R, C and D1 form a simple timing circuit which, with the values shown, is about 1 sec per flash. Capacitor C charges up through resistor R until its voltage is enough to bias D1 and turn Q1 on to light up the LED. The charge on C is slowly drained, until it is no longer able to keep Q1 on, at which point the LED turns off until C can be recharged by R again. More information on this circuit and others can be found at http://quantsuff.com.

Videos

This power supply was designed for ham-radio use and has been in operation for over 10 years.

Its design is very simple and practically immune to RF. It is assembled from discrete components, being the most costly element a 600VA toroidal transformer, which can be replaced by another with different characteristics, provided it has a voltage of between 17~20V and the right power for our needs.

Description

The reference voltage is obtained from a 7.5V Zenner diode (6V8 ~ 8V2) applied to Q2 which compares the voltage output by the resistive divider formed by R5, VR2 and R6, controlling the Darlington amplifier Q3 which is responsible for driving the output stage consisting of driver Q4 and 4 NPN power transistors 2N3771, which can be mounted directly to the heatsink without being isolated, because its collector is connected to ground.

The source has maximum current limiter (Q1) adjustable via VR1 and overload indicator LED (DL2) and total protection against short circuit, with zero consumption in case of short circuit, can be maintained in this state indefinitely without danger.
(Keep in mind that due to the high power of the PSU a occasional short with a high length or small section cable that has sufficient strength not to exceed the maximum current of 40A. Might not be considered a short circuit and cause the PSU supplying a high amount of power).

The power rectifier diodes (D1, D2) are recommended metallic threaded type with the cathode grounded, mounted on a large enough heatsink isolated from the chassis. For the power transistors using a heatsink as big as possible, without being isolated as discussed above, may be part of the aluminum chassis of the box. We also recommend the use forced ventilation with a thermostat.

I wish you every success.

Description

The following circuit Diagram of (DUAL VOLTAGE POWER SUPPLY ) can be used for Misc.. application.

It requires a few components to built. The most important components of this circuit are REGULATORS.

1 : (AN) 7812 and 2 : (AN) 7912 AN7812 is the Positive Voltage Regulator. It regulates the voltage from (almost) 24vDC to 12vDC (accurate). AN7912 is the Negative Voltage Regulator. It regulates the voltage from (almost) -24vDC to -12vDC. A transformer output must be between 12vAC to 24vAC @ 500mA. Input of transformer (Primary)
should be about 110vAc-220vAC. It also include some capacitors to filter the current.

Specifications

This kit provides a variable output power supply ranging from 1.2 to 15 V @ 3 A.  It uses Low Dropout Positive Regulator LM1084 in TO220 package for delivering variable output voltage.

• Input – 18 VAC/DC
• Output – variable output from 1.2 ~ 15 V @ 3 A Regulated low ripple DC voltage
• Heatsink for regulator IC
• On board bridge rectifier to convert AC to DC
• LED indication at input of IC
• Zener trimmed band gap reference, current limiting and thermal shutdown (provided by IC feature)
• Power Battery Terminal (PBT) for easy input and output connection
• Onboard PCB mounted Potentiometer (POT) for varying the output voltage
• Filter capacitors for low ripple DC output
• Four mounting holes of 3.2 mm each
• PCB dimensions 46 mm x 58 mm

Schematic

Parts List

LM1084 Datasheet

Description

If you own a motorcycle, a motor home, a caravan, a lawn mover, a day cruiser or maybe a vintage car you must at some point had to write off a lead acid battery. When a battery is improperly charged or allowed to self-discharge as occurs during non-use, sulphate crystals build up on the battery’s plates.

The sulphate preventing the battery from being fully charged and therefore it is unable to deliver its full capacity. When trying to charge a battery in this state it only gets hot and looses water, the gravity of the electrolyte is not increasing to its normal “full charge” state. The only thing you do is killing the battery completely. If a battery has a resting voltage of at least 1.8 Volts/cell and no cells are shorted, desulphation of its plates can be done. This circuit is an add-on and part for a modification of a normal charger and it takes care of the sulphate problem.

CAUTION: Before you begin a project like this remember: mains voltage is dangerous so if you are not 100% sure of what you’re doing consult a friend who has the skills or, don’t do it at all !

