Seeking suggestions for temperature switches

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I have an application where I'd like to turn a fan on and off based on a hardwired temperature.  The ideal device would have three pins:  Vcc, GND, and output, where output would source up to, say, 200 mA from Vcc when the temperature is above the threshold and floated (or shorted to GND) when below (with some hysteresis to prevent ringing, as in a thermostat).  The device can be screwed or glued to a heatsink and costs less than a dollar.  A cursory search yields very expensive, stainless steel type things and very inexpensive things that produce a logic signal but can't source what I need (while I could amplify the logic signal with a transistor, that significantly increases the complexity compared to what I'm imagining).  I thought I'd turn to the hive mind to try to save some time.  Does what I'm looking for exist?  Thanks for any pointers.

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What temperature?  There are little click switches that are electromechanical (coffemakers, etc).

 

automotive chips might be a good place to look.

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Thermistor and a 555?

 

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Last century bimetallic strip?

Ross McKenzie, Melbourne Australia

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avrcandies wrote:
There are little click switches that are electromechanical (coffemakers, etc).

That's a really interesting idea, and those tings are dirt cheap on Amazon.  I'll look into it.  Thanks.

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valusoft wrote:

Last century bimetallic strip?

Let's not be technological chauvinists.  ;-)  If it works it works.  I'd prefer a solid state solution, but if that technology doesn't offer something of comparable simplicity, reliability, and cost, why should I ignore the tried and true?  A thermistor and 555 doesn't conform to my idealized specification.

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Well, perhaps something similar to:

(Mouser Quanty 1 prices)

 

$0.44  ATTiny412   8 pin chip, 20 MHz, 3/5 V, has an ADC.   (ATTiny 1606 is $0.67 each)

$0.16  T2N7002BK NFet transistor, 60V, 400 mA

$0.25   NTC 81-NCU15XH103D60RC 

$0.10   A resistor for the NTC

$0.30   A by-pass cap for the micro.

 

$1.25  So, kind'a close without much searching.

You add the PCB...

Obviously you must be planning on building a lot of them, in which case the prices above drop.

If you were only building 1 /  a few who cares if it cost $1 or $2.

 

JC

 

Edit: Typo

 

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DocJC wrote:

Obviously you must be planning on building a lot of them, in which case the prices above drop.

If you were only building 1 /  a few who cares if it cost $1 or $2.

I'm only building 20 - 50, so you're right that it doesn't matter if it's $1 or $2; I just said $1 to have some boundary in the general vicinity of what I want.  The rest of the product is all off-the-shelf stuff that's plugged together with a basic wiring harness and held together with laser cut pieces.  It's more assembly than building.  Dealing with a BOM even as simple as the one you suggest would be a huge headache by comparison.

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OK how about something automotive - like a Radiator Fan Thermoswitch for example. Obviously they only come in a limited range of temperatures, but you didn't specify that.

 

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Hey Doc,

 

Using a micro is probably not the most reliable method.

 

seek out the series of articles by George Novacek back in the 2000's:

http://www.dtweed.com/circuitcel...

 

He breaks down the design of a safety critical system via an example of a hot tub controller. The techniques he uses are very simple and obvious in order to achieve redundancy.

 

Through searching for the link, I discovered that George passed away last year. His wonderful articles still live on. Thank you George.

 

 

 

Last Edited: Sat. Nov 14, 2020 - 11:54 AM
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Kartman wrote:
he circuit is highly unlikely to fail in a manner that would destroy the led.

Hmmm! The forum software has stitched you up in a similar manner that it did to me just recently. I.e. Attaching your comment to the wrong thread.

 

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More likely the nut in front of the keyboard! In my case,at least.

Last Edited: Sat. Nov 14, 2020 - 12:08 PM
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N.Winterbottom wrote:

OK how about something automotive - like a Radiator Fan Thermoswitch for example. Obviously they only come in a limited range of temperatures, but you didn't specify that.

Larger, more cumbersome, and more expensive than I want.  They also appear to be designed to screw into a fitting and sense liquid temperature (coolant).  Also, as I should have said earlier, I’d like a 150F set point, and the radiator-related stuff is set for higher temps closer to boiling.

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Hi 

 

Search for hysteresis, comparators and op-amps. As you have found you NEED hysteresis. Any buffering will automatically be taken care of by the op-amps you use.

 

Searching the brain I think there was a circuit floating around years ago utilising a 556 which will do what you want - Google is your friend.

 

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lautman wrote:
a 150F set point

So what is the vcc voltage of the fan?  5vdc?  Without spec's it takes 20 questions to get an answer!!

 

Jim

 

 

FF = PI > S.E.T

 

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from post # 2:    What temperature? 

 

Unlocking the combo vault is faster!!  Those aliexpress parts look really cool, er warm !!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Or search on ebay:

"Temperature Switch Bimetal Head Thermostat KSD301 NO 35℃-160℃ Auto Operate"