0-30V Stabilized Power Supply

[Guest 78L05]

I drew my own PCB design based on original schematic (with 3 TL081), and I will be very happy if I could fix it without major changes.
I tried to put 27k for R10, but the same thing hapeens. I didn't touch trimmer (it's now 100k). When I remove the capacitor on the output (C7) voltage is about - 0.15V. I put tantal capacitor at the same place but voltage is again about - 0.35V (like the first time).
What to do?

[audioguru]

I drew my own PCB design based on original schematic (with 3 TL081), and I will be very happy if I could fix it without major changes.

The TL081 has an absolute maximum allowed total supply of only 36VDC. The original circuit used a 24VAC transformer (that is too low) that produced a positive unloaded supply of 32.5VDC or more plus a negative supply of -5.6V so the total supply for the TL081 opamps is 38VDC or more which might blow them up!

I tried to put 27k for R10, but the same thing happens.

I showed that the datasheet for the TL081 uses 1.5k for R10.

I didn't touch trimmer (it's now 100k). When I remove the capacitor on the output (C7) voltage is about - 0.15V. I put tantal capacitor at the same place but voltage is again about - 0.35V (like the first time).
What to do?

1) Use a 20VAC transformer. Then the TL081 opamps will not have a total supply voltage too high but the max output from the original circuit will be only about +21VDC.
2) Change R10 to 1.5k.
3) Use a film capacitor for C7.
4) Adjust the trimmer for 0VDC output.

Or make the fixed and improved circuit.

[Guest 78L05]

I will try to make how you said to me, and then I will report results.
Thanks!

[Guest 78L05]

I change R10 into 1.5k and nothing, voltage on output is -0.3V.
I set voltage at 0.0V with potentiometer (10k), and measure resistance in that case, it's about 820R, so I added 820R resistor on the potentiometer and now when I turn potentiometer in the left side voltage is 0.0V, and when I turn it on the right side output voltage is 30.0V.

[audioguru]

I am glad you fixed your circuit but I do not understand why the offset voltage was as high as -0.3V and it could not be nulled with the offset adjust trimpot.

[Guest 78L05]

Maybe the problem is too high transformer voltage, but I built this circuit before three-four years and I could set offset voltage at 0.0V without problems. Transformer SEC is 24V, but in my place voltage is about 235V AC, so I get 26V AC on transformer output. I can't unwind transformer to get 24V AC (or less) because it's watered.

[audioguru]

Maybe the problem is too high transformer voltage, but I built this circuit before three-four years and I could set offset voltage at 0.0V without problems. Transformer SEC is 24V, but in my place voltage is about 235V AC, so I get 26V AC on transformer output.

A 24VAC transformer produces 24VAC with its rated load. It might be 25V or 26V with no load.
Your transformer is 26VAC (with or without a load?) but its peak voltage is 26V x 1.414= 36.8V and the rectifier bridge drops the positive unregulated supply to 35.4V or more. Your TL081 opamps are operating with a total supply of 35.4V + 5.6V= 41.0V or more but their absolute maximum allowed supply is only 36V so maybe they are destroyed.

[Guest 78L05]

My transformer is 26V without load. Current regulation work perfect, and I solved problem with offset voltage (added resistor 820R) and everything is alright.

[audioguru]

78L05, can your power supply project produce 30VDC at 3A with low ripple?
I think its maximum output voltage is about 25V or 26V at 3A.

The little 2N2219 driver transistor and the single 2N3055 output transistor will be VERY hot when the output voltage is low and the current is 3A.

[Guest 78L05]

I said that I made my own PCB, and changed few components.
For example, instead of 2N2219 I put BD139 (on heatsink), and for output transistors I use 3xTIP35C with emiter resistors.

[Guest Big zee]

dear all
you can see the different between the original and the other design
he dropped a few and change places then he added diode and resistor for the U1 (pin 7)
and there is the grounding and resistor on pin 6 of U2
and of course you can see he put the transistor back from the old design
and he fused it .
I colored for you the different between the two of them

[Guest Big zee]

I said that I made my own PCB, and changed few components.
For example, instead of 2N2219 I put BD139 (on heatsink), and for output transistors I use 3xTIP35C with emiter resistors.

can you attached the PCB please

[Guest 78L05]

It's in Sprint Layout 5.0, so I don't know how much it will be usefull for you.

