The AM Forum

I was trying to help W5TMG with his BTA 1R project and measured the DC R of my BTA 1R1 mod transformer to give him a reference number.

Problem is that my mod tranny measures 29 ohms to CT on one side and 44 ohms to CT from the other.   I was really expecting these to be similar values.  Does this mean I have a bad mod transformer?

73 Jack KZ5A

Not necessarily.  It could be connections to the terminals, etc.  How many times did you measure it, and with what were you measuring?  I've run into some interesting and similar issues when measuring DC resistance of transformers using a Simpson 260.  When you remove the probe, there is a big (relative to the battery voltage in the meter) reverse EMF, that tends to "weld" poor connections (temporarily) including those within the meter itself (like the battery).

A way I've tested mod transformers for shorts in the past is to put an incandescent lightbulb in series with the primary, and hook it up to 120V.  The bulb should light dimly or maybe even not at all, depending on the transformer.  If you short out the secondary, the bulb should light near full brilliance.

Any short, even between a small number of turns, will cause the light to be fairly bright when the secondary is open.

Anyway, another test to use.  Are there any clues that the transformer might be bad, so you're checking it?

The unequal DC resistance from the CT to either side of the winding is normal.  The reason is that although there will be the same number of turns either side of the CT, the length of wire is different since the winding radius gets larger as more turns are piled on. The outer turns have a greater circumference per turn.  This all adds up to greater resistance of the wire on the outside half of the center tapped winding.

To see if the transformer is good, drive the secondary with a filament transformer and then measure the voltage between the CT and either side.  The voltages measured from the CT to either side should be the same.  You can also check the turns ratio this way.

Chuck K1KW

Yep - I've seen this in most transformers too.  Makes sense.

It certainly is alarming the first time we come across it.

T

I agree completely with these comments.  I had the same thing happen to me when checking out a 30K-4 mod xfmr.  The transformer turned out to be fine, however, Later, it did fail and I had significantly different readings than when I measured it the first time.  

Thanks also Steve for the additional light bulb test!  Good info!

73,  Jack, W9GT

The DC resistances will always be different.  The amount of wire used to make the same number of turns on the core will be greater as the core diameter increases with each layer of wire.

You can check your mod xfmr by applying an AC voltage to a secondary winding and then measure the AC voltage on the primary.  Measure from the CT to each side.  The two voltages should be very close in value.

Fred