Re: OT: Housing Voltage in an Autotransformer


If the analog meter movement is located close to the PowerStat, the induced magnetic field into the sensitive meter movement may cause that effect. The PowerStat and other Variac devices are Variable AutoTransformers - hence they have magnetic fields.

Magnetic field strength is attenuated by the cube of the distance. Move the Analog Meter a foot or two distance from the PowerStat. See if that makes a difference.

If the PowerStat is housed in Aluminum as you indicated, Aluminum is transparent (ie. provided no H-Field Attenuation), from about 100kHz down to DC.

Just a possibility.


-----Original Message-----
From: <> On Behalf Of n4buq
Sent: Friday, May 20, 2022 8:27 PM
To: tekscopes <>
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer

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"
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 hmm.....

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

Barry - N4BUQ

Join to automatically receive all group messages.