Basic electronics question: is it bad to have a transformer that’s too big?

[John K Jordan]

Thanks,
I was a bit puzzled at seeing exactly 0 volts ac. Usually I get some very low random values.

I assume you tested to be sure the AC volt meter is working.

I also test transformers by measuring the resistance of both the primary and secondary windings (disconnected from everything, of course!)

JKJ

[Tim Otto]

You mean impedance, right?

[John K Jordan]

You mean impedance, right?

The simple answer is I generally use the resistance scale on my Fluke digital multimeter which puts a DC potential from a battery into the probes. I read the value in ohms.

I'm not an electrical engineer. While resistance and impedance are both measured in ohms, I understand impedance is the contribution of resistance and reactance. Since this is a static DC test the resistance check is more or less a check for an open conductor. Knowing a reasonable resistance for that coil can help determine if windings are shorted, sometimes useful when debugging motors.

Before believing either an DC or AC meter reading that reads zero I like to test the meter with a known good source, perhaps a battery for DC or an AC receptacle for the AC. This proves to me the meter is not broken. With some meters, of course, it's important to use the right scale too.

[Jim Koepke]

Thanks,
I was a bit puzzled at seeing exactly 0 volts ac. Usually I get some very low random values.

I assume you tested to be sure the AC volt meter is working.

I also test transformers by measuring the resistance of both the primary and secondary windings (disconnected from everything, of course!)

JKJ

John brings up my second point of disconnecting everything and checking the resistance of the primary and secondary of the transformer.

That will let you know if either of the windings is open or possibly shorted. Usually a shorted transformer is first detected by the smell or smoke.

Resistance is a DC measurement, impedance is an AC measurement. For measuring the resistance of transformer windings the DC measurement is a good beginning.

jtk

[Roger Feeley]

I did an autopsy on the failed transformer and found a bad thermal cutoff. I can’t explain why it blew. These transformers are old. The last time the house was remodeled was in the seventies..

My daughter wants to just go back to halogens until she replaces all the fixtures. So I ordered a replacement. Based on advice here, I chose one that’s a bit oversize.