It is quite normal to measure an AC voltage between an isolated circuit common and earth ground or neutral using a high impedance voltmeter. The voltage is caused by capacitive coupling between the primary and secondary windings on the transformer. If the end of the primary winding closest to the secondary is connected to the live terminal of the supply, the voltage can be quite high. Try reversing the mains connections to the transformer. It will probably reduce the measured voltage. Some high quality power transformers have a grounded electrostatic shield between the primary and secondary windings to reduce the capacitive coupling.
The best way to minimise this problem is by disconnecting the metal case of the supply from the circuit common and connecting it to the supply ground. This is the way commercial lab supplies are usually wired, with a ground terminal connected to the case, close to the output terminals.

Thank you for the advice. I will do so.

 

It seems to me, the transformer 230/24V 50VA can't be so leaky to cause a shock. What else- a small SMPS in black wrapping. It was said, the problem persist even it's was disconnected.

I see the only solution- to perform a hipot ( dielectric withstand ) test. Test voltage -500 or 1000VDC. Simply borrow a megger and check the resistance.

It's for your own safety !

Thank you for the answer. I am able to do such a test and I will, because we have a megger at the University.

 

Playing around I have found something interesting . What KeithWalker said about reversing the plug is true. I have come to the conclusion that I still have a lot to learn. I guess you are an eternal student in this field.

Thank you everybody for your contribution!

 

Playing around I have found something interesting . What KeithWalker said about reversing the plug is true. I have come to the conclusion that I still have a lot to learn. I guess you are an eternal student in this field.

Thank you everybody for your contribution!

The day I stop learning is the day they bury me!

 

Nothing makes sense at this point, IFF the transformer is serving as the power source for that power supply. That is what I see in the photo with the first post. But the wiring is such a maze that it is difficult to be certain about what is connected to the mains.
Consider that the mains power is evidently connected to something else if reversing the line plug solved the problem. The mains input to the transformer is isolated from the secondary quite well. THAT seems to remove the entire power supply PCB assembly from the suspects list. So what else is connected to the output and those screws on the front panel that deliver the shock??
Probably doing a hi-pot test with all of that low voltage semiconductor stuff will totally destroy enough that the supply will only have scrap metal value. So I suggest that is a very poor choice. Leakage has already been proven and the hipot test will not locate the fault or wrong connection.If there were not the isolation transformer in the system I would guess that the pass transistors remotely mounted on that heat sink were the source of shocks.I see no hint of insulation from the heat sink, and I see solder connections very close to it. And usually the center lead of a power tab device somehow gets connected to either the tab or the mounting surface.

 

Nothing makes sense at this point, IFF the transformer is serving as the power source for that power supply. That is what I see in the photo with the first post. But the wiring is such a maze that it is difficult to be certain about what is connected to the mains.
Consider that the mains power is evidently connected to something else if reversing the line plug solved the problem. The mains input to the transformer is isolated from the secondary quite well. THAT seems to remove the entire power supply PCB assembly from the suspects list. So what else is connected to the output and those screws on the front panel that deliver the shock??

MisterBill2, if you read my comments in answers #15 and #19, you obviously did not understand them.

I do agree that using a HI-Pot to test the transformer while it is in circuit will destroy most of the components, but there is absolutely no reason why it should be used anyway.

 

MisterBill2, if you read my comments in answers #15 and #19, you obviously did not understand them.

I do agree that using a HI-Pot to test the transformer while it is in circuit will destroy most of the components, but there is absolutely no reason why it should be used anyway.

The transformer is to provide the isolation from the mains that is not part of the switcher design.And we have no hint as to what the intended voltage range of the unit actually is. So an isolation/step down transformer is rather important. And it is probably not the source of the leakage. That can be verified with an ordinary volt meter.

 

The transformer is to provide the isolation from the mains that is not part of the switcher design.And we have no hint as to what the intended voltage range of the unit actually is. So an isolation/step down transformer is rather important. And it is probably not the source of the leakage. That can be verified with an ordinary volt meter.

If you examine the schematic of the circuit you will see that the negative output of the supply board is connected to the negative terminal of the mains fed bridge thru a 4700 ohm resistor. So that is not some static buildup. It IS a connection.

 

If you examine the schematic of the circuit you will see that the negative output of the supply board is connected to the negative terminal of the mains fed bridge thru a 4700 ohm resistor. So that is not some static buildup. It IS a connection.

