AC on DC power supply

[GRA]

I have a 24 volt dc power supply that intermittently has 120 volts AC on the DC side. I will see the AC at the DC output terminals for maybe a 1/2 second. My 12 volt DC power supply does not have that issue. This 24 vdc power supply provides power to a PLC used in my automation application. It only provides power to the PLC and 24 vdc is not used anywhere else in the circuit. What could b causing this anomaly and is it normal.

 

[Nigel Goodwin]

I would suggest there's actually no such anomaly, and your problem is with your meter or method of doing the reading.

What makes you think you have 120V AC on the output?, and why do you think the PLC hasn't blown in to a pile of little burning pieces?.

How did you even come to make such a 'discovery'?.

 

[GRA]

I checked the DC side using two different meters. The PLC has been working for about two months. As I was testing the system, trying to resolve occasional timing errors on the plc, I checked the DC output with a scope and of course saw some noise because it's a switching power supply. On a whim I checked the DC side with the volt meter selected to AC voltage. Intermittently I see the voltage go from .4 vac to 118 vac. It looks like a pretty steady pulse so I am wondering if it's the transistor circuit that I'm seeing.

 

[gophert]

Check this inside of the unit Is there a loose wire, a piece of foil that got sucked in, is the copper on the PCB delaminating and curling.. something else to cause a short?

 

[AnalogKid]

Power supply information, input and output wiring diagrams, schematics - **anything ** ?

ak

 

[MaxHeadRoom78]

If this is an industrial setting, personally I prefer linear supplies. Usually much more reliable.
Or is it one built into the PLC or separate?
M

 

[GRA]

It's on my boat. The power supply is a separate device. I've now tried two different power supplies from two different manufacturers, and both exhibit the same phenomenon. The AC appears when the supply is wired into the circuit and when the supply is setup on the bench with only a couple relays connected as the load. . The second supply is a multi-voltage supply... 5vdc, 12vdc, and 24vdc. The AC only appears on the 24vdc not the 5vdc or 12vdc. I am thinking this might be normal for the 24 vdc switching supplies.

 

[gophert]

It's on my boat. The power supply is a separate device. I've now tried two different power supplies from two different manufacturers, and both exhibit the same phenomenon. The AC appears when the supply is wired into the circuit and when the supply is setup on the bench with only a couple relays connected as the load. . The second supply is a multi-voltage supply... 5vdc, 12vdc, and 24vdc. The AC only appears on the 24vdc not the 5vdc or 12vdc. I am thinking this might be normal for the 24 vdc switching supplies.

Are you measuring when the supply is under load or idle? If idle, the duty cycle of the power supply may be zero and your meter is connected to ground and a floating reference (supply). At that point, a high Input impedence meter could be picking up a signal (as in antenna) from your 120vac source rather than a power surge. Try connecting a light bulb to your power supply and measure again The glitch will likely go away