redwire

0-30V Stabilized Power Supply

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sonycman,  U3 requires a voltage between 0 and 1.5V (for 3A output) and U2 requires a voltage between 0 and 11.2V to adjust the output voltage.  If you are using a microcontroller, couldn't  you delete U1, and U3 and feed a DAC signal to U2 of 0-5 V to control voltage (Yes you will need to adjust the gain for U2 by changing R12 to about  70K and R1 to about 10K so you get a gain of about 6 so that the 0-4.5V signal will provide the same amplification as the original project).  The microcontroller would also control the current by measuring  the voltage drop across R7 and  replace  U3.  If the voltage drop across R7 was greater than your set voltage, then the microcontroller could automatically drop the voltage to U2 until it matched or was lower than the set voltage.  You could also incorporate a temperature sensor that could also reduce current if a certain value is achieved.    Make sure you pick a processer that has enough pins to support these features and a LCD display.  Not sure if a digital potentiometer is necessary. 

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Guest sonycman

redwire,
Yes, using DAC instead digital potentiometer is another way to go, will see about it.
It must be the DAC with the same precision and external reference voltage as digital POT.

As to remove U3 and forcing to limit current in software by MCU - did`not agreed in that.
Current limiting must be done in hardware only by high speed opamps for reliability reasons.
MCU work is to set up references for opamps and to read voltage and current values via ADC for displaying it.

Thanks for helping!

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sonycman,    Good luck.    Did you check out http://www.electronics-lab.com/projects/test/007/index.html ?  It will give you a starting point for a MCU output display.  The hex file provided  is for a Atmega8 chip so don't purchase a Atmega88 chip. 

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Guest sonycman

redwire,
thanks again! I have some experience with MCUs, they are no problem  :)

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Guest electroguy

Unfortunately, both the op-amps MC34071 and TLE2141 is not available in my country but there is MC34074 available.
Are you using the single op-amps or the duals ?
Is it possible to replace the three MC34071s with one MC34074 ? but the problem remains in U2 for the pins 1 & 5

post-108796-14279144631804_thumb.jpg

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Unfortunately, both the op-amps MC34071 and TLE2141 is not available in my country but there is MC34074 available.
Are you using the single op-amps or the duals ?
Is it possible to replace the three MC34071s with one MC34074 ? but the problem remains in U2 for the pins 1 & 5

An MC34074 will probably get too hot.
You do not say what is your country. Go to www.farnell.com and click on the flag of your country. They have warehouses all over the world called Element14.

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Guest electroguy

I am from Bangladesh,it is a country in South Asia.
Unfortunately, again,  I don't see the name of my country in Element14.
If I use MC34074 with a big heat sink and a computer fan and did you use the offset null for U2 ?
On the other hand, MC34074 do not have any offset null.
Save me!!!

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I do not know why you cannot buy ordinary parts in your country.

An MC34071 quad opamp is in an epoxy package that cannot have a heatsink. You can glue on a heatsink but the epoxy will not conduct heat to it well.

The input offset adjustment on opamp U2 sets the output to exactly 0V when the voltage-setting pot is at zero.
Without adjusting the offset then the output could be plus or minus 20mV.

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Guest electroguy

Should I try this old modified one proposed by Ivan on 02/2010.
Oh, I forget to mention that I already bought the old parts last year :(

post-108796-14279144631873_thumb.jpg

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Should I try this old modified one proposed by Ivan on 02/2010.
Oh, I forget to mention that I already bought the old parts last year :(

The circuit you found uses a 7812 voltage regulator so that the original opamps do not have a supply voltage too high. Then opamp U1 will not work unless its circuit is changed to reduce its output voltage.
The maximum output from opamp U2 is about +9.5V and the maximum output voltage from the project is about +6V. If the original 24VAC transformer is used then Q2 will get extremely hot and might fail and the output transistors will get very hot.

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Unfortunately, both the op-amps MC34071 and TLE2141 is not available in my country but there is MC34074 available.
Are you using the single op-amps or the duals ?
Is it possible to replace the three MC34071s with one MC34074 ? but the problem remains in U2 for the pins 1 & 5

Audioguru wrote:  An MC34074 will probably get too hot.


