[ARC5] Homemade Isolation Transformer

[Roy Morgan]

Wayne,

Great find you have.  Good luck with it and all safety too with an isolation transformer setup.

Another nice use for a 1+ amp variac is to hook it up to operate in reverse - (i.e. lower output voltage clockwise) and feed a low voltage transformer primary in bucking mode. Now you can vary the reduction in line voltage from zero to maximum with the variac and the knob rotation makes sense.  A one-amp variac handles 120 watts or so, and that’s plenty for a 10-amp,12 volt filament transformer in bucking arrangement.

If you have ever come across any 400 cycle variacs, they can be used at 18volts 60 cycles and full current rating.  (That is 60/400 of normal voltage rating.) They are very much smaller than 60 cycle ones and can usually be had at hamfests for only a little money, because sellers don’t know what use they are. In the output of a 12-18 volt bucking transformer they make for handy line bucking adjustment, again arranged for backwards adjustment.

And while you are making handy bench accessories, make a small chassis or box with 150 volt AC voltmeter and moderate scale current meter.  Combined with the variable bucking variac and transformer, it makes an all-in-one line voltage control system

I have an odd-scaled military grade sealed AC voltmeter that goes from 90 volts to 130 volts. It is built for 400 cycle operation but has had a series resistor added which may correct its readings.  I’ll be testing that thing soon.  I hope it does not fry itself at 60 cycles.  It seems to have an unusual moving vane type movement, so I am not sure what will happen at 60 cycles.

I think you can leave the center taps  in your exit sign transformers unconnected.  To see if it is ok, check for voltage between the two leads before you connect them - this will avoid standing circulating currents if the ends of the transformer windings are not identical and have slightly different voltages on the half windings,

Roy

> On Apr 26, 2020, at 11:32 AM, Robert Eleazer <releazer at earthlink.net> wrote:
> 
> I acquired five power supplies for "EXIT" signs. ...
> I am wondering if there is any reason to hook the center taps on the transformers together.

Roy Morgan
K1LKY since 1958
k1lky68 at gmail.com