[Kevin Weddle]

The power supply draws an enormous amont of current. Voltage regulators have line and load reguation. It's a great 30V supply. Haven't built it yet.
Naw no way. Tell um.

[Guest Big zee]

The power supply draws an enormous amont of current. Voltage regulators have line and load reguation. It's a great 30V supply. Haven't built it yet.
Naw no way. Tell um.

why is that ?

[audioguru]

The power supply draws an enormous amont of current.

No.
It draws only a little more current than its output current.
With no load it draws about 20mA.
With a 3A load it draws 3.3A maximum.

[Kevin Weddle]

I was thinking of the low current IC's in regard to the high current output. Higher current means a lower beta, a lower impedance and a larger change in current. It wouldn't be an easy circuit to wire up. If any part of the design is wrong or a transistor is damaged or too far out of the design parameters it won't work.

[audioguru]

I was thinking of the low current IC's in regard to the high current output. Higher current means a lower beta

Yes, but the math is simple:
1) The two 2N3055 output transistors have a minimum beta of 41 when the load current is 3A.

2) Then the maximum current of the BD139 driver transistor is 3A/41= 73mA.

3) The minimum beta of the BD139 at 73mA is about 50 so its maximum base current is 73mA/50= 1.5mA.

4) The opamps can provide at least 10mA then they have no problem to provide only 1.5mA or less.

The parts will work perfectly if they are name-brand. Cheap Chinese copies sold on E-Bay might not work.

[Kevin Weddle]

I would want to increase the stability of the supply for small signal regulation. Before and in series with Q2 could be a fully biased NPN transistor so that the bias resistors can set the base voltage. Q2 would only need a BE diode to help limit the voltage drop and a VCE zener diode with a voltage best for average use.

[audioguru]

I would want to increase the stability of the supply for small signal regulation.

The modified project has perfect stability, why change it?

Before and in series with Q2 could be a fully biased NPN transistor so that the bias resistors can set the base voltage. Q2 would only need a BE diode to help limit the voltage drop and a VCE zener diode with a voltage best for average use.

Then you have removed opamp U2 that provides very accurate voltage regulation when the load changes and have removed the adjustability of the output voltage and current.

[Kevin Weddle]

A higher conduction voltage diode has a high impedance, it only shares it with the transistor. Probably around 15V, the impedance drops and the opamp only follows  the signal. So there is no small signal regulation. But you can always use an opamp to power low power DC circuits. But almost always they are used in filter circuits. Which isn't good at 1MHz because they are heavy and frequency sensitive and don't function.

[audioguru]

A higher conduction voltage diode has a high impedance

Where is the diode? The output stage has an opamp driving transistors configured as a darlington emitter-follower. The extremely high open-loop gain of the opamp causes this amplifier to be very accurate and have an extremely low output impedance.

.....So there is no small signal regulation.

No.
The small signal regulation is excellent.

[audioguru]

Hi, does that 'short circuit' LED should be called CV (constant voltage) or CC (constant current)?

The LED lights to warn you that current regulation (constant current) is reducing the output voltage.

[Kevin Weddle]

audio the gain of U2 should only be a liitle more than 2 for low frequency circuits. Only if the powered circuit is very high frequency will C6 lower the feedback enough.

[Kevin Weddle]

I was looking to modify Q2 with a BE diode and a VCE zener and an upstream transistor to drop the voltage for the zener. If Q2 has a low BE operating voltage and the parallel diode doesn't go low impedance, U2 will work fine. It's good to hold Q2 at a constant VCE and VBE. As long as the parallel diode doesn't lose impedance, Q2 can supply adequate current to function Q4 and small signal regulation will only be lost under higher current. But all is not lost if you don't exceed the maximum of U2. The voltage will only follow the small signal.