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Transformer question (puzzled)

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Transformer question (puzzled) Doug Miller 04-12-2007
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Posted by Doug Miller on April 12, 2007, 12:49 pm


Honeywell's published documentation for their R182C switching relay
http://customer.honeywell.com/techlit/pdf/95-0000s/95-6762.pdf
contains the following warning:

IMPORTANT: The transformer [120VAC primary, 24VAC secondary] on the R182C may
overheat when used with a series 20 thermostat if the total resistance of the
thermostat circuit exceeds 2.5 ohms. If the measured resistance of the
thermostat (including thermostat wire and thermostat contact resistance)
exceeds 2.5 ohms, add a 100 ohm, 10 watt resistor between the W and R
terminals.

I don't understand that -- if the total resistance is, say, 10 ohms, clearly
the current drawn will be *lower* than it would be at 2.5 ohms, or at 1 ohm.

How is the transformer in danger of overheating at a *lower* current? Seems
to me that the greatest danger of overheating would occur with a circuit
resistance of near zero ohms, i.e. a dead short across the transformer
secondaries. Someone please explain this to me.

[Please note that I *do* understand the purpose of the 100 ohm 10W shunt
resistor, in reducing the total resistance of the connected load. What I
*don't* understand is how a *lower* resistance avoids overheating the
transformer.]

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

AppliancePartsPros.com, Inc.
Posted by on April 12, 2007, 2:01 pm



>IMPORTANT: The transformer [120VAC primary, 24VAC secondary] on the R182C may
>overheat when used with a series 20 thermostat if the total resistance of the
>thermostat circuit exceeds 2.5 ohms. If the measured resistance of the
>thermostat (including thermostat wire and thermostat contact resistance)
>exceeds 2.5 ohms, add a 100 ohm, 10 watt resistor between the W and R
>terminals...
>
>How is the transformer in danger of overheating at a *lower* current?

Maybe this "transformer" is really some sort of constant-current device,
and pushing current into a higher resistance requires more power, which
could overheat it. Some control circuits work that way, eg 4-20 mA loops
which supply constant current regardless of wiring resistance.

Maybe the "transformer" just supplies enough current to operate the R182C
relay, regardless of wiring resistance?

Nick


Posted by on April 12, 2007, 4:35 pm



>Why the quotes around the word transformer?

Possible salespeak.

Nick


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