[GreenKeys] Off topic: variable transformer

Tue Aug 2 14:58:15 EDT 2016

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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