[GreenKeys] Off topic: variable transformer

Tue Aug 2 22:35:27 EDT 2016

There's nothing wrong with using a Variac in front of an isolation xfmr. In
fact, I do it all the time when dealing with hot-chassis designs (AC/DC
tube radios, many TV / CRT chassis, etc.).

Some Variacs do have line isolation - but they are quite uncommon, and will
typically advertise this fact on their labels as they are more expensive to
produce.

Want another quick Variac / Iso hack? Put a stout full-wave voltage doubler
on the output and use it as a high-current variable-voltage DC supply. You
should be able to get 300V or better.

On Tue, Aug 2, 2016 at 1:58 PM, Clay Archer <clay at archerservices.com> wrote:

> I have found it useful to bring up old equipment slowly for the first
> time.   I watch the Amp meter on the variable transformer (Eico 1078) to
> see if the equipment is drawing too much current.   This lets you back off
> the power without doing too much or any more damage to the equipment.
> Yes, it would be a problem if you ran it at too low voltage for a longer
> period, but to reveal a short in a power supply it’s a very useful tool.
> A surge of full power will usually take out the rectifiers or regulator
> circuit, or pop the shorted capacitor with lots of smoke.    There is
> always a transistor that will protect a fuse! ;-)
>
>
>
> For a poor-man’s isolation transformer you can take two identical
> transformers and tie the secondary’s together i.e.:  Line 120V|12V=12V|120V
> isolated.   Use transformers sized to handle the current that you need.
> Old discarded battery-backups (UPS’s) are a good source of free
> transformers.
>
>
>
> Clay
>
>
>
>
>
> *From:* GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] *On Behalf
> Of *Suhayl Khan
> *Sent:* Tuesday, August 02, 2016 8:52 AM
> *Cc:* GREENKEYS BULLETIN BOARD
> *Subject:* Re: [GreenKeys] Off topic: variable transformer
>
>
>
> 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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