How to get from 27 to 24v AC ?

Oct 26, 2005 6 Replies
W

What makes you think you need it to be lower? The furnace circuit board can probably take a wide range of voltages. Consider two facts:

  1. The output voltage of a transformer varies depending on the load. When there is no load (as when you measure it with a voltmeter, with no other load attached) it will be higher than when there is a load.
  2. The output voltage of a transformer varies depending on the input voltage. Common household line voltages (in the USA) range from 108V to
125V. Circuitry that is powered by transformers is designed to handle at least that much variation, often even more.

Don't take the "24V" requirement too literally. I'd bet your furnace will run on anything from 16V to 36V.

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J

If it's 12V it's probably DEAD ;-)

...Jim Thompson

| James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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M

Please don't laugh at the silly question, but what component should I buy to reduce the power output of a transformer from 26.4/27.4 volts, down to 24 ?



This is an AC transformer that supplies power to the circuit board of a home heating furnace. It takes line voltage on one end and should have an output of 24 Volts AC.



Two wires in, two wires out; grounding through its metal case. The actual voltage going in is 118v and the output is 26.4v to 27.4v (two voltmeters used).



The circuit board feeds the thermostat (among other things) and the output on the thermostat leads is 27.11v



Do I need to install a resistor? If yes, what capacity?



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P

Are you measuring this voltage with no load on the transformer ?

The rated voltage is usually the on-load voltage you see.

As WH says - it's unlikely to matter much anyway. Try measuring a car battery under different loads for example. It sure isn't 12V most of the time.

Graham

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M

WH and PB: Thank you for the comments!

The measurements were taken w/o load and now it seems so obvious that they are in the range during normal operation.

I only checked because I was tracing a power source for an inoperable vent motor and wanted to make sure it had the 24v it needs for its own reasons.

[ Oh, who am I kidding? I checked because of an uncontrollable compulsion to do it and in order to record the voltage output of every transformer in the house, ..as instructed by a voice coming from behind the furnace. ]

..MS

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J

If you\'re measuring the unloaded voltage of the transformer, there\'s a good chance the voltage will drop when you actually install it. In any case, since [expected] mains voltage variations of +/- 10% could cause the transformer\'s output voltage to vary from about 21.6 to 26.4V under ideal conditions, I don\'t think you\'ve got a problem. But... What happened that has caused you to ask the question?
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J

buy a smaller transformer.

probably not,

what's the impedance of the device that's being powered from the thermostat?

Bye. Jasen

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