Re: Help me solve this design problem PLEEEZ

funster wrote: 
There is an electromagnet at my job that has a 50% duty cycle.  That is
it may be on for up to 5 minutes and then off for 5 before using it
again.  I want to design a circuit that acts as a time integrator.  As
long as a relay is pulled in, the timer is counting up.  When the relay
is out, the timer counts down to zero and stays there.  If the operator
isn't paying attention, and he goes over 5 minutes, an alarm sounds to
alert him that he is operating the electromagnet outside its design
parameters.
How can this be accomplished?  Any suggestions would be appreciated.


OK, let's put on our thinking caps.
Why would anybody want to limit the duty cycle???

If it's heat, put a thermistor on the magnet.
When it gets too hot, turn on the "coffee break" sign.

If you can't get at the magnet, make a heater/heatsink/thermistor
with the same time constant. I'd suggest a snap-action thermal switch,
but make sure it's rated for lots of operations. The little round jobbers probably aren't.

Never infer a condition when you can (easily) measure it.
mike

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