Filament Isolation Transformer

Robert Simpson


Since this is more of a general topic, I thought it might be better in a separate message.
This topic came out of a different subject related to safely using an oscilloscope to look at home generator output. Chuck Harris suggest using a filament transformer to isolate the scope from the generator. This technique could be used in many applications.

Previous related messages:

Filament transformers are cheaper than a repair on your scope's
front end, and will reveal everything that matters on your
generator's output signal.

A second thing about instrumentation. When you are measuring
stiff power sources, you need to take stiff protection measures.
When you are measuring weak power sources, weak protection
measures will suffice.

Your generator is a fairly stiff power source. It can dump
way more than 4000W into a dead short circuit.
I think I have a 120 to 12V transformer. So I might try some tests with motor loads, such as my table saw and hand electric saw.
Question, should the output leads of the transformer be open or have a light load say 10K oms?
I usually just leave the 1M scope load to set the
output impedance.

But, since it is easy, try a few different values,
and see what looks the best.

50 ohms would not be a good idea, as it would draw
almost a watt from a 6.3V transformer.

How do you connect your probe leads to the transformer output. Isn't the output side now independent of a ground reference? At my workbench, I use a 7A22 with two probes in differential setup. But my 465M is what I am using outside near the generator.
I found a 120 to 34V transformer I can use.
The filament transformer complete isolates the primary from
the secondary electrically. So, I put a household plug on
the 120V side, and usually use a Pomona binding post to male
BNC adapter for the low voltage secondary.

Nothing to stop you from simply soldering a piece of RG58 that
is only terminated on one end, or soldering a BNC female to the
transformer, and using a BNC-BNC cable to connect that to the

-Chuck Harris

OK, I rigged up my 120 to 34V transformer through an on off switch. The output is connect to a terminal strip with a 100K ohm resistor across the output. I am getting a lot of distortion and wonder if I just have a cheap transformer.
Should the frame of the transformer be grounded?

Also, I am looking into buying a filament transformer and a I found some older filament transformers as as well as newer power transformers for sale. Is there a difference?

Ed Breya

If this is just to make a quick and dirty way to monitor the generator, the simplest perhaps is to get a wall-wart that puts out AC. Lots of stuff use this kind, so a good collection of old ones in a well stocked junk department should yield something about right. Just cut off the output connector and hook the wires to a binding post to BNC adapter and into the scope. Then plug the wart into one of the generator's outlets - done.

If it's now evolving (as many things do) into a more general purpose, clean line monitoring probe, then you should revisit my previous post #172658 here:


Robert Simpson

Thank you for the reply. Actually based on all this new information to me, I am looking into several possibilities. Although I hadn't planned on monitoring the generator, your suggestion would make that safe and easy. I hove boxes of wall warts and likely have an AC only version.
I am also looking into using this technique for other types of testing. Belonging to this group often opens new worlds to explore which helps to keep my aging brain active.