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Message from discussion burned out element detection
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ehsjr  
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 More options Nov 5 2009, 2:04 pm
Newsgroups: sci.electronics.design
From: ehsjr <eh...@nospamverizon.net>
Date: Thu, 05 Nov 2009 01:04:34 -0500
Local: Thurs, Nov 5 2009 2:04 pm
Subject: Re: burned out element detection
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lektric....@gmail.com wrote:
> On Nov 3, 11:48 pm, ehsjr <eh...@nospamverizon.net> wrote:

>>I had the same situation. CTs, as others have mentioned (with
>>a burden resistor, a current limiting resistor and a LED for
>>each CT) were the correct solution. A glance tells you instantly
>>if all elements are heating, or, if not, which element(s) is/are not
>>heating,  whether due to an open element, blown fuse, failed
>>contactor, whatever.  Mine lit the LEDs when working properly,
>>so it required no power source - the CT's powered the LEDs. Yours
>>will require power to run the inverter circuits, if you must
>>have led on = failure. Simpler to have led on = working.

>>Ed

> I hate to re-post to replies, but I think I have an idea that might
> work.  I can use a current transformer to drive a low-power relay,
> possibly a reed relay.  A neon lamp in parallel to the heating element
> would go through the NC relay.  When current flows through the
> element, current/voltage is produced in the current transformer,
> activating the relay, and keeping the light from coming on.  If the
> element burns out, the relay stays closed and the neon lamp will
> light.  Any problems w/this?

Maybe.  What happens if the element is ok, but the control circuit
fails?  For example, a burned point on the contactor or a blown fuse
could interrupt power to the element. The neon can't light in those
cases, so you won't know that there is a failure.

Assuming that is not a show stopper, you still may have a little
more complexity with the relay idea.  You likely need to add a
diode and filter cap to power the reed relays, and your expense
will be higher for the relay approach.

Ed


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