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I have a large transformer and I want to measure its coil resistance to find the primary and secondary. I set my multimeter in the 2000 ohm setting and measure. At first, it reads 500 something, and then the resistance slowly dies off to 0. What am I doing wrong? Why can't I make a steady measurement?

My multimeter: Craftsman 82141 My transformer: TKP0278-E142Z

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  • \$\begingroup\$ Maybe the coil has inductance? Use a lower ohms range like 200 ohms or 20 ohms and take the (low) value when it has settled. \$\endgroup\$ – user_1818839 Nov 11 '20 at 15:51
  • \$\begingroup\$ Curious what you are measuring the resistance for? It won't tell you much about the transformer other than it's "probably good" or "probably bad". It definitely won't give you information such as turns ratio or power handling capability. \$\endgroup\$ – Kyle B Nov 11 '20 at 16:35
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What am I doing wrong?

Nothing. This is actually the expected result. You have discovered inductance. Congratulations*. Had you done it about 200 years ago you'd be famous. But you can call yourself Joe or Mike for the day with my permission.

Why can't I make a steady measurement?

You can, if you're patient. The transformer is an inductor, which resists sudden changes in current. Most ohmmeters are really voltmeters that put a constant current** into the device under test and then read the resulting voltage.

So when you hook up the thing, it tries to force the current through and can't -- that "force" is the voltage which it reads. As you continue to force current through, the effect decays depending on the meter characteristics and the inductance of the transformer winding.

To get an accurate reading, wait a while until the inductive response fades.

As a fun experiment, do the measurement on the primary with the secondary coil open and note how long it takes to settle. Then do it again with the secondary coil shorted. It should take significantly less time to settle. Repeat this while measuring the secondary, with the primary open and then shorted.

The reason this effect happens is because the short circuit is "reflected" back to the opposite coil, by the same mechanism that makes a transformer primary draw power when you draw power from its secondary.

* I can never tell with the written word if I'm coming off as snide or not -- I don't mean to in this case. Inductance is pretty cool.

** As constant as they can, up to the limit of the voltage the designer is willing to put on the wires.

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  • \$\begingroup\$ So basically, I just wait for the value to settle down, and then eventually it will go back up from 0 to the actual reading? \$\endgroup\$ – DragonflyRobotics Nov 11 '20 at 16:41
  • \$\begingroup\$ It's an inexpensive meter that only goes down to a 200\$\Omega\$ range -- when it says "0" it could be anything below an ohm or so. If you really need to know you need to put a healthy, known current through it (I'd go with about 1/10 the rated AC current for the winding), then measure the voltage, then do some math. Or you need a tester that's specifically designed to measure low resistances (which will do the same thing, just more conveniently). \$\endgroup\$ – TimWescott Nov 11 '20 at 17:36
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It's not unheard of for even a moderately sized transformer to have a primary inductance of circa 100 henries. It's also quite common for a multimeter to inject 1 mA when testing resistance so, If I simulated 100 henry in parallel with 1 kohm (to ensure my simulator isn't damaged by excess voltage) I would see this graph with time: -

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It's taking about a second to properly settle down.

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    \$\begingroup\$ "to ensure my simulator isn't damaged by excess voltage" Very wise of you. I've lost a few co-workers for this very reason, due to secondary USB port and LCD detonations. Be safe out there. \$\endgroup\$ – Chris Knudsen Nov 11 '20 at 16:30

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