[GreenKeys] Off topic: variable transformer

[Suhayl Khan]

I am wondering if there are any other useful applications for having a
variable transformer on the bench. For example, my isolation transformer is
supposed to provide an output voltage of 117 to 124 V for a 120V input. I
noticed that the output can be a little higher than specified (122V input
results in about 130V output under no load). I have my isolation
transformer plugged into the variac and adjust the output so the isolation
transformer remains at 120V before load and then I raise the voltage on the
variac to ensure 120V remain on the output of the isolation transformer
after load. Would this be a valid use of a variac?

Suhayl

On Thu, Jul 28, 2016 at 12:29 AM, Jeffrey D Angus <jdangus at att.net> wrote:

> On 7/27/2016 10:09 PM, Paul Heller wrote:
>
>> I have some electronic and computing equipment that I want to
>> safely power up. I'd like to use a variable transformer. Can
>> anyone recommend a particular model to buy?
>>
> My recommendation: None.
>
> This a bad idea despite what you've been told.
> With regards to vacuum tube gear, there will NOT be enough filament
> voltage on the rectifier tubes to conduct until you reach 80-90 VAC on
> the variable transformer. Then you will "hit" the equipment with 60 to
> 70% of full B+ voltage. However, anything that requires the correct B+
> and any biasing voltages is going to be way off.
>
> If you're doing this to "reform" electrolytics, you're wasting your time.
> They're either good or bad, if the "reform" then they'll probably fail
> shortly afterwards. Electrolytic capacitors are cheap. The stuff they
> damage when they fail are not.
>
> One "computer gear" what ever that means.
> If they have old linear power supplies, the voltages are going to be
> way off until you hit a certain percentage of the line voltage. Having
> wrong or miss-matched voltages is a bad idea.
> On the other hand, if they are switched mode supplies, they tend to
> set fire to themselves when they don't have the right voltages on the
> inputs.
>
>
> --
> Jeff-1.0
> wa6fwi
> http://www.foxsmercantile.com
>
>
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