ron_swanson

Variance when testing two like used transformers
« on: July 25, 2019, 07:52:30 PM »
I have two used Triad HS-56 transformers that I'm trying to confirm whether or not all is well before attempting to repurpose.

Using a DMM,  600ohm configuration for both primary and secondaries, both measure ~60ohms across the primary.

When measuring the secondaries, one measures ~60ohms, the other ~50ohms.

I would assume I should also see about 60ohms on the secondaries of both units just like I do when measuring the primaries.

Understanding that a DMM is not 100% indicative of the condition of a transformer as a test, would a 10ohm difference  (~15% discrepancy) between these two units be something to be concerned about?

Short of testing in an actual purpose built circuit, are there other tests I can apply to better understand the condition or is the 10ohm mis-match bad news before I even start?

Thanks in advance!

Cheers,
Greg


Audio1Man

Re: Variance when testing two like used transformers
« Reply #1 on: July 26, 2019, 12:15:58 AM »
This is only a simplistic example of a Transformer:

To verify the transformer you need to make AC and DC measurements.

By your ohmmeter measurement of the DCR tells you only that the resistance of the primary might be a step down type of transformer, however this is only 50% correct.

A transformer is a power matching or impedance matching device. It a step up or step down in voltage or impedance. It is related to the turns of the primary vs the turns of the secondary. If primary has 10 turns and the secondary has 1 turns, then the output voltage is 10 times lower than the input voltage. The transformer secondary has 10 times more current than the primary.

Since the mean diameter (length of wire that wraps around a coil form) the first coil winding is smaller than the next coil windings mean diameter it need a longer piece of wire and the DCR is higher if the wire size is the same.

I hope this give you some help.
Duke :)

gyraf

Re: Variance when testing two like used transformers
« Reply #2 on: July 26, 2019, 03:37:14 AM »
so yes, variance is expected..
..note to self: don't let Harman run your company..

moamps

Re: Variance when testing two like used transformers
« Reply #3 on: July 26, 2019, 06:25:11 AM »
I would assume I should also see about 60ohms on the secondaries of both units just like I do when measuring the primaries.
That's correct if transformers are "matched", made in a same production run. If they are made in different years or decades maybe there are some difference in wire diameter or place position on bobin, etc.
Quote
..would a 10ohm difference  (~15% discrepancy) between these two units be something to be concerned about?
Maybe. You should check AC parameters also.


ron_swanson

Re: Variance when testing two like used transformers
« Reply #4 on: September 03, 2019, 12:34:19 PM »
HI All!

Thanks for the replies. 

After some poking around, I was able to verify using AC as suggested.

~ Greg


 

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