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Hi all

I really like this scheme and I am thinking about reproducing it for my own use. Even though, there is something that I miss and it's that the capacitors are too small in the out and I need them to be bigger. I think add a transistor in order to avoid problems with discharge of 0.1F cap.

Svetozar

schem1.gif

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You do not need a huge output capacitor because the circuit has voltage regulation.

Actually this original old circuit from a Greek kit has many overloaded parts and is not reliable. We fixed in and you can see the improved circuit in the forum.

A Chinese kit copy (from Banggood or something) is sold at a very low cost but it also has some overloaded parts.

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Thanks for the warning. I actually first saw Hristo scheme, and which I will be adapt to 0-20V 0-10A.

PS_sch2.thumb.png.2e0a9063ead3aa4e79076f

Clear that one of the purposes of this circuit is to maintain a stable output voltage, but how it would behave if the load is 9A pulses? My idea for laboratory power supply is a device in which the pulses of output V are kept to a minimum in all conditions, and when consumption exceeds the set limits - lowering out Instantly. So I think an improvement add a large capacitor on the output and the possibility of discharge Instantly. And if they are not really necessary discharge transistor it is, to discharge the capacitors in the device that we connected to our source of power. Here I leave as an example an old but very good Bulgarian scheme which implements the discharge transistor.

 TES-18.thumb.png.18327db752d51f72effa189

 

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When a capacitor is discharged by a high load current then its voltage begins to drop immediately since it has no voltage regulation.

When you want the output capacitor discharged then you are using a transistor that will vaporize. The only thing limiting the current in the discharge transistor is its current gain since the opamp can supply its base with only about 20mA .

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I can see myself forced to put a resistor 10R, but according to manufacturer seems not.

BD140-hFE.thumb.png.6b7c20a6339b5e6e6623

To test the reaction of the discharge transistor of a TES I set I = 0.01A and U = 15V. When connecting a 0.1W LED Light Peak is not noticeable, just lights. I know it's something old test this way, but I did over 25 years ago. I would love to get the same but with large capacitors in the output. This might not be the most appropriate transistor and connection. It is the first thing that occurred to me, basic and seems to work. The idea is to eliminate the charge on the capacitors even in the device we tested when something goes wrong to protect circuits behind them.

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Hi audioguru.
Now that I'm done with the other conversation I would like to continue this.
To me the truth I do not like to limit the transistor with a resistor if possible. You do not think it is possible to hold without burning? From what I see in the manufacturer PDF
, gain low wen collector current up. I would try immediately if possible, but for now have only the PCB, I hope the components, and am content with theory.
Please excuse my bad English,
it is the google translator.. :-[

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Your main filter capacitor value is much too low for an output of 10A. use at least 20,000uF.

The transformer voltage is too high for a maximum output voltage of only 20V and its current rating is much too low.

The poor little BD139 driver transistor will smoke and die if you are unlucky to find those very old output transistors with low current gain.

Q1 is completely wrong. Its maximum allowed reverse bias on its base-emitter is only 5V.   

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First thanks for the comments.
Just I get started to modify the scheme and did not have much time to review and change everything I need. Also I expect your comments. It is the first time I use Proteus. I always try to build recycling scrap, bought mainly old PCB 1 / 1kg. The TIP35C have them in large quantities, with pin collector cut but not be a problem. The toroid transformer is 2x12v / 300w, C1 will probably be 2x18000uf, my friend has 4 also scrap. So far I have not decided, we'll see.

What I had not thought is the reverse bias of BD140, I'll have to protect it, thanks for advice.  BD139 is 12W 1.5A, TIP35C gain to 1.5A is 25-50. If consumed 10A output to 0V, in the collector BD135 would have 0.4A for 30Vce = 12W. I'm wrong? I know, it is at the limit... You can suggest another transistor to facilitate me?

 

Why R7 is the wrong value for this configuration???