The project: get hold of an old charger, big or small it’s your choice depending on the size of batteries you normally handle (bigger is better). There are some tricks to boost the performance if you need it. Start by ripping out everything except the transformer and the rectifier. Some older chargers are equipped with fin rectifiers, which have high voltage drop and must be replaced. Replace with a rugged bridge rectifier that can cope with the amperes. All wiring on secondary should be short and heavy wire. The rectifier should be bolted to the chassis to keep cool. If the charger have a high/low switch it’s a bonus, if not you can in some cases add a few turns of wire on the secondary winding. The circuit; a 14-stage ripple counter and oscillator IC 4060 produce a pulse, which is the heartbeat of the circuit. The pulse is feed to the 555 timer that deicide the length of the active output. With the switch you can select long or short pulse output. The output of the 555 timer triggers the zero-cross optoisolator triac driver MOC 3041 via a transistor. This gives the charger transformer a soft start via the triac and the snubber circuit. A small power supply is necessary for the circuit and consists of T1 a transformer 15V 0.1A secondary, a bridge rectifier, a regulator and two caps. Because this project include a charger that is (X) the outcome can differ in performance from one case to another. However this do not mean that your project doesn’t work, but the efficiency can vary. Some notes the snubbercap is a high voltage AC type (X) and the resistors on the mains side is at least 0.5W type. Use a triac that can take 400V+ and 10A+, I use BTA 25.600 but this is overkill in most cases. No PCB sorry!

How it works

Well the short version. The object is to get the cell voltage high enough for the sulphate to dissolve without boiling or melting the battery. This is achieved by applying higher voltage for shorter periods and let the battery rest for a while. The pulses on short range is about 0.5s on / 3s off and the long pulse range is 1.4s on / 2s off. These times can vary depending on component tolerances. Start on long pulse and if you discover “boiling” (more than with normal charging) in the electrolyte switch to short puls. Don’t leave the process unattended, at least until you know how your specific version of this project turns out. I built ver.1 of this circuit some 10 years ago and have experimented with it but I’m sure someone can improve it further.

Good Luck! Ante

Schematic

Description

This is a simple TRIAC AC load dimmer used to control the power of a resistive load such as incandescent lamp or heater element. The max load it can handle is 400VA. Such a circuit is often found on cheap commercial light dimmers and is proven to work reliable for the rated power.

The circuit is working by controlling the phase of the 220VAC voltage allowing the load to be powered for less than 360º of the full sine wave. Powering the load for smaller period than full sine wave delivers less power, so it has a dimming effect on the load.

Control is done through R2 potentiometer which controls the time needed for the C2 to be charged through R1-R2. C2 is charging until it reaches the breakdown voltage of DIAC D1 which then fires the TRIAC T1. Once the TRIAC is contacting the circuit is closed and the load receives power.

The rate which C2 charges determinate the point on 220VAC sine-wave where it reaches the breakdown voltage of DIAC. So, slow C2 charging means that the DIAC conducts at the end of AC period and fast C2 charging means that DIAC conducts at the begging of AC sine.

Notes

• Components L1 and C1 act as interference suppression filter reducing RF emissions.
• Circuit is directly connected to mains voltage, so lethal voltages present all over the board. Take care!
• The potentiometer should have a plastic spindle to provide sufficient isolation.

Parts List

Measurements

Copyright of this circuit belongs to smart kit electronics. On this page, we will use this circuit to discuss improvements and we will introduce some changes based on the original schematic.

General Description

This is a very useful project for anyone working in electronics. It is a versatile power supply that will solve most of the supply problems arising in the everyday work of any electronics workshop. It covers a wide range of voltages being continuously variable from 30 V down to 3 V. The output current is 2.5 A maximum, more than enough for most applications. The circuit is completely stabilized even at the extremes of its output range and is fully protected against short-circuits and overloading.

Technical Specifications – Characteristics:

• Input voltage: 24V AC / 3A
• Output current: 2.5 A
• Output voltage: 3-30V DC

How it Works

The power supply is using a well known and quite popular VOLTAGE STABILIZER IC the LM 723. The IC can be adjusted for out put voltages that vary continuously between 2 and 37 VDC and has a current rating of 150 mA which is of course too low for any serious use. In order to increase the current handling capacity of the circuit the output of the IC is used to drive a darlington pair formed by two power transistors the BD 135 and the 2N 3055. The use of the transistors to increase the maximum current output limits the range of output voltages somewhat and this is why the circuit has been designed to operate from 3 to 30 VDC. The resistor R5 that you see connected in series with the output of the supply is used for the protection of the circuit from overloading. If an excessively large current flows through R5, the voltage across it increases and any voltage greater than 0.3 V across it has as a result to cut the supply off, thus effectively protecting it from overloads. This protection feature is built in the LM 723 and the voltage drop across R5 is sensed by the IC itself between pins 2 and 3. At the same time the IC is continuously comparing the output voltage to its internal reference and if the difference exceeds the designer’s standards it corrects it automatically. This ensures great stability under different loads. The potentiometer P1 is used to adjust the out put voltage at the desired level. If the full range from 3 to 30 V is desired then you should use a mains transformer with a secondary winding having a rating of at least 24 V/3 A. If the maxi mum voltage output is not desired you can of course use a transformer with a lower secondary voltage output. (However, once rectified the voltage across the capacitor C2 should exceed by 4-5 volts the maximum output expected from the circuit.

