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?
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?, ...
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.
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?
Assuming the power supply is designed to provide galvanic isolation between the AC input and DC output, ask yourself this: "When measuring the potential difference (the voltage) between two nodes that are galvanically isolated, what voltage should one expect to measure?" The answer is, "It could be just about anything."
Figure 1. What voltage does VM1 measure between Earth ground (neutral) on the primary side and either transformer wire on the secondary side, when the transformer's primary and secondary are galvanically isolated from each other?
Figure 2. What voltage does VM1 measure between Earth ground (neutral) on the primary side and signal ground on the secondary side? (NB: Earth ground and signal ground are galvanically isolated; they are not at the same potential.)
Imagine measuring the potential difference (voltage) between the AC neutral line and the negative "-" terminal on a battery, where the AC power source and battery are galvanically isolated. What voltage would one measure in this case? Very likely it'll be an AC voltage: but what does that measurement mean when the meter is connected to two nodes that are galvanically isolated?
And for what it's worth, isolation transformers have parasitic capacitance between the primary and secondary windings, and between the windings and the transformer's metal enclosure/chassis. This capacitance can and does contribute to the potential difference (the voltage) between Earth ground on the primary and signal ground on the secondary. See Andy aka's answer to this post on EE.SE, Isolation transformer and AC potential between grounds.
simply I can answer for above mentioned issue that consider first fundamentals of electricity, there in diagram,I'm observing that the tester voltmeter is connected with nutral in mains and negative in DC, for this condition voltmeter can read only loop voltage and for open circuit it shows zero, however for all negative or another half cycle voltmeter can show the deflection with respect to negative of dc as it provides the reference to that of mains for voltmeter, also the overall dropped voltage/RMS voltage will be shown by meter due to their electrical structural configuration and the way/order of its connection. Keep in mind that voltmeter always needs reference voltage that is zero and this can read only loop voltage where as for ameter these conditions are not applicable.
Good observation. In my opinion the DC side negative is referenced to a floating ground not with respect to the ac mains ground. Why should it be any way ? If one takes care of measurement of DC side voltage through an isolator its fare enough is it not ? Only thing is in the case given by you if the voltages are measured with out isolating the ac ground and dc reference. The probes used may get damaged because 90 volts is applied across it.