 

TIP35C.pdf

BD140.pdf

BD139.pdf

0-20v,0-10a1.bmp

0-20v,0-10a1.bmp

Edited by CBETO3AP

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I'm sorry, I should have said I think it's the wrong value for this configuration.  I'd have to work out the math for it but I think you'd do better with a much lower value at 10A output.  Doing that would require also adjusting the resistor values around the voltage reference and current regulation circuit.

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The datasheets for the BD139 and BD140 show that the chip will be at its maximum allowed temperature if it dissipates 12.5W and its case is cooled down to 25 degrees C somehow (with a huge heatsink in liquid nitrogen?). In free air a pretty big heatsink will get hot then the chip will be too hot.

 

EDIT: I forgot. I do not know of a power transistor that can dissipate more heat and is as fast as the BD139. The problem with using two BD139 transistors in parallel, each with an emitter resistor, is that they will overload the opamp that drives them. 

Edited by audioguru

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The 10V zener diode reduces the total supply voltage of U3 from its absolute maximum of about 44 V to about 34V. Then opamp U3 will be cooler and it will live longer.

EDIT: But it is not needed in your circuit since your input and output voltages are lower.

Edited by audioguru

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The kit from Banggood is a copy of the original kit from Greece with a couple of parts changed. It will not produce 30VDC at 3A and the TL081 opamps will have a total supply voltage higher than they are allowed when the load current is low. The single output transistor will get much too hot when the output voltage is low but the current is high. If you calculate the power in many resistors then you will see that they are overloaded.

The schematic is from the Greek kit and does not show the 7824 regulator that would drive a fan. The regulator is also not on Banggood's parts list. 

 

If you like to gamble then you can build it and maybe you will be lucky that it does not become destroyed.

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Limited by the 24V / 300W transformer this will give out 24 or 20V-10A.
I bought fake China MC34071 and now I'm doing tests with UA741, the truth is that there are going pretty well. I expect a TLE2141AIP and if it is the original'll ask the rest. What I will possibly do after testing is remaking the scheme to put LM311, open collector 50V out at currents as 50mA. This and a more powerful transformer allow me more output
. The use of such power is for example the CNC machine cutting expanded polystyrene with hot wire.

p.p. By my calculations with 1 or 2 driver transistors BD140 is enough.

LM311-n.pdf

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The peak voltage from a 24VAC transformer is 34V. It charges the main filter capacitor to this peak voltage and heats the load, rectifiers, series pass transistors, driver transistors and current sensing resistor. Then this 300W transformer is at its max current when the DC output current is 8.8A.

 

The inputs of some 741 opamps fail to work when their voltage is within 4V from the positive or negative supply so the 1.3V from the two diodes is not enough. Use the negative -5.6V supply in the original circuit.

The LM311 is a comparator, not an opamp. It its voltage gain is 200,000 times and it oscillates if it has negative feedback.

 

Maybe you calculated with "typical" current gain. Then you must buy a few hundred transistors, test their gain and use only the best ones. We usually design circuits so that all of them work well even if the transistors have minimum specs.

 

We do not use "China" transistors or ICs. Instead we use Name Brand genuine parts.

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BD140 driver emiter current tested - 85mA max at 10A load  8) (200mA expected)

The peak voltage of this toroid transformer (filtered, no load) - 38v, i modifed the secondary from 12v/25A to 24V/12.5A

741 opamps is on negative -5.6V (4.7V in my case) supply, ofcorce

The LM311 is a comparator I know, will see...

"Then you must buy a few hundred transistors, test their gain and use only the best ones" I usually buy scrap and test :)

"Maybe you calculated with "typical" current gain" nop, i calculate well and have some luck with the gain :)

Edited by CBETO3AP
orthography and add

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Yes, I have been forced to change the shunt resistor. Now this place metal shunt 10A / 0.1V.Thus I am saving me 1.4V. Plus something like 0.5V to have 10 emiter resistor in parallel. Low Vdrop is because the shunt is bolted directly to the output. And the sensing cable to the out.

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