Re: OT: Housing Voltage in an Autotransformer


The stray capacitance between the winding and the frame and core is passing current and acting as a voltage divider. This is common with transformers and motors. I have a few military surplus power supplies that have .1 MFD. condensers in the power line filters. If you don't ground the case, there is about 60 volts on it and capable of giving a noticable sting, but not enough to trip a GFI. Using a three wire power cable eliminates the problem.

  Bruce Gentry, KA2IVY

On 5/20/22 22:27, n4buq wrote:
I put a 47k across the Fluke's leads and that drops the reading to about 200mV which I expect is normal.

I also used a Simpson 269 analog meter and, oddly, no matter what AC range I select, there's a small rise in the meter reading. That seemed odd but maybe "ghosts" do weird things?

At any rate, I think I'm good now. It just surprised me to see that.

Barry - N4BUQ

----- Original Message -----
From: "Frank DuVal via" <corvairduval@...>
To: "tekscopes" <>
Sent: Friday, May 20, 2022 9:10:05 PM
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer
Did you use the Fluke for this voltage measurement? Try shunting the red
and black leads with a 10 to 100 K resistor to remove "ghost" voltage. I
see this all the time using a Fluke in house/commercial wiring. Just
capacitance coupling. If with resistor shunt, the voltage remains,  then

Frank DuVal


On 5/20/2022 9:48 PM, n4buq wrote:
Please pardon the OT but I don't know where else to turn except to groups like
this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have
rewired it as well). This one of the models with just a two-wire plug and I
decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to
make sure nothing was leaking through to the frame. To my surprise, when I
checked between neutral and the frame, depending on where I test, I'm getting
some significant voltages there. From neutral to the main frame, I'm getting
50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and
it shows infinity with a Fluke DMM so I don't know why there would be voltage
on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where
the windings sit on an insulated disk and a conical inside that rises up inside
the windings. Is it possible that what I'm seeing is just an induced voltage
from the windings? If not, I just don't see where/why there's voltage like
that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Barry - N4BUQ

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