Construction

First of all let us consider a few basics in building electronic circuits on a printed circuit board. The board is made of a thin insulating material clad with a thin layer of conductive copper that is shaped in such a way as to form the necessary conductors between the various components of the circuit. The use of a properly designed printed circuit board is very desirable as it speeds construction up considerably and reduces the possibility of making errors. Smart Kit boards also come pre-drilled and with the outline of the components and their identification printed on the component side to make construction easier. To protect the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and makes soldering easier. Soldering the components to the board is the only way to build your circuit and from the way you do it depends greatly your success or failure. This work is not very difficult and if you stick to a few rules you should have no problems. The soldering iron that you use must be light and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose come very handy specially made sponges that are kept wet and from time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip can not be cleaned, replace it. There are many different types of solder in the market and you should choose a good quality one that contains the necessary flux in its core, to assure a perfect joint every time.
DO NOT use soldering flux apart from that which is already included in your solder. Too much flux can cause many problems
and is one of the main causes of circuit malfunction. If nevertheless you have to use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after you finish your work. In order to solder a component correctly you should do the following:
Clean the component leads with a small piece of emery paper.

Bend them at the correct distance from the component body and insert the component in its place on the board. You may find sometimes a component with heavier gauge leads than usual, that are too thick to enter in the holes of the p.c. board. In this case use a mini drill to increase the diameter of the holes slightly. Do not make the holes too large as this is going to make soldering difficult afterwards. Take the hot iron and place its tip on the component lead while holding the end of the solder wire at the point where the lead emerges from the board. The iron tip must touch the lead slightly above the p.c. board. When the solder starts to melt and flow wait till it covers evenly he area around the hole and the flux boils and gets out from underneath the solder. The whole operation should not take more than 5 seconds. Remove the iron and allow the solder to cool naturally without blowing on it or moving the component. If everything was done properly the surface of the joint must have a bright metallic finish and its edges should be smoothly ended on the component lead and the board track. If the solder looks dull, cracked, or has the shape of a blob then you have made a dry joint and you should remove the solder (with a pump, or a solder wick) and redo it. Take care not to overheat the tracks as it is very easy to lift them from the board and break them. When you are soldering a sensitive component it is good practice to hold the lead from the component side of the board with a pair of long-nose pliers to divert any heat that could possibly damage the component. Make sure that you do not use more solder than it is necessary as you are running the risk of short-circuiting adjacent tracks on the board, especially if they are very close together. After you have finished your work cut off the excess of the component leads and clean the board thoroughly with a suit able solvent to remove all the flux residues that may still remain on it.

Start building the circuit by placing the pins on the board and soldering them. You must be very careful when soldering the components that are going to carry heavy currents as your joints must be capable of withstanding the maximum current without getting hot. Solder the IC socket in its place taking care not to insert it the wrong way round and then put the resistors in their places on the board. Resistor R5 should be soldered in such a way as to keep its body slightly separated from the p.c. board to let the air circulate around the component and cool it. Continue your work with the capacitors. Be careful not to insert the electrolytic the wrong way round. The polarity is marked on the capacitors and the p.c. board is also marked accordingly. Insert the rectifier bridge in its place. The bridge is a heavy duty type and has leads made of heavier gauge wire than usual. If you have any difficulty inserting them in the p.c. board you can enlarge the holes with a mini drill. (Automatic production of p.c. boards requires all the holes on the board to be of the same diameter).

Do not however make the holes too wide as you are going to find soldering the leads much more difficult afterwards. Solder TR1 in its place and mount TR2 on the heatsink following the diagram and making sure there is no electrical connection between the heat sink and the transistor. Don’t forget the insulators, and use heat transfer compound between the transistor body and the heat sink. Using heavy gauge wires connect TR2 to the board and finally using a flat ribbon cable connect the potentiometer with the rest of the circuit. Insert the VOLTAGE REGULATOR in its socket and your power supply is ready. Now make a final inspection of your work to ensure that there are no mistakes that could cause a lot of trouble later. If everything looks OK you can connect the input of the circuit (it is marked «24 VAC» on the board) to the secondary winding of the transformer. Connect a voltmeter to the pins marked «OUT 3-30 V» and using a mains lead connect the primary of the transformer to a convenient power out let. If everything was done properly the voltmeter should give a reading and turning the potentiometer should make it change.

Slight variations from the minimum and maximum voltages specified are normal, are caused from component tolerances and should not worry you. Although the circuit works with low voltages and is quite safe to touch any part while it is in operation it needs a mains transformer to supply this low voltage and the primary of the transformer is connected to the mains which makes it very dangerous. The best idea is to use a case for everything in order to make a complete stand alone power supply for your experiments. Smart Kit also makes a suitable case for this supply with a printed front panel, ready drilled for the output connectors, switches, fuse holder and panel instruments.

