Analog circuit to cut power - Page 1
Forum section: electronics / beginners

author: diltsman
author_rank: Newbie
timestamp: November 10, 2017, 02:42:11 am
content: I am trying to figure out how to go about designing an analog circuit to use temperature to cut power to a different circuit that could be dissipating a decent amount of power.I can figure out how to linearize the thermistor (I don't need large temperature ranges, only 20-30 degrees C). I was planning on using a latching SPDT relay as the actual switch.What I am uncertain on how to do is to get the thermistor circuit to cut the other circuit. I could probably use an op amp with a DC bias to get the voltage from the thermistor circuit to the range that I would need, but I don't know if that would allow me to trigger the relay at a precise enough temperature.The ideal circuit would be a step function from 0V to Vcc (whatever is needed to activate the relay). In school I never got past LRC circuits, and I can't see a way to accomplish this with just those components.Could someone suggest what direction I should start looking to figure out how to proceed? Pointing to a blog post, suggesting components to look at, or anything like that would be appreciated.Thanks.

author: MasterT
author_rank: Frequent Contributor
timestamp: November 10, 2017, 03:26:34 am
content: Start from google, "thermostat circuit diagram". Select one, that you like, and come back with questions if not clear.

author: diltsman
author_rank: Newbie
timestamp: November 11, 2017, 04:02:53 am
content: That was exactly the direction that I needed. Thanks! In retrospect that should have been obvious. Now to do some math!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2017, 04:14:52 am
content: How much power? The switching alone can be worth a lot of thought! What if the load itself becomes faulted, can the switch safely restart (and hopefully open gracefully again) into such a condition?Using a constant current source (if you can arrange to handle the power dissipation) or protected switch (everything integrated on an IC, the transistor, current protection (sometimes) and internal thermal limit) is a darn good start.Place a schmitt trigger in front of one, then a thermistor resistor divider, and you've got logic on/off, external temp limit, internal temp limit and current limiting all in one place.Tim

author: diltsman
author_rank: Newbie
timestamp: November 15, 2017, 01:51:45 am
content: I'm looking at 5-10 amps AC mains into a resistive heater. So, something like 500-800 Watts. The actual heating element has no controller and is designed to plug directly into a wall outlet. So, load is pretty constant. Mostly I just need to turn it on and off so that things don't get cooked.This particular circuit is an emergency shutoff in the case that the primary controller faults on and tries to cook things. I am looking at using a triac for the primary controller. I am concerned with noise driven back into the will due to the triac switching on, but I am not certain how much to be concerned or how to deal with it (I have seen suggestions around to use a few hundred uH inductor inline with the triac and heater).

author: Ian.M
author_rank: Super Contributor
timestamp: November 15, 2017, 02:35:10 am
content: Use a 16A rated Klixon manual reset thermal cutout in series with the heating element for the over-temperature safety cutout. Its simple, reliable and there's no electronics to fail on.Seehttp://www.sensata.com/klixon/thermostat-hvac-1nt.htm

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2017, 10:28:29 am
content: If there's a risk of fire, burns or damage to property, there should be a safety cut-out/thermal fuse, in addition to the primary thermostat, in case it fails permanently on.