From: Lizard Blizzard
Subject: Re: V regulator input cap size?
Date: Thu, 24 Oct 2002 17:17:52 -0700
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> On Thu, 24 Oct 2002 12:39:36 -0700, Lizard Blizzard wrote
> (in message ):
>>But I have used
>>transformers similar to these control transformers, and they put out
>>about 42 volts at the filter capacitor. You have to remember that the
>>transformer gives 28VAC at full load, so at much lower load, the
>>secondary AC voltage will be substantially higher. And after
>>rectification, it can go as high as 44 or so VDC which is about 31 to
>>32VAC times 1.414. If a regulator with 40V max is used, it will still
>>have an input voltage that exceeds its maximum.
> I measured the control transformer voltage and with no load (HVAC off) at the
> thermostat, where I will take the voltage - it measures 28v rms. I would
> assume that under load it will go a bit lower, but as long as it doesn't go
> above this measured value I think the 7824 and 7812 solution will be safe by
> a margin.
It's good that you are taking the precautions of measuring and finding
out what you will be getting for AC voltage. One other important thing
is to put a fuse inline somewhere close to the point where you will be
tapping into the AC. Seems like a quarter amp slow blow should do it,
unless there is such a high surge current when the power is turned on
that it causes that small a fuse to blow.
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