Parts List

CAUTION

This circuit works from the mains and there are 220 VAC pre sent in some of its parts. Voltages above 50 V are DANGEROUS and could even be LETHAL. In order to avoid accidents that could be fatal to you or members of your family please observe the following

rules:

• DO NOT work if you are tired or in a hurry, double check every thing before connecting your circuit to the mains and be ready to disconnect it if something looks wrong.
• DO NOT touch any part of the circuit when it is under power.
• DO NOT leave mains leads exposed. All mains leads should be well insulated. -DO NOT change the fuses with others of higher rating or replace them with wire or aluminium foil.
• DO NOT work with wet hands. -If you are wearing a chain, necklace or anything that may be hanging and touch an exposed part of the circuit  BE CAREFUL. USE ALWAYS a correct mains lead with the correct plug and earth your circuit correctly. If the case of your project is made of metal make sure it is properly earthen. If it is possible use a mains transformer with a 1:1 ratio to isolate your circuit from the mains.  When testing a circuit that works off the mains wear shoes with rubber soles, stand on dry non conductive floor and keep one hand in your pocket or behind your back.  If you take all the above precautions you are reducing the risks you are taking to a minimum and this way you are protecting your self and those around you. A carefully built and well insulated device does not constitute any danger for its user. BEWARE: ELECTRICITY CAN KILL IF YOU ARE NOT CAREFUL.

Here are some photos from this power supply finished and installed in a box.

I use the lm10 IC because it has a reference voltage and that’s useful for dc power supply. With two ICs can take different output voltage and amperage. This circuit is protected from short circuit.P2 is for controlling the current at the range of 0-2A. Stabilize the output voltage with R4 on negative pin on op-amp and with R2 & P1 on positive pin.

Op-amp output controls T1 that not let ripple of voltage.T1 increase or decrease ampere of R6 and control the voltage of T5 & T4. Pin 1 is the reference voltage and reference voltage is losing some voltage on R1 that has 100uA . This current passes through P1 too.

Vlose p1=100uA*Rp1

This lose voltage regulate output voltage rate of output current is compare between reference voltage of P3 and lose voltage on R11.T3 is protecting short circuit with R11. For reduce out put voltage to 0v should parallel one resistor 470 ohm in out put. Minimum voltage is 0.4v. The maximum output voltage is fixed with R1b and should not become over of 50v. Therefore your transformer should give 36V, 3A with 4700uF capacitor. T6, T5, T7 need heatsilk.

Parts list

R1a = 2,2 K
R1b = read the text
R2 = 10 K
R3, R7 = 3.3 k
R4 = 390 Ohm
R5 = 47 K
R6 = 3.3 K 1Watt
R8 = 180 Ohm
R9, R10 = 0.47 Ohm 3Watt
R11 = 0.075 Ohm 2Watt
R12 = 470 Ohm
P1 = 500K liner potentiometer
P2 = 4.7 K potentiometer
P3 = 10 K potentiometer
C1 = 1nF
C2 = 10nF
C3 = 22nF
C4 = 47mF 63v electrolytic
C5 = 4700mF 80v electrolytic
T1, T2 = BC161
T3, T4 = BD141
T5 = BD241
T6, T7 = 2V3055
D1, D2 = 1N4148
D3, D4 = 1N4001
IC1, IC2 = LM10C

Regulated DC power supply, short circuit safe, and with current limiter.

This PSU has been especially designed for current-hungry ham radio transceivers. It delivers safely around 20Amps at 13.8V. For lower currents, a separate current limiting output, capable of 15ma up to a total of 20A has been added. Let us see what we have got here. The power transformer should be capable to deliver at least 25A at 17.5 to 20V. The lower the voltage, the lower power dissipation. The rectified current will be “ironed” by the C1, whose capacity should not be less than 40.000uF, (a golden rule of around 2000uF/A), but we recommend up to 50.000uF. This capacity can be built up by several smaller capacitors in parallel. The base of this design is a simple 12V regulator (7812). The output voltage can be brought to desired value (here 13.8V) by two external resistors (R5 and R6) using this formula:

U= 12(1+R5/R6)

The low currents (here 15mA) will keep the 7812 in its regular function. As soon as the current rises over 15ma, the voltage drop on R4 will “open” the Q3, actually handling the high output current. This is a PNP transistor (Ic>25) and current amplification factor of at least 20. The one that has been tested and proven here is the 2N5683. The current limiting resistance RL, for the maximum output of 20 Amps should be 0.03 Ohms, rated at least 15W. You can use the resistance wire or switch several resistors in parallel, totaling the resistance/power values. Values for other currents can be calculated by the rule:

RL=0.7/Imax

The RL and Q2 (3A PNP such as BD330) form a short circuit “automatic fuse”. As soon as the maximum current reaches 20Amps, the voltage drop over the resistor RL will open Q2, and thus limit the B-E Current of Q3. Parallel to Q2 is Q1, which lights the LED 1 whenever the current limiting circuit is active. When the “fuse” is active, the Q2 bridges the R3, so the full current would flow through the IC1, and damage it. Therefore the R4 is inserted, as to limit the IC1 current to 15mA. This makes it possible to run the IC1 without any cooling aid. The LED 2 will light up every time the PSU is switched on.