The bridge is not mains fed. He mentioned that he is using an industrial 24V transformer, so the circuit is isolated from ground.

 

That was my other point.
So the voltage is coming from some other connection. And since we are not given any hint as to the rest of the circuit, it might be anywhere. It could be from a digital meter that we do not even know about. Or even a strand of wire in one of the terminal strips that has gone astray.. I have seen that mistake made by a panel builder.

 

MisterBill2, if you read my comments in answers #15 and #19, you obviously did not understand them.

I do agree that using a HI-Pot to test the transformer while it is in circuit will destroy most of the components, but there is absolutely no reason why it should be used anyway.

I agree that Hi-Pot test would destroy the electronics. In which case your recommendation for this test should really go with a warning to the TS so that he would be aware that he is about to destroy all his hard work, in which case he may decide to delve much deeper into finding where the leakage is coming from before destroying most of his work, even though it is dangerous in its current state.

 

Can't figure out what those white bits under the power in the connector/earth tab are. They look like high-wattage ceramic resistors. They look like they are connected to the mains input and going to the PCB?

 

Those "white bits" might be the wires going to the remotely mounted pass transistors, if I am seeing the same as you are describing.
And now the bad news, which is that if reversing the mains plug removes the shock source, then it seems that there may be an accidental connection between the mains and the four screws. THAT could be a big deal sort of problem. I suggest marking the plug to note which insertion is safe.

 

Those "white bits" might be the wires going to the remotely mounted pass transistors, if I am seeing the same as you are describing.
And now the bad news, which is that if reversing the mains plug removes the shock source, then it seems that there may be an accidental connection between the mains and the four screws. THAT could be a big deal sort of problem. I suggest marking the plug to note which insertion is safe.

Reversing the mains does not "remove the shock source". It reduces the voltage to ground that is capacitively coupled to the secondary. You obviously still don't understand what I explained in post #15.
The voltage that is measured between a power transformer isolated secondary and ground is caused by the capacitive coupling between the outside layer of the primary winding and the inside layer of the secondary winding.
If the end of the primary winding closest to the secondary is connected to the live terminal of the AC supply, the full AC voltage will be capacitively coupled to the secondary.
If the power connections are reversed, there will only by the small voltage that appears across the outside layer of the primary winding, which is what will be capacitively coupled to the secondary.
If you find this difficult to comprehend, I suggest that you try it for yourself, using a mains transformer that does not have a grounded electrosttic shield between the primary and secondary windings.
To remove potential shocks the circuit should be isolated from the case and the case should be grounded. That is the way commercial lab supplies are wired. A ground terminal is connected to the case for use when the supply is not required to be isolated.

 

I am fully aware that reversing the ends of essentially a voltage divider changes the voltage. I have worked with a lot of different systems and understand that fully. And it might even be a problem with the transformer. But given the view of the wiring I suspect the problem is someplace else.
Removing the transformer and doing a hipot test on it would be reasonable, but doing it inside the assembled package is what I see as a poor choice.

 

I am fully aware that reversing the ends of essentially a voltage divider changes the voltage. I have worked with a lot of different systems and understand that fully. And it might even be a problem with the transformer. But given the view of the wiring I suspect the problem is someplace else.
Removing the transformer and doing a hipot test on it would be reasonable, but doing it inside the assembled package is what I see as a poor choice.

Why bother? I don't think there is anything wrong with the transformer or the circuit. It behaves in the way I would expect. It just needs the case grounding to avoid any problems.

 

Hello,
Those white bits are power resistors, I have used them since I have two transistors in paralell. No mains connection there.That was more of an experiment since the PSU will not be used for more than 2Amps max and 20 volts. I have been super careful with the wiring and checked for shorts. The transistors have mica pads to isolate them from the heatsink, you just cannot see them. I also checked to see if the thermal paste is conductive. It is not. . And I do understand that if I want to perform the test mentioned previously I would need to extract the transformer out of the circuit.

I do agree that the wiring is a bird's nest and I need to make it more professional, but this PSU was more of a learning experiment for me. Thank you and sorry for the eventual misunderstandings. English is not my first language and that fact might generate confusion.
A sincere thank you to everybody who took their time to help me!

 

OK, and no apology needed.
It seems that somehow there is a high resistance connection from one side of the mains connection to those front panel screws. Reversing the plug reduced the voltage, but that has not solved the problem of the resistance connection to the screws.