Voltage across transformer primary to secondary - ways to mitigate

Question

I am trying to galvanically isolate my electronics from the Mains and isolate Earth, so have placed an isolation step-down transformer (230:20) in between the Mains and my electronics.

If I measure from load (which is across the secondary) or the secondary terminal to primary terminal, I am seeing Voltage. The two terminals of the secondary show different voltages with respect to the two terminals of the primary, but both the secondary terminals show voltage with respect to both the primary terminals.

The above readings differ from multimeter to multimeter, some not showing any voltages at all. I am sticking to Fluke multimeter.

I guess this is capacitive coupling.These voltages do not disappear even when the secondary is loaded.

I got my transformer maker to make a transformer with shield in between and grounded the shield. Still the cross-voltage only worsened.

As the source of this voltage is capacitive coupling, I guess this will allow AC noise to be transmitted to the load.

I would like to know if it is possible at all to completely decouple this cross-voltage from the secondary and if yes, by what means ? Can this coupling convey primary side noise and fluctuation to secondary side ?

On the secondary side, I have a switching regulator supplying regulated voltage to an electronics instrument. I do not want secondary side to refer to Earth.

Thanks in advance.

Answer 1

I think your expectations are wrong. If the primary and secondary are isolated, you should expect to see some random voltage develop between them.

If the voltage between them were consistently 0 V, that would be a sign that they are electrically connected and that the isolation has failed.

As another answer points out, you may not want perfect isolation in order to protect the secondary from damage due to electrostatic discharge. But even so you might only constrain the floating voltage with back-to-back TVS diodes or a spark gap, which would still allow the voltage between the two sides to vary by 100's of V.

And many isolated systems won't have any such coupling at all. In fact if the isolation is for safety reasons they will likely have to pass a "hipot" test to show that 100's or 1000's of volts can be applied between the primary and secondary without appreciable current flowing.

Answer 2

You're measuring the voltage using a high impedance device - the multimeter probably has a 10MOhm input impedance. Due to capacitive coupling to all the wiring in the walls etc., it will measure voltage even with no transformers involved. Just measure between the tip of your finger and ground.

In other words: what you're seeing is normal, and the design of your device must be able to deal with it, because everyone else's designs deal with it too. And it's not too hard to deal with unless you're measuring minuscule currents or very high impedances. Even bioelectrical measurements are easy to perform without putting the whole setup into a Faraday cage and running it from a battery :)

If you want to figure out what current capacity this voltage you worry so much has, divide the voltage by the input impedance of your multimeter (e.g. 10MOhm).

Say you see 1mV of voltage, the corresponding current flowing into the voltmeter is 1E-3/1E7 = 1E-10 = 100pA.

You haven't told us what application you have in mind, but it will be fairly specialist if a 100pA 50-60Hz current induced into the enclosure is a concern.

Answer 3

It is normal that between two isolated sytems there is a voltage, because of static charge. The secondary is always biased with some voltage referenced to the ground, because we are sorrounded with it. Leaving the secondary floating can make things bad, because there is always a constraint in dielectric strength: if you will touch the secondary electronic, this could destroy the insulation of the transformer, because the voltage difference could rise to tens of kV due to electrostatic discharge. Usually all electroncs is referenced to the ground, even galvanicly isolated.