There is an adjustable current limiter in parallel to the fixed output, thus providing adjustable current source for smaller currents.

This circuit is very simple too. You will notice that there is no current sensing resistor. But it is really there, in a form of the Rds-on resistance of the N-channel FET, which actually handles the load cutoff from the source. The function of the FET is shown in the diagram 2. When the current Id is rising, the tension Uds over the resistance Rds rises very slowly in the beginning, but very fast after the knick. This means, that before the knick the FET behaves as a resistor but after it, works as constant current source.

The D2, R3 and B-E connection of the Q4 will sense the Uds voltage of the FET1. When the voltage rises enough, the Q4 will shortcut the FET1 gate to mass, and cut the current flow through the FET 1 off. However, to enable the FET1 to open, there is certain gate voltage necessary, which in this case is brought up by the voltage divider consisting of R8, Z1, P1 and R9. So the maximum Gate voltage will be the one of the Z1, and the minimal will be around 3V6. The Z1 voltage (Uz1) will thus determine the max current flowing through the FET 1.

The diagram 2 will show that for 5 Amps the Uz1 should be 5V6, and for 20Amps around 9V6. The Capacitor C4 will determine the “velocity” or the reaction time of the limiter. 100 uF will make the reaction time to be around 100ms, and 1n will make it 1us.

Within the designed limits, the P1 will limit the current output in the range of 15mA to 20A. You can use both output simultaneously, but the total output current will be limited by the value of the RL. This PSU can be built also for higher outputs, as long as the transformer will handle the current requirements, and you provide sufficient cooling for the Q3.

If somebody will be interested, there is a PCB design ready.

REV1.

I have received several requests for some modifications, and the one I find useful is the addition of an amp meter. Therefore the slightly modified diagram is included in this revision. All elements within the dotted border are now placed on the pcb. There is also elements placement design included. Should one have an 25Amp instrument on hand, there is nothing easier. Just mount it in line and there it goes. However, a ham would probably find an instrument somewhere in his “junk-box”, but the scale would be something completely different, let say an “S” or Voltmeter. No problem. We already have a “shunt” for the amp-meter, and it is there as the Current limiting resistor “RL”. As already known from before, there is a voltage drop of 0V7 over the resistor at current flow of 20A. What we now have to do, is to simply measure the voltage drop over the resistor, and co-relate it with the current. Let us say that our instrument has an internal resistance of 13R, and has a full scale reading of 60mV. The voltage drop over the RL is 0V7 for 20Amps. Therefore, we need another resistance in line with the instrument, that would bring the 0V7 to 60mV, or an voltage drop of 640mV.

The formula is simple:

U1:R1=U2:R2

60:13=640:X

X=13×640/60

X=138.66

Therefore, the resistance that has to be inserted in line is around 140R. I suggest to insert a trimmer (VR1) of around 200R, to fine trim the reading when calibrating the instrument. Using your favorite drawing software, design your scale to your likings, (at least 20A for the full scale) and insert it in the instrument you have. Due to the many requests for the PCB Layout, I have included the design here. The exact dimensions of the pcb are 160x100mm. Please remember, that the pcb has to be printed as a mirror image, to obtain higher quality when transferring it to the copper side of the board.

Wish you a good time.

Bob

Copyright of this circuit belongs to smart kit electronics. In this page we will use this circuit to discuss for improvements and we will introduce some changes based on original schematic.

General Description

This is a high quality power supply with a continuously variable stabilised output adjustable at any value between 0 and 30VDC. The circuit also incorporates an electronic output current limiter that effectively controls the output current from a few milliamperes (2 mA) to the maximum output of three amperes that the circuit can deliver. This feature makes this power supply indispensable in the experimenters laboratory as it is possible to limit the current to the typical maximum that a circuit under test may require, and power it up then, without any fear that it may be damaged if something goes wrong. There is also a visual indication that the current limiter is in operation so that you can see at a glance that your circuit is exceeding or not its preset limits.

