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0-30 Vdc Stabilized Power Supply


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Hi Arman,
Did you substitute any parts?

The circuit is easy to fix if you list voltage measurements and have it turned on without a load and with the voltage and current pots turned up to max.
Connect the negative wire of a DC voltmeter to the negative wire of C1. Measure the voltages:
1) The positive wire of C1.
2) Pin 4 of U3. (It should be a negative voltage)
3) Pin 6 of U3.
4) Pin 4 of U1.
5) Pin 6 of U1.
6) Pin 3 of U2.
7) Pin 6 of U2.

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  • 2 weeks later...

Hi all,

I need your help.

What hfe should the BC548 (or BC547) and the BC557 (or BC327) have?

A search in this thread didn't turn up any hits, which I find surprising.
Hasn't anyone else wondered whether a BC548A, BC548B or BC548C
is the right one? Or doesn't that matter?


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  • 2 weeks later...

Can I use a 28V transformer?

Yes if you upgrade certain parts.
A 28VAC transformer will be about 29VAC without a load. Its peak voltage is 41V and it will be rectified and smoothed into 39.6V. To have a 3A output then the transformer must be rated at 41V x 3A= 123VA which is 28VAC/4.4A.

The opamps use the smoothed 39.6V for their positive supply and also use a negative 5.6V supply, so their total supply voltage is 39.6V+5.6V= 45.2V. The TL081 opamps spec'd for this project have an absolute max allowed voltage rating of only 36V so they will probably fail. A few other opamps are rated for a max 44V supply but their rating also too low.

The 28VAC transformer will make probably 37VDC at max current. Then if the transistors have low current gain the max output voltage from this project will be about 28VDC.

The output transistor must dissipate 37V x 3A= 111W if the output is set for 3A and is shorted or is set to a low voltage. It can dissipate only about 70W if it has a huge heatsink or a high velocity fan, so two output transistors should be used to share the heat. The driver transistor will also dissipate far more power than the original little transistor can and should be upgraded.
The rectifier diodes will become extremely hot and should be upgraded to a full-wave bridge rectifier module bolted to the chassis.

A few resistors will become too hot and should be bigger. The main filter capacitor should have a higher value.

A recommended upgraded list of parts is here:


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  • 2 weeks later...

Hi Subumann,
It is good to hear that your original circuit works great. Other people have found that its max output is about only 25VDC and about only 2.5A at the same time. They also found that the transistors and rectifier diodes get very hot when the voltage is set low and the current is high.

The 24VAC original transformer has an output of about 25V or more without a load, so its rectified output is 35.35V peak and its unregulated positive voltage is 34V. Two opamps use that 34V as their positive supply voltage and -5.6V as their negative supply voltage and therefore the TL081 opamps are operating with a total supply voltage that is higher than their absolute max allowed of only 36V.

Two of your substitute opamps need a compensation capacitor. All of the dual opamps don't have an offset voltage adjustment needed for U2. Some of the opamps have diodes between their inputs. Most of the opamps are much slower than the original TL081 opamp and therefore might cause the circuit to oscillate.

I recommend using a 30VAC/4.3A transformer, high voltage and wideband OPA445AP opamps, two 2N3055 transistors and a few other parts changes for this project to meet its specs, stay cooler and be reliable. 

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Thanks Audioguru !
But i was triyng to replace the ops with cheaper versions because, for a dual suply the costs of the entire project will be very close (or beyond) to a "store ready made" suply 0-30V/0-3A.
If i replace 081 with LM 301 (with compensator caps) will there be a problem ?
Or maybe LM318 ?

PS: 6*OPA...=6*14$=84$
      2* TRAFO=2*22$=44$
      the rest=~20$
TOTAL=~150$ - displays not included, heatsink not included, case not included...
And a suply with 2*LCD is 140-150$ !!!

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If i replace 081 with LM 301 (with compensator caps) will there be a problem ?

The very old LM301A is 33 times slower than a newer TL081 so the circuit might oscillate. It also has a high input bias current.

Or maybe LM318 ?

It has a high input bias current. The TL081 and OPA445 have FET inputs.

PS: 6*OPA...=6*14$=84$

Where in the world are you? You are being ripped off! Digikey sell them for "only" $9.50US.


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Hi All:

I've been studying this schematic and reading the two primary threads about this power supply for a few days now (and may I say special kudos to Audioguru and Kain for their fortitude in producing the 5A version).

The circuit makes sense except for the current limiting function.  I'm hoping somebody can explain to me how the current limiter acts in a linear manner.

As I understand the project specs I can set the current limiter to a specified amount (e.g. 2 A).  If the load's resistance causes it to draw more than 2A, then the voltage is reduced so that the current is maintained at 2A. 

What I don't understand is how this linear effect is achieved.  It seems to me that U3 is the op amp (wired as a comparator since there is no feedback) used to kick in the current limiter when the voltage drop across R7 exceeds the the preset value assigned by the voltage divider of R18, P2, and R17 (11.2 Volts across 66.033 KOhms is not a lot of current).  When V of R7 is greater than the wiper of P2, the comparator output goes negative to just a volt or so above -5.6V (-4.6V).  This will forward bias D9 and make the voltage on the non-inverting input of U2 about -3.9V

This is a low voltage which I'm surprised doesn't turn off the output completely but in any case I do not see how the voltage into U2 could be anything other than this value when current limiting is on.  The output of U3 is near the negative rail (as expected from an comparator) and there is a fixed voltage drop across D9 resulting in a fixed value into U2.
There doesn't seem to be any components that can vary their voltage or current to reduce the bias into the pass transistor just enough to maintain the 2A.

I've looked closely at the parallel return path thru R9, R8, P1, R7, and D7 but the resistance thru D9 and U3 is much lower than the other path and should dominate the value of the voltage into U2.

I do realize that this circuit has been built, does work and obviously my analysis is the problem here.  If somebody could explain to me the voltages around the current limiter when it is on I think I'll understand the circuit completely.  I'm pretty sure that R9 and R8 will play into this somehow but if their purpose can be highlighted I'd be very grateful.

Thanks in advance for the help!!!!!


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