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Help with thermistor and hot/cold alarm.

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JB_

New Member
Woo hoo..just found this forum and it might just be what i'm looking for :)

Check this site. It's a frost alarm to close/open a relay when things get cold, but what i want to do is reverse this and have it operate when things get hot, typically above 30-35 degrees. Its hopefully going to be used to start a fan running when the air inside my pc gets above that temperature and stop itself when the air reaches a low enough temperature. But only having a fairly rudimentary knowledge of electronics, not beginner, but not advanced, i dont know the correct thermistor to replace the 15k one. If indeed this would be neccesary, if anyone can help then i'd be most grateful.
 

mechie

New Member
High Temperature Alarm

Welcome JB_
Still using the 15k thermistor (Maplin part No. FX22Y) ...
Swap the thermistor and 100k preset potentiometer for each other (so thermistor goes from 0v to op-amp input, 100k preset goes from op-amp input to 12v supply.
This will reverse the sense of the circuit so the relay will energise for a high temperature.

:idea: I would also suggest swapping the 100 preset for a 4k7 resistor in series with a 22k preset. This will allow the operating temperature to be adjusted somewhere between 10deg C with the preset at maximum resistance to 50deg C with the preset at minimum resistance.
I believe such a resistor should have been shown in the original circuit [a design error ?] - if the preset was set to min resistance the thermistor would be connected directly across the supply lines. :oops:
 

JB_

New Member
Works a treat, thanks very much that man. If i ever see you about i'll buy you a drink.

/me buys fake beard, hat and glasses 8)
 

ljcox

Well-Known Member
The hystersis is set by the 270 k feedback resistor.

Reduce it to increase the hystersis.

Len
 

bryan

Member
Thanks, but how much would I need to reduce the resistance, to achieve a hysteresis of 10 degrees. Ballpark?

ljcox said:
The hystersis is set by the 270 k feedback resistor.

Reduce it to increase the hystersis.

Len
 

ljcox

Well-Known Member
I can't answer that question unless you tell me how much the resistance of the thermistor changes when the temp changes by 10 degrees.

Post that info and I'll do the calculation for you.

Len
 

bryan

Member
Thanks Len:

If you can provide the calculations I would be happy to do the math. I will play around with the thermistor tonight and post tomorrow in any event.
 

ljcox

Well-Known Member
bryan said:
Thanks Len:

If you can provide the calculations I would be happy to do the math. I will play around with the thermistor tonight and post tomorrow in any event.
I asked the wrong question yesterday. Instead of asking for the thermistor resistance change, I should have asked for the voltage change (for a 10 degree temp change) at the negative input to the Op Amp.

With the existing feedback resistor (270k), the positive threshold will be about 6 + 0.22/2 = 6.11 Volt (see the 270k entry in the table below). The negative threshold will be about 6 - 0.22/2 = 5.89 Volt.

So, I suggest you use the table below to determine feedback resistor you need for the 10 degree hystersis. ie. determine how much the voltage changes when the temp changes by 10 degrees and select the Rf from the table.

Hystersis (Vh) for various values of Rf (currently 270 k)

Rf Vh Rf Vh Rf Vh
560 0.11 150 0.39 47 1.15
470 0.13 120 0.48 39 1.36
390 0.15 100 0.57 33 1.58
330 0.18 82 0.69 27 1.88
270 0.22 75 0.75 22 2.22
220 0.27 68 0.82 18 2.61
180 0.32 56 0.98 15 3.00
k Ohm Volt k Ohm Volt k Ohm Volt
 

bryan

Member
Thanks for the help. can I assume that from the data in the table a 270K resisitor would offer a hysteresis of approx 0.22 degrees, or am i interpreting it wrong.
 

ljcox

Well-Known Member
Thanks for the help. can I assume that from the data in the table a 270K resistor would offer a hysteresis of approx 0.22 degrees, or am i interpreting it wrong.
Wrong. The figures are voltage, not degrees. See my second paragraph and the units under the table.

I suggest that you measure the voltage at the negative input of the Op Amp with the thermistor at a temp of say 10 degrees and again at 20 degrees (I'm assuming that you are working in Celsius not Fahrenheit).

The difference between these readings is the hystersis that you want (in volts). Then look at my table, and select the Rf you need to provide this hystersis. Then all you have to do is to adjust the pot in order to set the upper or lower trigger point.

Len
 
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