Hi, I am working on understanding a simple thermistor temperature switch using a 741 Op Amp for a comparator.
The circuit I have attached does not have all the parts assigned a value, except for the thermistor and related reference resistors.
First comments and questions:
I choose a 100K thermistor because (if I've figured correctly) it will be 9.4k ohms at 212 degrees F. Using this figure, I chose a 10k resistor in series with the therm. This should bring the voltage to approx 1/2 the supply at 212 deg F.
I chose a 20 K Trimmer for the reference adjustment. This would seem to me that when the wiper is in the middle, I would be in the range of my 212 deg F trip point Does this make sense?
Also you will see that I am using a TIP 120 transistor. The sole reason is that I have some already.
The 741 spec sheet says I can have a 20m amp continuous draw at the output. If I take the 1000 amplification factor for the TIP 120, I am way beyond what is needed to close a relay with a 100 ohm coil (.12 amps @ 12 V)
Do I need to put a current limiting resistor between the 741 and Tip120?
Also I am curious on how to handle the offset null pins. Can I let them float?
I would also appreciate any comments on anything else you might see that I have done wrong.
I think that's it for now. Thanks!
Steve
The circuit I have attached does not have all the parts assigned a value, except for the thermistor and related reference resistors.
First comments and questions:
I choose a 100K thermistor because (if I've figured correctly) it will be 9.4k ohms at 212 degrees F. Using this figure, I chose a 10k resistor in series with the therm. This should bring the voltage to approx 1/2 the supply at 212 deg F.
I chose a 20 K Trimmer for the reference adjustment. This would seem to me that when the wiper is in the middle, I would be in the range of my 212 deg F trip point Does this make sense?
Also you will see that I am using a TIP 120 transistor. The sole reason is that I have some already.
The 741 spec sheet says I can have a 20m amp continuous draw at the output. If I take the 1000 amplification factor for the TIP 120, I am way beyond what is needed to close a relay with a 100 ohm coil (.12 amps @ 12 V)
Do I need to put a current limiting resistor between the 741 and Tip120?
Also I am curious on how to handle the offset null pins. Can I let them float?
I would also appreciate any comments on anything else you might see that I have done wrong.
I think that's it for now. Thanks!
Steve