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How can I regulate amperage from going over a set point?

I have a very expensive IP addressable thermostat and the relays can only handle .5 amps, not very much at all, but for a 24 volt thermostat it is plenty. The house was hit by lightning 1 year ago and now I am getting spikes of .75 amps and I have fried 2 more $400.00 thermostats, can I use a resistor or some type of regulator to control this?

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seandogue7 years ago
Lighting strikes are going to be much too fast for most standard over current protection. In fact, there is a special category of protective circuits known as lighting protectors. For normal surge current, fuses, poly-resettable fuses, circuit breakers, and crowbar protection offers moderate to high levels of security against over current situations, and each of the preceding terms can be searched on the wider net for commercial components. You might want to look at a fast blo fuse rated @ 1/2A between the 24V supply and the thermostat for a low cost solution, but for lighting strikes, you're going to need a whole house solution, which, if you get alot of activity in your area, would be a good investment not only for your thermostat, but also for your TV, computer, and almost every other piece of electronic equipment in your household. I would also consider that the thermostat might not be all that great after all, if it doesn't have onboard protection for such a small surge. Warranty comes to mind.
pioneer407 (author)  seandogue7 years ago
Thank you for the info, but let me add a bit more. The old thermostat is back in place and it works fine and the house now has a lightning arrestor system installed by the power company. My problem is explaining to a customer that my $750.00 therm. I charged them wont work but this $29.00 thermostat works fine. I have to believe that I can have a sort of amperage limiter or something like that. The .5 max amp load is not much, but the readings we were getting would only go up to .75 amps and then back down to .225 amps under mormal work loads. The thermostat is only 24 volts, do you think there is anything I can add or do, or do I walk away from this and refund thier money?
Honestly? Having learned that lightning suppression was installed and a failure occurred again, it sounds like the 24V supply is the culprit at this point. There's no way that without a pre-damaged thermostat that it should be pulling more than it's designed to. If so, then you have no choice but to take it back and send it to the mfg for a refund at their end, since it would be a warranty failure due to a design flaw. But if the 24V supply isn't regulating and outputting voltages in excess of the input range for the thermostat, then it's the most likely source for the failure and replacement with a more robust supply is in order. Was the supply thoroughly tested or replaced following the lightning event? BTW, I was hasty with the recommendation for a fuse...Having a fuse pop probably isn't the best solution in the long run anyway, because when it fails (during cold weather for instance), you're hosed without heat until the fuse is replaced, and it could just pop again as soon as you install a new one. A poly resettable wouldn't be such a bad idea anyway, since it will auto reset and continue powering the device when the load drops again, but a fuse is a one shot and will require manual replacement every time there's a problem. Not physically being there to properly troubleshoot is difficult, but I'm guessing that you really need to look hard at that supply...In the long run, it's a hell of a lot cheaper than a $400 thermostat.
OK, just add another relay, triggered by the little one, but capable of switching a lot more current - say 10A - should cost you about 10 bucks. If the old thermostats just have busted relays, they should be repairable for another couple of bucks each. Steve
Looking at WHY you are getting such big assed spikes might be more beneficial than adding protection. It looks to me like something further down the line has been damaged by something. Why SHOULD a tiny little switch circuit (which is really all a sensible thermostat would expect to switch) is suddently shifting a 75 W load ???
pioneer407 (author)  steveastrouk7 years ago
Thank you for the info, but let me add a bit more. The old thermostat is back in place and it works fine and the house now has a lightning arrestor system installed by the power company. My problem is explaining to a customer that my $750.00 therm. I charged them wont work but this $29.00 thermostat works fine. I have to believe that I can have a sort of amperage limiter or something like that. The .5 max amp load is not much, but the readings we were getting would only go up to .75 amps and then back down to .225 amps under mormal work loads. The thermostat is only 24 volts, do you think there is anything I can add or do, or do I walk away from this and refund thier money?
orksecurity7 years ago
I agree that the real question is where those spikes are coming from. Sounds like something else was damaged that you haven't yet repaired. That's the place to start, before it gets worse and starts blowing out other things... or causes a fire.
kelseymh7 years ago
You're looking for a "current limiting" circuit.
http://www.google.com/#hl=en&source=hp&q=current+limiting+circuit