I have a circuit that I'm trying to put together to turn the fans on in my car at a lower temperature than what the car's computer is set to turn them on at. (I'm using a lower temp thermostat and trying to prevent large temperature fluxuations while idling).
I would like to use the stock temperature sensor and leave it connected to the stock engine computer (PCM) so that the computer can still turn the fans on if my circuit fails. The stock temp sensor is a thermistor that varies from about 10k ohm at 0 deg F to less than 180 ohms when at 212. The PCM applies a 5 volt reference and monitors this voltage. I would like to use a 741 op amp set up as a comparator so that when the resistance gets below 270 ohms, the relay will trip to ground the fan relay (not shown) turning on the fan.
I built this one on the breadboard and applied a 12 volt power supply. I then applied 3 volts to my reference and 1.5 volts to my input and the relay trips. So everything seems to work as I expect.
My question is, does anyone see any problems with the circuit, or any improvements that they might suggest? Do I need to use a higher rated variable resistor and ground the other leg to provide the correct voltage reference? I've tried to protect the circuit with a zener and a capacitor as a filter on the 12 volt power supply as shown. Any other suggestions?
I would like to use the stock temperature sensor and leave it connected to the stock engine computer (PCM) so that the computer can still turn the fans on if my circuit fails. The stock temp sensor is a thermistor that varies from about 10k ohm at 0 deg F to less than 180 ohms when at 212. The PCM applies a 5 volt reference and monitors this voltage. I would like to use a 741 op amp set up as a comparator so that when the resistance gets below 270 ohms, the relay will trip to ground the fan relay (not shown) turning on the fan.
I built this one on the breadboard and applied a 12 volt power supply. I then applied 3 volts to my reference and 1.5 volts to my input and the relay trips. So everything seems to work as I expect.
My question is, does anyone see any problems with the circuit, or any improvements that they might suggest? Do I need to use a higher rated variable resistor and ground the other leg to provide the correct voltage reference? I've tried to protect the circuit with a zener and a capacitor as a filter on the 12 volt power supply as shown. Any other suggestions?
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