[ARC5] Homemade Isolation Transformer

[Scott Robinson]

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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