# Problem in controlling the device temperature using an op-amp

OBJECTIVE: I have a device to which voltage is applied and this voltage has to be controlled according to the temperature detected by the thermistor that is placed on the device. If the temperature of the device increases the voltage applied to it should be reduced and vice versa. The max voltage that can be applied to the heater is 30V.

METHOD: To achieve this I'm using a P-MOSFET and an OP-AMP as shown below. When the resistance of the thermistor changes due to change in temperature, the fraction of the voltage across the thermistor will also change which is fed to the non-inverting terminal of the Op-Amp. At 25C the thermistor resistance is 10KOhm.

PROBLEM:

1. Is my approach correct?
2. When I simulate the circuit, I am always having the voltage of 23.9V at the output. It's not changing with the change in the thermistor resistance.

Could someone help me with this?

Thanks

• That is not a PNP transistor. Oct 16, 2020 at 11:28
• The single-supplied op-amp should be properly biased. Oct 16, 2020 at 11:33
• Another thing is, the way you've got it set up now is basically a follower, feeding whatever the opamp sees on the divider straight into the FET's gate. Even if you were to wire the (horizontal) 10k gate resistor to 3.3V instead of the opamp output, or to 0, you'd be getting the same result. (As the Vgs would still be high enough to turn the FET on.) I'm afraid you'll have to completely redesign this. Oct 16, 2020 at 11:36
• 1) Why this circuit? 2) Where did you get it? 3) there are PLENTY of examples of temperature control circuits to be found, similar to yours but working. 4) if you "designed your own" show the calculations that proves that it should work. Oct 16, 2020 at 12:42
• If your intention is to produce a kind of PWM you need much less negative feedback, only a few degrees worth, and enough positive feedback to switch the output smartly and keep the cycle time reasonable. Oct 16, 2020 at 13:34

It should be obvious the opamp output is restricted to the range (0,3.3V) or in practice much less (about 2,3.3V given the "741" tag) giving (12V, 13.5V) range or (13,13.5V) on the MOSFET gate.

At both ends of this range the MOSFET is fully ON, connecting 24V directly to the load. Which is what you are measuring.

So : Take a more organised approach to designing, rather than throwing some random bits together and wondering why it doesn't work.

First define what you really want.

At what temperature, what voltage do you want applied to the load?

I expect a function defining the relationship between output voltage and temperature. Or a table with at least three temperatures (too cold, normal, too hot) and the expected output voltages.

What thermistor resistance corresponds to each of these temperatures?

What gate voltage corresponds to each desired output voltage? Or rather, what range of gate voltages can you expect, given the variation in characteristics for your chosen FET? This will involve reading and understanding the datasheet.

Consider a FET with the minimum Vgs(th) and another with the maximum Vgs(th), and work out the gate voltages you expect for each FET at each desired output voltage.

Now you can design an amplifier that can take input voltage from that range of resistances and produce at least that range of gate voltages. You can use negative feedback (sensing the drain voltage) to eliminate the errors caused by the variation in transistor characteristics.

Finally you will have to consider system stability. you have three large time constants : the response time of the load (heater), the response time of the thermistor (sensing temperature) and the response time of the amplifier (dominated by R? and C? ... that's another thing : from now on, give every component a NAME aka reference designator, like R7 and C2 so you don't have to say that capacitor between the negative input and GND).

Anyway, those three time constants could easily make this a phase shift oscillator if you get them (and the gain of that amplifier) wrong, so it oscillates, alternating between too hot and too cold.

The calculations required for system stability are also known as "control theory" and I'm not going into them in this answer.

Taking @Brian Drummond's suggestions into consideration, I modified my design as below and its working now. The thermistor readings are as below. With the increase in the temperature the gate voltage controlled by the Op-Amp increases and thus the output voltage decreases.

At 55C temperature, gate voltage reaches to 22.6 V and the transistor turns OFF, so the output voltage is almost zero.

The transistor I used here is fdc5614p from Fairchild and op-Amp is CA3140A from Renesas.

This transistor has Vgs(th) of -1.6V typical.