Electronics Forum

Electronics Circuits & Projects discussion forum. Get help with electronics.


Temperature switching w/Thermistor

Discuss all your electronics engineering related technical problems and other questions pertaining to design problems in any field of Electrical/Electronics/Computer Hardware

Moderator: pebe

Temperature switching w/Thermistor

Postby davter » Mon Sep 01, 2014 12:15 am

I am experimenting with a temperature switch using a thermistor as the base switch. I will list details below, but the main problem is the differential switching point or as some call it, the hysteresis point. I am simply wanting to set up a fan switch relay to operate when the ambient goes above about 80 C, and to remain on for say 3-5 minutes after the cooling fans have lowered the ambient below the set point. I am using a suppliers PCB with a dual OP Amp (358) with the set-point set via a pot from a ref voltage, which is fed into the non inverting i/p. The thermistor is a 10K, which is about 8.7K @ 25C. I have used a couple of different therms, but as this is not a critical cct, well the exact trip point is not a problem. But the differential is. This cct has a 1M pot fed from the output of the OP amp thru a 1N4148 diode back to the non inverting I/p. I find that with this pot set high, ~ 800K, then the amp switches instantaneously, with a low setting, ~ 250K then simply does not reset, the amp does not return to low state O/P. Would appreciate advice please.
davter
 
Posts: 1
Joined: Sun Aug 31, 2014 11:45 pm

Re: Temperature switching w/Thermistor

Postby pebe » Mon Sep 01, 2014 2:17 am

I don't see why there should be a diode in the feedback loop, or how the thermistor is wired into the op-amp, and why there are two op-amps.
Could you post a circuit diagram, or a photo of the PCB, please?
pebe
 
Posts: 1058
Joined: Tue Dec 09, 2003 11:12 pm
Location: Ellon, Scotland


Return to EE quiz questions & problems



Who is online

Users browsing this forum: No registered users and 1 guest

Parmak HS-100 Horse Surround Electric Fence Charger, 5-Mile, Low Impedance,
$85.0
Parmak HS-100 Horse Surround Electric Fence Charger, 5-Mile, Low Impedance, pictureHP Agilent 4291B 1 MHz-1.8 GHz RF Impedance/ Material Analyzer
$2100.0
HP Agilent 4291B 1 MHz-1.8 GHz RF Impedance/ Material Analyzer  picture1 Panasonic FM 3300uf 6.3V 3300mfd Impedance Electrolytic Capacitor
$7.45
1  Panasonic FM 3300uf 6.3V 3300mfd  Impedance Electrolytic Capacitor pictureED&D Millenium Series GC 2000S Automatic Ground Impedance (OHMS) Tester AS-IS
$499.0
ED&D Millenium Series GC 2000S Automatic Ground Impedance (OHMS) Tester AS-IS picture


cron