[AMRadio] [AM Radio] Old Iron

[Mel Farrer]

One of the things I learned early was that a transformer is just that, it transforms the primary voltage to what ever the ratio is to the secondary.  I have used the light bulb method, but with unknown conditions, I prefer this.

Set up the transformer where all of the leads are stable not moving around and take a standard variac 0-120 VAC and hook it to the primaries.  Put a VOM on the primary and while watching for smoke slowly increase the voltage to the UUT until you reach 12 VAC.  Now set the VOM on the highest scale and work down to get a reading on each secondary.  All of the measured voltages on the secondary are divided by 10.  Easy and no stress on either the transformer or the person testing.  This is the only way I test unknown condition transformers.

Mel, K6KBE

--- On Fri, 2/3/12, JAMES HANLON <knjhanlon at msn.com> wrote:

From: JAMES HANLON <knjhanlon at msn.com>
Subject: Re: [AMRadio] [AM Radio]  Old Iron
To: "AMradio" <amradio at mailman.qth.net>
Date: Friday, February 3, 2012, 3:03 PM


A good way to test a transformer with a 120 volt "primary" winding is to put a 120 volt light bulb in series with that primary and then to power it up with no load on any of the secondary windings.  If there are no shorts in any of the windings, primary or secondary, there will be very little current pulled by the primary and the bulb will stay "dark."  There will also be very low voltage drop across the bulb.  One can, if you wish, measure the resulting voltages across the primary and the secondaries to see how the transformer is doing.
 
On the other hand, if there is a short in any one of the windings, the current drawn by the primary will be high and the bulb will light to nearly full brilliance.  Almost all of the voltage drop will then be across the bulb and very little voltage will appear across the transformer primary winding.  The bulb will act as a variable resistance to protect a faulty transformer from self destruction.
 
If the transformers are all OK, it's not completely obvious that the entire 32V might not be recoverable.  A good clean up might do wonders.
 
Jim, W8KGI                           
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