Silly question here.

I've wired up a full-wave rectifier using this dual-secondary transformer. If you want to check my wiring, I connected red-to-orange to "create" the center tap (I assume this is OK?), and use black and yellow to provide +/- 24VAC. The center-tap is my circuit ground, obviously. See the rough idea below.

rectifier circuit

This circuit works nicely. Using some filter caps and linear regulators, I get a nice DC voltage. Everything is as expected. I was running some relatively high current through a load (about 1 amp).

After a few hours of use, I noticed I was getting a "tingly" feeling when I accidentally touched the circuit ground. My multimeter says 77VAC between circuit ground and earth mains! Yikes!

I don't understand how it floated so high above earth ground. I understand that when you have two unconnected grounds, you have no right to expect them to not float with respect to each other.

But, in this case, it seems to me there should be no way for current to flow from myself (at earth ground) to the circuit ground--hence there could be no voltage developed between "us". I thought this was entirely the point of isolation transformers: voltage is developed with respect to each lead of the secondary, but not with respect to earth ground--so one is safe when working with high voltages. My transformer claims 4000VAC dielectric resistance between primary and secondary. So, what is going on here--how could there be such a voltage? And, should I connect earth mains to circuit ground here to fix this--or will I blow a fuse?


2 Answers 2


That's a big fat unscreened transformer, so there is plenty of capacitive coupling between the input and output. A high impedance meter could easily read a fraction of mains voltage between the center tap and earth. If you flip the input connections you'll likely get a somewhat different measurement.

The measure of health of the transformer is the leakage current at near maximum isolation voltage. It's not possible to measure that easily with a simple multimeter. The voltage measurement alone is not an indication of a problem, however the tingle could be. It would only take microamps to make the meter read something like 77VAC, but a perceptible tingle would reportedly take more like 1mA.

There is no problem in earthing the secondary center tap- if it blows a fuse, the transformer is faulty.

  • \$\begingroup\$ I see; can I model this as a capacitor between primary and secondary? If that is the case, wouldn't there be a large current always flowing to ground if I connected the center tap to main's ground? Well, "large" I suppose would depend on the amount of capacitance. This current, flowing constantly would cause 60hz voltage noise in my ground plane, no? (eventually this design is going on a PCB) \$\endgroup\$
    – Paul L
    Jan 23, 2015 at 6:54
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    \$\begingroup\$ More like two caps, one from each side of the mains. The current should not be large because the capacitance should not be very large. Say 50pF (WAG) at 120VAC/50Hz would be a couple uA, which should not be perceptible. \$\endgroup\$ Jan 23, 2015 at 13:08
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    \$\begingroup\$ Or one cap to half the supply rail (i.e. halfway up the primary). I observed about 1000pf on a smallish (probably 20VA) toroidal transformer.After some surprise, a colleauge did a WAG calculation based on its dimensions and a guess at permittivity ... he came up with 1100pf! So it's real - and pretty harmless to humans, though probably not to microprocessors if you're not careful. \$\endgroup\$
    – user16324
    Jan 23, 2015 at 19:07
  • \$\begingroup\$ Thanks Sephro. You're right--I just measured the current and it's about 38uArms @ 60Hz, from circuit-ground to earth-ground, steady-state at 1A load, \$\endgroup\$
    – Paul L
    Jan 23, 2015 at 20:19
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    \$\begingroup\$ @PaulIL The best thing to do depends on what else you're connecting. If all your connections (in/out) are differential or floating then grounding it directly is probably best. If some are single ended, then it might be better to have it grounded in one spot. You could also consider grounding it through (say) a 100 ohm resistor so any ground loop current would be small, and the voltage would only be mV. \$\endgroup\$ Jan 23, 2015 at 20:41

Earth it after the filter - at your output, not before the filter if you do it at all. Earthing is about protecting things from transients, lightning strikes ... not about everything having some "same 0v". Most of the time that's not necessary and isolating different circuits with different potential is more the point. Safety is about cutting power if there is a differential showing a fault. Having earth to the chasis or any large amount of metal helps for some safety if it prevents potential big spikes during a storm or event from jumping to a human body rather than heading to earth by a shorter, lower resistance path. I too have been obsessed about earthing but I discovered that its really about managing over-voltage and transients - protecting your equipment. Connecting all your shit to earth is not the point - you have to be pretty sure why. For audio applications you really tend to want to isolate and use other means to be safe, so you might earth the chasis but avoid 0v and chassis from being connected and bringing interference, hum and other dangers to your amplifier, pre-amp, processing.

  • \$\begingroup\$ You can also put in some diodes so that current coming into your 0v is routed to your pos or negative lines - accidentaly backwards connection to output etc .. find a power supply circuit with protection diodes. \$\endgroup\$ Dec 21, 2020 at 11:36

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