I have a very simple question. I have an array of AC to DC converters from well known companies such as Meanwell, Delta Electronics, Recom Power, XP Power etc. They take my 230V 50Hz AC mains and convert it to a DC output (24V, 100W in my case). However, when I use an AC voltage tester on the DC lines, I detect the presence of AC voltage. In my case I get an AC voltage of 90-100V (RMS) on the voltmeter when I measure the AC voltage of the DC ground/ 24V with respect to the neutral. Is this because of poor AC mains or is the AC-DC converter not good enough in the sense that it does not filter AC mains well enough from the DC lines? Or am I missing something quite obvious?
Electronic – AC mains on DC ground
acpower supply
Best Answer
You have not provided enough information for a certain answer - eg is output DC -ve connected to input / chassis ground?, are you using a 3 wire mains lead with ground or a 2 wire lead?, is the psu designed to use a 3 wire mains lead with ground?, ...
However ....
In many cases this sort of problem is caused by operating a power supply designed to have a 3 wire power lead with a ground connection with 2 wire mains input with no ground connection, plus output negative is connected to chassis ground and to the (missing) ground wire.
The remaining part of the puzle is the provision of two "Y capacitors" between each mains lead and chassis ground as part of an input noise filter. When a ground lead is connected the chassis ground is at ground potential and all is well.
If the ground wire is missing the capacitor midpoint is at about 1/2 mains potential and so the chassi ground AND the output negative are also.
If you use a typical DMM set to AC volts the meter impedance (typically 1- M Ohm ish) and the capacitor impedances form a voltage divider and so the measured voltage is somewhat less that 1/2 mains.
Solutions:
Correct: Add a ground lead in the input cord and / or power socket.
Naughty: Remove the 2 x Y capacitors - typically 0.001 uF or 0.01 uF or thereabouts.
Did it work?