Electronic Circuits and Projects Forum

20th September 2002 09:21 PM

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.

23rd September 2002 11:33 AM

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.

ops:

23rd September 2002 09:06 PM

Thanks very much, i'll go buy me bits tomorrow and try it out

25th September 2002 10:08 PM

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)

4th July 2004 08:30 AM

How could the circuit be modified to allow for a large hysteresis, say 5-10 degrees?

5th July 2004 04:46 AM

The hystersis is set by the 270 k feedback resistor.

Reduce it to increase the hystersis.

Len

5th July 2004 06:14 AM

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

Originally Posted by ljcox

The hystersis is set by the 270 k feedback resistor.

Reduce it to increase the hystersis.

Len

9th July 2004 04:05 AM

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

9th July 2004 04:56 AM

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.

10th July 2004 03:08 AM

Originally Posted by bryan

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

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Electronic Circuits and Projects Forum

Help with thermistor and hot/cold alarm.

Help with thermistor and hot/cold alarm.

High Temperature Alarm