Reliable circuit to detect over-temperature...

#12

Joined Nov 30, 2010
18,224
The device above will do everything that the OP requires at lower cost and a fraction of the effort than most of the other solutions put forward.
This device is not adjustable, comes in increments of 10 degrees C, and has a mechanical switch in it.
I will admit I have only replaced about 3 Klixon sensors in a 40 year career, but I have never repaired the LM723 based thermostat in 37 years of continuous operation, and it is adjustable to a tenth of an F degree.
 

Lyonspride

Joined Jan 6, 2014
137
I'm going to suggest the KISS principle....... Try using a thermal switch like the KSD-9700.

They're really cheap, really small and you'll have more time to spend on something that hasn't already been done.
 

Thread Starter

MikeA

Joined Jan 20, 2013
362
I'm going to suggest the KISS principle....... Try using a thermal switch like the KSD-9700.
I have actually come across these same exact thermal switches while searching the interwebs since posting this thread. They are indeed very cheap, at about 20 cents a piece. And I can probably vary the trigger temperature by placing it a bit farther or closer to the heat generation source.

The reason why I thought of using a thermistor is because I also wanted to tap the thermistor to get a temperature reading to control the variable speed cooling circuitry. But I just might build a completely separate cooling circuitry, and a separate over-temperature fail safe using a bimetal switch.

Now, for example the KSD-9700 is rated 250V 8A. Is that for AC? Will it work fine with DC voltage? What about 12V DC? Or 5V DC?
 

ian field

Joined Oct 27, 2012
6,536
I have actually come across these same exact thermal switches while searching the interwebs since posting this thread. They are indeed very cheap, at about 20 cents a piece. And I can probably vary the trigger temperature by placing it a bit farther or closer to the heat generation source.

The reason why I thought of using a thermistor is because I also wanted to tap the thermistor to get a temperature reading to control the variable speed cooling circuitry. But I just might build a completely separate cooling circuitry, and a separate over-temperature fail safe using a bimetal switch.

Now, for example the KSD-9700 is rated 250V 8A. Is that for AC? Will it work fine with DC voltage? What about 12V DC? Or 5V DC?
Some of the early "quiet" ATX PSUs had a fan speed control tacked on as a tiny square of PCB and a bead thermistor clamped to a heatsink., more recent versions include it in the rest of the circuitry on the main PCB - if you can find an old PSU to salvage the fan PCB, its pretty much ready made for you. All you need to add is a comparator to trigger the over-temp shutdown and whatever circuitry you need to accomplish that.
 

Thread Starter

MikeA

Joined Jan 20, 2013
362
So am I right to assume that the KSD-9700 switch can work with low voltages?

And would be rated similar like a relay contact? Say if it's rated 250V 8A AC then it would be around 4A DC? And would work for any voltage/current below those thresholds?
 

#12

Joined Nov 30, 2010
18,224
Will it work fine with DC voltage? What about 12V DC? Or 5V DC?
What Albert said. Very low current contacts are called, "dry" contacts. They are usually gold plated.
Sears refrigerators had a problem with this because they changed the controller to a microprocessor and used the same old 3 amp, "defrost terminator" switch. I don't know if it was phosphor bronze, zinc plated, or what, but it wasn't reliable at 5 volts and 1 milliamp.
I also wanted to tap the thermistor to get a temperature reading
In that case, the LM35 is much easier to work with than a thermistor.
 

ian field

Joined Oct 27, 2012
6,536
So am I right to assume that the KSD-9700 switch can work with low voltages?

And would be rated similar like a relay contact? Say if it's rated 250V 8A AC then it would be around 4A DC? And would work for any voltage/current below those thresholds?
There is a potential problem that under utilised contacts can form an oxide layer that insulates them.

Purely a guess - but I'd think anything over 1/3 of the rated current should keep the contacts in good condition.
 
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