beginner: adjusting a thermostat circuit

[nyoo]

Please find attached a circuit. This circuit turns on a 500mA relay when the Th1 temperature, controlled by P1, is exceeded.

As a very beginner, I'm trying to adjust this.

Instead of the 500mA relay, I will use a 12VDC 500mA pump motor. The datasheet for the 2N2222 describes it as "designed for a high speed switching application at collector current up to 500mA". Do I need something bigger, to handle start/stop spikes?

Instead of turning on the pump when it reaches a particular temperature (65 degrees C), I want to turn it off. Is it as simple as swapping the leads going to pins 2 and 3 of the LM741?

The R2 resistor handles hysteresis. A range of 120K ohms to 470K ohms was specified. I'm okay with 1.5 degrees C during which the state does not change. Would that be the high end of the R2 range?

Thanks very much.

 

[on1aag]

Hi Nyoo,

Instead of the 500mA relay, I will use a 12VDC 500mA pump motor. The datasheet for the 2N2222 describes it as "designed for a high speed switching application at collector current up to 500mA". Do I need something bigger, to handle start/stop spikes?

You will need something bigger than a 2N2222.
I suggest to replace R4, R5 and Q1 by a IRF540 mosfet.
Connect the gate (of the mosfet) to the output of the opamp,
the source to gnd and the drain to the load.
It doesn't need to be a IRF540, see what is available with similar
specifications.

Instead of turning on the pump when it reaches a particular temperature (65 degrees C), I want to turn it off. Is it as simple as swapping the leads going to pins 2 and 3 of the LM741?

It's something like that but it's a little different, swap R1 and th1.

The R2 resistor handles hysteresis. A range of 120K ohms to 470K ohms was specified. I'm okay with 1.5 degrees C during which the state does not change. Would that be the high end of the R2 range?

Probably.


on1aag.

 

[nyoo]

Thanks very much. I shall built with an IRF540 mosfet.