Technical Specifications

• Input Voltage: ……………. 24 VAC
• Input Current: ……………. 3 A (max)
• Output Voltage: …………. 0-30 V adjustable
• Output Current: …………. 2 mA-3 A adjustable
• Output Voltage Ripple: …. 0.01 % maximum
• PCB dimensions: 123 x 85 mm

Features

• Reduced dimensions, easy construction, simple operation.
• Output voltage easily adjustable.
• Output current limiting with visual indication.
• Complete protection of the supplied device against over loads and malfunction.

How it Works

To start with, there is a step-down mains transformer with a secondary winding rated at 24 V/3 A, which is connected across the input points of the circuit at pins 1 & 2. (the quality of the supplies output will be directly proportional to the quality of the transformer). The AC voltage of the transformers secondary winding is rectified by the bridge formed by the four diodes D1-D4. The DC voltage taken across the output of the bridge is smoothed by the filter formed by the reservoir capacitor C1 and the resistor R1. The circuit incorporates some unique features which make it quite different from other power supplies of its class. Instead of using a variable feedback arrangement to control the output voltage, our circuit uses a constant gain amplifier to provide the reference voltage necessary for its stable operation. The reference voltage is generated at the output of U1.

The circuit operates as follows: The diode D8 is a 5.6 V zener, which here operates at its zero temperature coefficient current. The voltage in the output of U1 gradually increases till the diode D8 is turned on. When this happens the circuit stabilises and the Zener reference voltage (5.6 V) appears across the resistor R5. The current which flows through the non inverting input of the op-amp is negligible, therefore the same current flows through R5 and R6, and as the two resistors have the same value the voltage across the two of them in series will be exactly twice the voltage across each one. Thus the voltage present at the output of the op-amp (pin 6 of U1) is 11.2 V, twice the zeners reference voltage. The integrated circuit U2 has a constant amplification factor of approximately 3 X, according to the formula A=(R11+R12)/R11, and raises the 11.2 V reference voltage to approximately 33 V. The trimmer RV1 and the resistor R10 are used for the adjustment of the output voltages limits so that it can be reduced to 0 V, despite any value tolerances of the other components in the circuit.

Another very important feature of the circuit, is the possibility to preset the maximum output current which can be drawn from the p.s.u., effectively converting it from a constant voltage source to a constant current one. To make this possible the circuit detects the voltage drop across a resistor (R7) which is connected in series with the load. The IC responsible for this function of the circuit is U3. The inverting input of U3 is biased at 0 V via R21. At the same time the non inverting input of the same IC can be adjusted to any voltage by means of P2.

Let us assume that for a given output of several volts, P2 is set so that the input of the IC is kept at 1 V. If the load is increased the output voltage will be kept constant by the voltage amplifier section of the circuit and the presence of R7 in series with the output will have a negligible effect because of its low value and because of its location outside the feedback loop of the voltage control circuit. While the load is kept constant and the output voltage is not changed the circuit is stable. If the load is increased so that the voltage drop across R7 is greater than 1 V, IC3 is forced into action and the circuit is shifted into the constant current mode. The output of U3 is coupled to the non inverting input of U2 by D9. U2 is responsible for the voltage control and as U3 is coupled to its input the latter can effectively override its function. What happens is that the voltage across R7 is monitored and is not allowed to increase above the preset value (1 V in our example) by reducing the output voltage of the circuit.

This is in effect a means of maintaining the output current constant and is so accurate that it is possible to preset the current limit to as low as 2 mA. The capacitor C8 is there to increase the stability of the circuit. Q3 is used to drive the LED whenever the current limiter is activated in order to provide a visual indication of the limiters operation. In order to make it possible for U2 to control the output voltage down to 0 V, it is necessary to provide a negative supply rail and this is done by means of the circuit around C2 & C3. The same negative supply is also used for U3. As U1 is working under fixed conditions it can be run from the unregulated positive supply rail and the earth.

The negative supply rail is produced by a simple voltage pump circuit which is stabilised by means of R3 and D7. In order to avoid uncontrolled situations at shut-down there is a protection circuit built around Q1. As soon as the negative supply rail collapses Q1 removes all drive to the output stage. This in effect brings the output voltage to zero as soon as the AC is removed protecting the circuit and the appliances connected to its output. During normal operation Q1 is kept off by means of R14 but when the negative supply rail collapses the transistor is turned on and brings the output of U2 low. The IC has internal protection and can not be damaged because of this effective short circuiting of its output. It is a great advantage in experimental work to be able to kill the output of a power supply without having to wait for the capacitors to discharge and there is also an added protection because the output of many stabilised power supplies tends to rise instantaneously at switch off with disastrous results.

Construction

First of all let us consider a few basics in building electronic circuits on a printed circuit board. The board is made of a thin insulating material clad with a thin layer of conductive copper that is shaped in such a way as to form the necessary conductors between the various components of the circuit. The use of a properly designed printed circuit board is very desirable as it speeds construction up considerably and reduces the possibility of making errors. To protect the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and also makes soldering easier.

