Steve's Mains Power Isolation Station


Dateline - Year 2000: electrolytic capacitors reach their expiration date...  Hehe...

Seriously, the reality is, vintage equipment is now - and has alwayse experienced failure of components. Be it TVs, radios, recorders, test equipment or whatever, aging components are breaking down. Sitting in attics, basements, garages, and barns is still ample numbers of antique electronics waiting to be restored. One of the tools to perform checks and diagnoses is an invaluable lifesaving piece of equipment called a line power or mains isolation station with current limiting.

Since I was in my first high school electronics class, Iíve always used some kind of current limiting device. In the electronics classroom there was mounted on a large panel on the wall a number of light bulb testers. It was from these experiences where I learned the value of limiting AC current in powering up unknown devices including stuff that was given to me by people who knew I enjoyed tinkering around with old radios and such. It was common to see the white smoke when a student would bring in items to be fixed and they would just plug them in.

I've always had some kind of tester that I used to check out appliances people would bring me to fix in my fix-it shop I had as a young teenager in my parentís basement. Most kids worked at the local golf course caddying or at other after school jobs. I on the other hand fixed appliances to earn extra money.

SE-03 Mains Power Isolation Station

I built this isolation transformer station from a number of salvage components as well as a surplus metal box which was most likely used in the manufacturing of power control equipment. I surmised this from the silkscreen printing which is now on the back side of the front plate as well as a cutout that accommodates a US style duplex outlet.

The power switch is brightly lit when the power is switched on and applied to the variac. Along with the meter, the other pilot light indicates output.

The variac type variable transformer I used is something I got at a garage sale. It originally was damaged on the front, so it ended up in my junk bin where later I it used part of it here.

The very heavy isolation transformer can handle up to 6 amps for all windings, though I am only using it for 4 amps of isolation. According to the fellow who sold this boat anchor to me for a couple of bucks at a Hamfest, it was damaged and was then removed from service from a piece of medical equipment. One of the 16 volt secondary windings was open due to a gash in the side of the transformer. I did repair the winding, but I ended up not using either of the 0-16 volt @ 0.2 amp windings. I did use the 0-40 volt winding which I wired in series with the two 0-110-220 volt windings which were already wired in series on the connector block

I wired in the transformer on the output of the variac because I wanted to be able to vary the output of the various secondary windings of the isolation transformer.

For line conditioning and filtration, I installed a heavy duty line filter I took out of a mainframe power control unit. I also used a 4 amp breaker mounted on the front panel instead of a fuse.

The box is mains grounded throughout. So if I want to totally isolate a device that has a three prong plug, I use an adapter as seen above that negates the ground pin. The secondary of the transformer is totally isolated and is shielded.

This box does not have an amp meter. I figured I could insert a Kill-a-watt in line with the output, which is in its own little box.


DBT-02 Dim Bulb Tester

As for current limiting, I built a dim bulb test panel using three mogul light sockets from some old floor lamps. I had some large 300 and 500 watt bulbs along with a used spare 3-way 50-100-150 watt mogul bulb which I kept for an old floor lamp I got from my childhood home.

With the dim-bulb tester, I can select 50, 100, 300, and 500 watts of load-limiting in series with the device being tested. The rotary switch is this heavy porcelain thing that can handle up to 15 amps.

I sometimes fix old tube-type ham gear. If I have any doubts, I plug inline this current limiter and I set it to the lowest setting of 50 watts and work my way up if needed.

The plug and wire is from a sump pump float switch. The hot side of the plug and socket are wired to two conductors which are in series with the selected dim bulb.


The four serially connected banana jacks on the front of the SE-03 can be used for various voltages ranging from 0 to, if needed, up to 300+ volts AC at 2 amps.

The box will sustain an output at 4 amps at 110 volts for many hours. It can do 130 volts for shorter intermittent periods. This slightly exceeds the voltage rating printed on the isolation transformer.

I made up a pigtail with clips so I can clip on to AC inputs on various devices I am building or testing.

As for the red neon pilot light above the outlet, I say, when you start to see it light up, the output can kill you.

SteveS November 15th 2018