"David Nebenzahl" wrote in message
.com...
On 2/8/2009 4:13 PM Ralph Mowery spake thus:

"mm" wrote in message

Coil current with AC applied will be much lower than the value
calculated from DC resistance ... by perhaps a factor of ten or more.

Even at only 60 Hertz?

Inductive reactance is the dominant contributor to impedance here
and it is proportional to frequency and isn't there at DC.

I agree with mm's comment about 24vac relays. Your HVAC thermostat
may have a solid-state relay driver that won't be happy with the
unexpected load.

Probably not 10 times but more like 3 or 4 times more reactance than
resistance for the relays.

As pointed out, at a low frequency like 60 cps*, DC resistance = impedance
to a good first approximation (in other words, close enough for gov't.
work).

When the reactance is 3 or more times the DC resistance that is nowhere near
enough for gov't work. The coil will smoke if you use the DC resistance
even at 60 cps. if the power supply will put out the current.

Oops , forgot about the big bailout going on now by the government and the
big crash that is going on now. That banking and loan calculations were
good enough for gov't. work.
It smoked too.