U1 and U3 need to provide very little output current.  U2 is the real driver in this circuit.  If electroguy utilized a darlington power transistor instead of the 2N3055 then the BD 139, and subsequently the MC3074 would need very little current to drive the BD139.  I think the quad opamp could handle this load.  I would suggest the MJ11016.  Yes, you don't have the input offset pins, so setting the voltage to exactly zero may be impossible but a few millivolts should be a problem?  I think this would be more preferable than building the old circuit.

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Guest electroguy

Instead of redesigning the circuit for the quad one, what if I just place all the pins: 2;3;6 of U1,U2,U3 at the input and output of the quad one. will it work ?
The design is shown below.
The three red/ blue/ green dots represents the input and output of the quad one.
As I also bought the old parts, is it going to be wasted?
The transformer is 24V 3A, if I buy MC34074 and use it for 24V, less than 30V will not be a big problem for me.
@redwire: a few millivolts is not a problem for me

post-108796-14279144632047_thumb.gif

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I finished with working on power supply and made a test yesterday but just one think is not working properly, i can adjust the current only up to 3A then current blocking turning on.
I use elements for 5A project (Redwire) as i mentoined couple of times above but i have not idea where is the problem, i hope that i didn't overlook some element value, i will make a review of all elements in circuit once again.

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Guest MAntunes

Hello!
Would a 24VAC transformer be good for this power supply?
I am building the 3A one.
Regards :)

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So what would I get?
I don't mind getting a little bit less than 30V!

The original project could not produce 30VDC at 3A and when it was tried, its little 72VA transformer and many other parts were overloaded and they overheated. Maybe its maximum output was 25V at 3A or 20V at 3.75A.

The improved project uses a 28V transformer rated at 118VA and many parts are stronger. It is reliable.

We do not know which circuit you are making.

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Guest liquibyte

I'm using these 130VA transformers from Digikey.  At $25 each they are fairly economical and will produce the voltage and amperage required.  I searched high and low for 118VA 28V transformers and these were as close as I could get that were a stock item.

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Guest MAntunes

Sorry for that.
I am making the newer one (3A), I got the parts and the schematic from the first post of this topic, but I only got a 24VAC transformer i think, it may be a little bit more.
I wanted to know what voltage would I get with that tranformer!
Thank you for the help so far :)

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Sorry for that.
I am making the newer one (3A), I got the parts and the schematic from the first post of this topic, but I only got a 24VAC transformer i think, it may be a little bit more.
I wanted to know what voltage would I get with that tranformer!
Thank you for the help so far :)

The main filter capacitor, R7, the output transistors, the driver transistor and opamp U2 all reduce the maximum output voltage when the project has a load. With a 24V/102VA transformer the improved project will make a maximum of 24VDC at 3A.

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Guest MAntunes

Thank you very much audioguru.
I will give feedback here when my power supply is ready!

Another two questions, the BD139 is the only transistor thay should have a heatsink or should all of them have one?
About the opamp, there are 4 references (TLE2141CP, TLE2141CPE4, TLE2141IP and TLE2141IPE4) which one should I get?
Regards and thank you for the help :)

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...the BD139 is the only transistor thay should have a heatsink or should all of them have one?

Heat in a transistor is caused by the voltage across it times the current in it. The result is in Watts.
Each of the two output transistors has a voltage across it of 36V and a current of 1.5A when the current from the project is set to 3A and the output voltage is low or is shorted. Then the heating is 54W in each transistor so a pretty big heatsink is needed. The bridge rectifier can be bolted to this heatsink. 
The BD139 will have a max voltage of 35V and a max current of 75mA so its max heating is 2.6W and a medium heatsink is needed.

About the opamp, there are 4 references (TLE2141CP, TLE2141CPE4, TLE2141IP and TLE2141IPE4) which one should I get?

Get the CP. The I and M have slightly better spec`s that you do not need and a much higher price.

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I finished with working on power supply and made a test yesterday but just one think is not working properly, i can adjust the current only up to 3A then current blocking turning on.
I use elements for 5A project (Redwire) as i mentoined couple of times above but i have not idea where is the problem, i hope that i didn't overlook some element value, i will make a review of all elements in circuit once again.


What is the value of R7?  if it is 0.47 Ohms,  you need to use  0.27 Ohms.  Did you adjust the 100k pot to allow a higher current?

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