0-30 Vdc Stabilized Power Supply

[jhanus]

Hello,

There was a suggestion to put two pots in serial for better control precision, but they are connected with all 3 terminals, as divider.

To connect the second pot to adjustment terminal of first?!

 

[audioguru2]

I don't recommend using two pots in series for fine and coarse control.
I recommend adding trimpots or fixed resistors in series with the existing pots to reduce the max voltage to exactly 30.0V and to reduce the max current to exactly 3.0A.

All 3 terminals should always be used on pots. Then if the slider loses contact the resistance will be the max resistance of the pot instead of infinite resistance.

View attachment 40420

 

[jhanus]

Thank you,

I understand first and the second recommendation and I will implement calibration trimpots.

But just for my knowledge, how to connect two pots in series if one has all three terminals connected.
If each terminal comes to each terminal that is parallel, but how to realize connecting in series(in this circuit)?  ???

 

[audioguru2]

I showed trimpots with three terminals but wired for two terminals, connected in series with pots that have three separate terminals.

Nothing is in parallel. The series trimpots reduce the max voltages.

 

[jhanus]

Thanks,

OK, correct me if I'm wrong, with trimpots I will reduce max voltage to 30V and current to 3A, to existing trimpots I will add another pot in series connected exactly like trimpot for fine tuning and that should be it.

 

[audioguru2]

You don't need fine-tuning controls on a variable power supply.

 

[jhanus]

Audioguru, thank you for information, I will implement your suggestions.

 

[eggy1]

Before you said the output was +33V with Q1, and 0V without Q1.
What is the putput voltage and what is the voltage of pin 6 of U2 when the voltage pot is turned? Also, what is the voltage of pin 6 of U3 when the voltage pot is turned?

Okei, I was away from the PSU for a while.
Voltage at the pin 6 on U2 varies from 4,1 t o 26.6V when voltage pot is turned. Pin 6 on U3 does hold its voltage at near the power supply voltage.
I tested opamps within one test circuit and the worked fine, so the should be alive.[atleast at the moment] Somehow I have "lost" this 5.6V negative power supply...I tested diodes, resistors-the seems fine. Voltage drop across D7 zener is 0.8V [should be 5,6V?].
One more interesting thing...when I removed Q2, then output voltage of PSU vary from 4.1 to 26.6V. (It comes directly from op-amp?`No real current....) When I connect Q4 output voltage rises and is not changeable.

PS! Sometimes R15 heats up quite quickly. This could happend if Q1 is shorted? It connects one end of R15 to GND and otger goes through D10 to Vcc`? 

 

[audioguru2]

Somehow I have "lost" this 5.6V negative power supply...I tested diodes, resistors-the seems fine. Voltage drop across D7 zener is 0.8V [should be 5,6V?].

The circuits won't work properly without the -5.6V negative supply. C2 and C3 should be replaced to see if they are bad.

when I removed Q2, then output voltage of PSU vary from 4.1 to 26.6V.

The output of the opamp connects to Q2. Q4 cannot work and the output cannot work without Q2. Something is shorting the output of the opamp to Q4 or to the output. maybe D10 is backwards.

When I connect Q4 output voltage rises and is not changeable.

Double check the pins on Q2 and q4. If they are connected correctly then replace Q2 and Q4 because one is defective.

Sometimes R15 heats up quite quickly. This could happend if Q1 is shorted? It connects one end of R15 to GND and otger goes through D10 to Vcc`? 

Whatever is wrong with Q2 or Q4 is causing R15 to be hot. If Q1 is shorted or if it is normally turned on because the -5.6V negative supply is off (yours is off) then U2 would get hot and R15 would be cold and would turn off Q2 and Q4.

 

[eggy1]

Okei, Replaced Q4[actually connected wires to another transistor]. D10 was backwards...C2 and C3 seemed to be fine. I have replaced D5,D6,C2,C3,D7(R3 is OK, 230Ohms)...nothing-voltage drop across D7 is still  near 0.7V...all op-amps has same voltage across pin 4 and 7. At least one thing is working...kind of...I connected 1.5Ohm resistor in series with digital ammeter[is it right term in English? ] and another digital multimeter showed me output voltage. First it showed 33V, resistor connected, voltage dropped to ~4,5V...although current pot has no effect...

Still trying to get it work....

PS! I use that kind of PSU. AC for negative power [originally from AC pin 2]is taken before one diode...

PS2! I had this negative 5.6V by using the same scheme...

 

[audioguru2]

The voltage across your D7 is only 0.7V because it is also probably connected backwards. The cathode end of diodes is marked with a black band.

33V across 1.5 ohms is a current of 22A! When the voltage dropped to 4.5V then the current was 3A.

 

[eggy1]

So, reversed D7. Voltage across it is 5.6V, but....voltage across pins 4 and 7 of U3 and U2 is ~5 volts lower than U1's voltage at the same pins...it should be higher, not lower, right?

 

[audioguru2]

I think your -5.6V supply is actually +5.6V because the other two diodes are backwards.

 

[eggy1]

Non of the diodes are connected reverse...when I revers any of[ D5,D6D7] diodes the current limit led stops working.

 

[audioguru2]

Is your -5.6V supply at pin 4 of U2 and U3 a negative voltage or is it a positive voltage?

The LED is supposed to light only when the project is reducing the output voltage because it is regulating the current. It warns that the output voltage is being reduced. It also warns that the load wants to draw more current than the current pot's setting.

 

[eggy1]

Is your -5.6V supply at pin 4 of U2 and U3 a negative voltage or is it a positive voltage?

Just to be sure....I have to measure it between pin 4 and...pin 7 or negative rail(the one with R7)?

The LED is supposed to light only when the project is reducing the output voltage because it is regulating the current. It warns that the output voltage is being reduced. It also warns that the load wants to draw more current than the current pot's setting.

I know, but when D7 is reversed then current led will not light up when 1,5Ohm resistor is connected between output terminals of PSU. Its like Op-amps stops working.

 

[eggy1]

Okei, negative supply is now OK. I had some bugs with the AC wire from the transformer to R2.

 

[audioguru2]

Hi Eggy,
Good, you fixed it.
Didn't you use a 24VAC transformer?
How high does the project's DC output voltage go when it has a 3A load?
What is its max output voltage without 100Hz ripple when it has a 3A load?

 

[rufinus]

Finally i managed to put it all together. Everything was working fine. I connected 1 ohm resistor to the output and measured up to 2.5 amps through it. Voltage and current regulation was excellent. Then i decided to insert ammeter between emitter of Q4 and output, as audioguru suggested.
And i got a blown fuse. After replacement, i can regulate the voltage. But with the same resistor connected voltage drops to 0 and does not regulate anymore. Current is something like 30 maA. 
Any ideas??

 

[eggy1]

Hi Eggy,
Good, you fixed it.
Didn't you use a 24VAC transformer?
How high does the project's DC output voltage go when it has a 3A load?
What is its max output voltage without 100Hz ripple when it has a 3A load?

Actually transformer output has 25V AC before diodes and caps. After, 31V. Capacitors are 4x2000uFx50V. Have to test this 3A load part, but what do you mean by "without 100Hz ripple"?
PS! Now the R2 is heating....even without op-amps...
PS2! Voltage on U2 pin3 is varying from 5.1 to 16.4V not from zero to 11V...Output voltage changes from zero to 31,5V, BUT there are only two stages...zero and 31.5V. Each other in one end of the voltage pot. No smooth  voltage change.