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I have a Comfort home control system. This takes 15V AC via a transformer (from 240V). It's been working for years, but last week the fuse blew. It's a 250mA fuse on the mains side of the transformer.

With power off I checked the impedance on the 240V side of the transformer and it shows about 80 Ohms. Is that too low? By my reckoning that would mean 3A, which would blow the fuse. I'm guessing that insulation in the windings could have broken down and caused a short. Does this sound reasonable or am I misunderstanding how the transformer works?

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up vote 1 down vote accepted

Your understanding of transformers is a bit off, but the root cause may be as you suspected...

A transformer (usually) consists of two coils of wire, a primary and a secondary, wrapped around a ferromagnetic core. An AC voltage applied to one coil creates a changing magnetic field in the core, which induces another AC voltage in the other coil. The ratio of the primary voltage to the secondary voltage is determined by the ratio of the number of turns in each coil (the turns ratio). When AC is applied, the impedance across the primary coil is that of the load circuit multiplied by the square of the turns ratio.

An ohmmeter applies a known DC voltage to the circuit being tested and measures the current flowing into the circuit. Because it's DC, the magnetic field in the core doesn't change, so there's no voltage induced in the secondary coil. What you are measuring is simply the resistance of the primary coil, which should be low.

Barring some unusual circumstance like an electrical storm causing a surge or a teenage son turning everything up to 11, you probably have a problem somewhere in your system. I think I would call in the service people at this point (I'm not familiar with your system, so don't know if it can be serviced by end-users), but if you're up to troubleshooting, and if you can disconnect or turn off everything on the secondary side of the transformer, you could try replacing the fuse and re-applying power. If the fuse blows again, you know it's the transformer at fault. If not, reconnect or turn on devices one-by-one until the problem manifests again.

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The problem turned out not to be the transformer. The X10 interface had died and was shorting out. Thanks – steevc Oct 1 '10 at 12:59

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