Soldering the components to the board is the only way to build your circuit and from the way you do it depends greatly your success or failure. This work is not very difficult and if you stick to a few rules you should have no problems. The soldering iron that you use must be light and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose come very handy specially made sponges that are kept wet and from time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate on it.

DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it. There are many different types of solder in the market and you should choose a good quality one that contains the necessary flux in its core, to assure a perfect joint every time.
DO NOT use soldering flux apart from that which is already included in your solder. Too much flux can cause many problems and is one of the main causes of circuit malfunction. If nevertheless you have to use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after you finish your work.

In order to solder a component correctly you should do the following:

• Clean the component leads with a small piece of emery paper.
• Bend them at the correct distance from the components body and insert he component in its place on the board.
• You may find sometimes a component with heavier gauge leads than usual, that are too thick to enter in the holes of the p.c. board. In this case use a mini drill to enlarge the holes slightly. Do not make the holes too large as this is going to make soldering difficult afterwards.
• Take the hot iron and place its tip on the component lead while holding the end of the solder wire at the point where the lead emerges from the board. The iron tip must touch the lead slightly above the p.c. board.
• When the solder starts to melt and flow wait till it covers evenly the area around the hole and the flux boils and gets out from underneath the solder.
• The whole operation should not take more than 5 seconds. Remove the iron and allow the solder to cool naturally without blowing on it or moving the component. If everything was done properly the surface of the joint must have a bright metallic finish and its edges should be smoothly ended on the component lead and the board track. If the solder looks dull, cracked, or has the shape of a blob then you have made a dry joint and you should remove the solder (with a pump, or a solder wick) and redo it. Take care not to overheat the tracks as it is very easy to lift them from the board and break them.
• When you are soldering a sensitive component it is good practice to hold the lead from the component side of the board with a pair of long-nose pliers to divert any heat that could possibly damage the component.
• Make sure that you do not use more solder than it is necessary as you are running the risk of short-circuiting adjacent tracks on the board, especially if they are very close together.
• When you finish your work, cut off the excess of the component leads and clean the board thoroughly with a suitable solvent to remove all flux residues that may still remain on it.

Construction (… continued)

As it is recommended start working by identifying the components and separating them in groups. Place first of all the sockets for the ICs and the pins for the external connections and solder them in their places. Continue with the resistors. Remember to mound R7 at a certain distance from the printed circuit board as it tends to become quite hot, especially when the circuit is supplying heavy currents, and this could possibly damage the board. It is also advisable to mount R1 at a certain distance from the surface of the PCB as well. Continue with the capacitors observing the polarity of the electrolytic and finally solder in place the diodes and the transistors taking care not to overheat them and being at the same time very careful to align them correctly.

Mount the power transistor on the heatsink. To do this follow the diagram and remember to use the mica insulator between the transistor body and the heatsink and the special fibber washers to insulate the screws from the heatsink. Remember to place the soldering tag on one of the screws from the side of the transistor body, this is going to be used as the collector lead of the transistor. Use a little amount of Heat Transfer Compound between the transistor and the heatsink to ensure the maximum transfer of heat between them, and tighten the screws as far as they will go.

Attach a piece of insulated wire to each lead taking care to make very good joints as the current that flows in this part of the circuit is quite heavy, especially between the emitter and the collector of the transistor.
It is convenient to know where you are going to place every thing inside the case that is going to accommodate your power supply, in order to calculate the length of the wires to use between the PCB and the potentiometers, the power transistor and for the input and output connections to the circuit. (It does not really matter if the wires are longer but it makes a much neater project if the wires are trimmed at exactly the length necessary).
Connect the potentiometers, the LED and the power transistor and attach two pairs of leads for the input and output connections. Make sure that you follow the circuit diagram very care fully for these connections as there are 15 external connections to the circuit in total and if you make a mistake it may be very difficult to find it afterwards. It is a good idea to use cables of different colours in order to make trouble shooting easier.

The external connections are:

• 1 & 2 AC input, the secondary of the transformer.
• 3 (+) & 4 (-) DC output.
• 5, 10 & 12 to P1.
• 6, 11 & 13 to P2.
• 7 (E), 8 (B), 9 (E) to the power transistor Q4.
• The LED should also be placed on the front panel of the case where it is always visible but the pins where it is connected at are not numbered.

When all the external connections have been finished make a very careful inspection of the board and clean it to remove soldering flux residues. Make sure that there are no bridges that may short circuit adjacent tracks and if everything seems to be all right connect the input of the circuit with the secondary of a suitable mains transformer. Connect a voltmeter across the output of the circuit and the primary of the transformer to the mains.

