Potentially dangerous Mains Power supply

Thread Starter


Joined Oct 19, 2020
I recently had a fault occurring whilst connecting my laptop to my mixing desk whilst having multiple new DMX fixtures connected to a USB interface. Whilst checking for faults I discovered a voltage of about 80-90V between the DMX port on the fixture and the Earth Connector. Knowing that the ports on my mixing desk were earthed I assumed that this was the cause of the fault. I then discovered that all my older fixtures seemed to have the same issue and Wasnt quite sure why this hadn't happened to me before. I then discovered many other power supplies had this issue. I assume the problem is some issue involving isolation between the high and low voltage side of the circuit. My question would be simply is this anything to worry about and where might I get a slightly safer power supply as I'm sure many of the cheap power supplies on eBay have the same issue.

I've attached images of the power supply board from one of the fixtures.IMG_20201019_112041272.jpgIMG_20201019_112227203.jpg


Joined Jun 22, 2012
Remove the capacitor that holds the Negative mains side to the output Negative, it usually breaks down, then try reading the voltage again.

It's common in these cheap PSUs.


Joined Jun 4, 2014
It is usual for the output of such isolated supplies to be about half the input supply voltage when measued with a high impedance meter such as a standard DMM. The current available is very low so there is no danger. This due to the stray capaitance between the primary and secondary side - probably mostly between the windings on the transformer.


Joined Jul 18, 2013
What you can do to greatly reduce the high impedance that may account for your measurement, is to place a 1k resistor across the leads.


Joined Aug 7, 2020
If you take out the negative-mains-to-negative-output capacitor as Dodgy Dave suggested, remember to put it back, because it is the most important capacitor in the circuit for suppressing the interference from the switcher.

It passes the relevant British and European safety standards if the current from either of the output terminals to mains earth is <700uA.
However, those same standards require a 5mm clearance gap between live parts and output, and Class-Y capacitors, and that looks nowhere near 5mm to me, and that capacitor also has to withstand a hi-pot test of 4kV without breaking down.

If you're in a 230V area (which I am guessing you are because of the brown and blue cable) than that PSU should go back to the supplier and Trading Standards should be informed.

Now, even if you have a PSU with a proper capacitor and proper spacing, a safety problem arises when you get a dozen of them connected together (such as by the screen of the DMX cable). Then you get a potential shock current of almost 10mA which is getting dangerous, and you also get the possibility of zapping the DMX input when a new unit is connected, because 90V DC at several 10s of nanofarads is suddenly discharged into the RS485 line receiver.
There is nothing wrong with using multiple double insulated power supplies as long as the negative is connected to earth somewhere.