Malfunctioning Power Supplies

Hi,

Two power supplies are malfunctioning. Both have problems with their variable DC output voltages. The DC output voltages can be varied using a potentiometer.

The first supply gives a fixed negative output (which is much lower than the maximum +ve output obtainable) for all possible values (That is the values which can be got using the potentiometer).

The second supply gives higher output than it should be as the shaft of potentiometer is rotated and reaches the maximum value before the shaft of the potentiometer got completely rotated after that if the shaft is further rotated the output starts to increase from the start again.

Can anyone make any educated guesses about the potential causes/problems? Or give any suggestions?

No circuit diagrams available!!!

The first supply gives a fixed negative output

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You can no longer adjust output voltage? This could be a bad connection at one of the potentiometer terminals.

If not then it is the control circuitry, which could be anything.

after that if the shaft is further rotated the output starts to increase from the start again.

Click to expand...

Are you saying it can be turned around and round, without reaching the end of travel? This sounds like a broken stop inside the potentiometer.

You'll need to replace it. Or if you wish, you can disassemble it, and see if you can attach (or re-attach) a small part to either the rotating part or the stationary part.

BradtheRad said:

You can no longer adjust output voltage? This could be a bad connection at one of the potentiometer terminals.

If not then it is the control circuitry, which could be anything.



Are you saying it can be turned around and round, without reaching the end of travel? This sounds like a broken stop inside the potentiometer.

You'll need to replace it. Or if you wish, you can disassemble it, and see if you can attach (or re-attach) a small part to either the rotating part or the stationary part.

Click to expand...

In the first case the voltage is fixed and -ve (it must be +ve). The supply gives an output of -4V. It should be able to give variable dc voltage in the range of +(0-20)V. Can you tell why it is -ve? Also what do you mean by control circuitry?
In the second case the shaft of the potentiometer is ok. That is, it doesnot let me keep it rotating.

The first supply may have one or more shorted diodes, which is a likely way for negative polarity to get into the system.

Just a guess on my part.

Control circuitry is a general term for components which adjust (or regulate, etc.) the components which carry power.
It may be simple, or it may include several components including IC's.

Hi,
Are your power supplies discrete componet based or IC based? Voltage regulation in each case is different. The former the potanciometer middle terminal is generally wired to a transistor base and the latter it is wired to an IC pin through resistors wired as voltage divider. If one of these components fails, the output voltage gives odd readings.
Narciso Peleida