[ARC5] Homemade Isolation Transformer

[Rich Post]

One the cheapest isolation transformer options is a pair of the older UPS
(uninterruptible power source) that are still sold for computer backup.
Often these show up in resale shops such as Goodwill for cheap. Some are
still sold new although many of the new ones use a switch-mode derived 60
Hertz.

The ones to look for are still heavy with the battery removed. The weight
is for the transformer. Obviously you will need two of the transformers for
isolation, the second one can usually be placed in the battery
compartment.  If you find just one, use it as a bucking system to cut line
voltage down.   That won't of course be isolated.  I tend to grab these if
cheap at resale shops and pass them on to friends.

Stay safe,
Rich KB8TAD

On Tue, Apr 28, 2020 at 2:41 PM Scott Robinson <spr at earthlink.net> wrote:

> Hi Roy,
>
> Your Variac voltage booster will work, but only at low currents. The
> Variac current rating is primarily brush current, so if you want to run
> a Variac as a 10:1 step-up, your rated output current will be 1/10 of
> the original rating.  10A brush current at 12V will require a 10A Variac
> to generate 120V at 1A safely.
>
> Peace and health,
>
> Scott
>
> On 4/28/20 2:02 AM, Roy Morgan wrote:
> > 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
> >
> >
> >
> >
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