The voltmeter should measure a voltage between 0 and 30 VDC depending on the setting of P1, and should follow any changes of this setting to indicate that the variable voltage control is working properly. Turning P2 counter-clockwise should turn the LED on, indicating that the current limiter is in operation.

Data

Adjustments

If you want the output of your supply to be adjustable between 0 and 30 V you should adjust RV1 to make sure that when P1 is at its minimum setting the output of the supply is exactly 0 V. As it is not possible to measure very small values with a conventional panel meter it is better to use a digital meter for this adjustment, and to set it at a very low scale to increase its sensitivity.

Warning

While using electrical parts, handle power supply and equipment with great care, following safety standards as described by international specs and regulations.

CAUTION

This circuit works off the mains and there are 220 VAC present in some of its parts.
Voltages above 50 V are DANGEROUS and could even be LETHAL.
In order to avoid accidents that could be fatal to you or members of your family please observe the following rules:

• DO NOT work if you are tired or in a hurry, double check every thing before connecting your circuit to the mains and be ready
• to disconnect it if something looks wrong.
• DO NOT touch any part of the circuit when it is under power.
• DO NOT leave mains leads exposed. All mains leads should be well insulated.
• DO NOT change the fuses with others of higher rating or replace them with wire or aluminium foil.
• DO NOT work with wet hands.
• If you are wearing a chain, necklace or anything that may be hanging and touch an exposed part of the circuit BE CAREFUL.
• ALWAYS use a proper mains lead with the correct plug and earth your circuit properly.
• If the case of your project is made of metal make sure that it is properly earthen.
• If it is possible use a mains transformer with a 1:1 ratio to isolate your circuit from the mains.
• When you are testing a circuit that works off the mains wear shoes with rubber soles, stand on dry non conductive floor and keep one hand in your pocket or behind your back.
• If you take all the above precautions you are reducing the risks you are taking to a minimum and this way you are protecting yourself and those around you.
• A carefully built and well insulated device does not constitute any danger for its user.

If it does not work

Check your work for possible dry joints, bridges across adjacent tracks or soldering flux residues that usually cause problems.
Check again all the external connections to and from the circuit to see if there is a mistake there.

• See that there are no components missing or inserted in the wrong places.
• Make sure that all the polarised components have been soldered the right way round. – Make sure the supply has the correct voltage and is connected the right way round to your circuit.
• Check your project for faulty or damaged components.

Parts List

Feedback

Another implementation of this power supply is located here – in Czech language

here is a board made by Sam Carmel and worked nice

Daniel’s build of PSU – front view with LCD voltmeter
Potensiometers for coarse and fine voltage adjustment and current regulator

Daniel’s build of PSU – internal view. A mobile phone charger is used as power supply for voltmeter

Daniel’s build of PSU – internal view. He is going to change 2200uF a 6800uF capacitor to reduce ripple in high load.

Daniel’s build of PSU – internal view. new capacitor (6800uF x 40V) to improve ripple filtering

Daniel’s build of PSU – internal view. Modification to protect the LM311

Received the following email from Daniel  on 06/2012:
I only face a problem with one of the biggest plagues in electronics now… Fake components. I bougth a fake 2N2219 and it last 100ms (or less) in my first try. As the piece was new I never suspected on it. I spent 2 hours looking for the problem and I could not believe when I test it… I had more two I bougth togeter, they had the same destiny… For my lucky I had a box with old components (some dates from the 70´s) and there I found a genuine Motorola 2N2219… This is running perfect. This was the only difficulty I found…

Received the following email from Ivan on 02/2010:
Ok. I bulid your project about a day ago. Mounted all the parts on the pcb and then concluded that there is some serious problems in this schematics. First, 2N3055 will overheat, so you have to connect two of them in parallel with emitter resistors 0.1ohm/5w. Second, maximal voltage between ‘+’ and ‘-‘ of TL081 is 36VDC.If you connect them as it is shown in this circuit diagram that voltage will be about 45VDC, so they will burn down immediately. To fix this problem you have to reconnect all pins number 7 of U1, U2 and U3, emitter of Q3 and ‘upper’ end of R19 to out of an 7809 with 18V zener diode between ‘common’ pin and ‘-‘ of 3300uF cap, and input of 7809 connect to ‘+’ of the same cap. Now, on pin number 7 and mentioned parts you’ll have 27VDC, and total voltage will be 32.6VDC. Third, instead of using 3300uF, use 4700 or 6800uF/63VDC to reduce the ripple on higher currents (2-3A). The rest of the circuit is perfect. I like it cause it is so inexpensive and easy to make with those simple reconstructions i mentioned.

Notes

• A great addition to your PSU would be a Voltmeter + Ammeter LCD panel. Check it here!
• LCD multimeter – Volt & Amper meter