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0-30 Vdc Stabilized Power Supply


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that colored ring is pointing to ground (0V)

Here are some more measurments on -5.6V elements:


Ur14=-220mV (common of instrument was on Q1 base)
Uc2=40V (common of instrument was on negative of cap.)
Uc3=650mV (common of instrument was on negative of cap.)
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Hi Tedy,
Your Zener dagram is confusing. It shows the polarity of ordinary diodes, not zener diodes.

Your D7 is installed correctly. D5 also has its cathode to ground.
Your lowest voltage is at C3, maybe it is shorted.
Do you have a 4-diode full-wave-bridge rectifier for D1 to D4?
Is the transformer's terminal #2 on my sketch accidently grounded?


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  • 2 weeks later...

Hi again Rodrigo,
Before your supply's problem was that its output voltage was too low. Now it is too high. What happened that changed it?
Is Q2 or Q4 shorted?
What is the output voltage of U2 when the voltage control pot is turned down? It is normal for it to go low but U2 will overheat if left for much time.

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Hi Rodrigo,
The input voltage to U2 should change smoothly from 0V to about 11.2V from the voltage adjust pot. Without a load, U2's output voltage should change smoothly from about 1.2V to about 36V (if the supply voltage is high enough) since Q2 and Q4 are emitter-followers and since R11 and R12 apply negative feedback to U2 so that the output stage has a gain of 3.1.

U2 has a gain of 200 thousand so its inputs should have just about exactly the same voltages. Since yours has its - input voltage too low at 10.5V then it is trying to force its output as low as it can. Replace U2 and turn off the power if it gets hot.

Which opamp did you use for U2?

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Hi Rodrigo,
It is time to troubleshoot your project:
1) Remove R15 (1k or 100 ohms), R12 (56k), C6 (100pF) and C9 (100pF).
2) Temporarily connect the 56k resistor where C9 was.
3) Turn on the power without a load and measure if the output pin 6 of U2 goes smoothly from 0V to 35V when the voltage adjustment pot is turned.
4) Measure the project's output voltage, it should be 0V.

If U2's output works properly then it is fine.
If the project's output has high voltage then either Q2, Q4 or both are bad.

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I build the original 30V x 3A PSU and it works pefectly, but after some (test ;D) short circuits the output fall to 0V. Searching I found Q2 transistor (2N2218A) damaged. This happened two times and I don't know what is the problem ???.
Somebody can help me?


PS: sorry for English ;D

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Hi Mix,
Welcome to our forum. ;D
When the project's output is shorted or set to a low voltage at 3A. Q2 has about 32V across it and up to 100mA through it. That is heating of 3.2W. A little 2N2218A will be at its absolute max internal temperature with about only 1.5W with the clip-on tiny heatsink recommended in the original project.

That is why I recommend using a real little power transistor like a TIP31A that has a metal tab for bolting to a real heatsink.


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