RE: [SI-LIST] : Why 110 VAC on my circuit

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From: Ingraham, Andrew ([email protected])
Date: Wed Nov 01 2000 - 05:07:58 PST

> I have problem with a circuit i have designed uses a linear power supply
> with a transformer 220-12 VAC 1A , when i measure on any point on the
> circuit with respect to the earth point i found 110 VAC on the scope. I
> need to know why i see this 110 VAC or this is due to the transformer
> interwinding capactance, if it's so how i can remove this signal from my
> circuit. I think that this is a common mode signal.
Presumably the scope's input is hi-Z. SOUNDS like coupling through the
interwinding capacitance, but there are other possibilities. You need to
consider either a fault, a bad component, or a circuit design flaw that
places a dangerous voltage on the power supply output. What's really
important here is the current capability behind that 110 VAC. If your scope
is very hi-Z, you can measure large voltages in a floating wire, induced by
the lighting in the ceiling ... but with ~no current capability to cause any

Some old equipment used to have two ~ 0.01 uF capacitors on the primary, one
from each line to chassis ... a nice voltage divider that would put the
chassis at 1/2 the AC line potential if the safety ground connection broke
(or if there was none ... before 3-prong AC plugs were common). You could
draw some pretty nasty sparks between equipment with that arrangement!

If your circuit has nothing more than interwinding capacitance and a very
high impedance from the other side to ground, can't you just connect your
circuit to earth ground and eliminate the 110 VAC?

A better transformer with an inter-winding shield or a lot less capacitance,
or adding an isolation transformer (perhaps with secondary center-tap
grounded), are other ways to reduce or eliminate it.

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