[Boatanchors] LS-183

[Michael D. Harmon]

Hi Ernie,

I know it's sort of a pain, but when I am trying to map out an unknown 
power transformer, I use the same essential technique as Alex 
recommended, with the following addition:

I start from a reference, say the first lug, and measure the DC 
resistance between that lug and every other lug on the transformer. Then 
I move the common meter lead to the second lug and repeat the process 
for all lugs on the transformer.  I keep doing this until I've measured 
the DC resistance between every lug and every other lug.  Now I have a 
list of resistances, and it's relatively easy to figure which lugs are 
attached to each winding.  After that, you should be able to identify 
the taps on each winding.  Just keep in mind that resistances of say, 
less than 5 ohms are probably filament windings, 5-20 ohms are probably 
primaries, and anything much higher than that is probably a plate (or 
bias) winding.

Once you have a ball-park idea as to which windings are what, hook up a 
dim-bulb tester in series with the suspected primary winding. Start from 
zero on a Variac and slowly advance the voltage a few volts at a time, 
watching the other windings with an AC voltmeter. You should be able to 
get a relative idea as to the ratio of the input and output voltages.  
If you accidentally apply power to a filament winding (or the winding 
has a shorted turn), the dim-bulb tester should keep you from doing any 
damage - the bulb will just glow brightly.

It's a little bit time consuming, but it has always worked well for me.

Mike, WB0LDJ