I need to design a regulator for my thesis, but I really need help, Please!
The topic: - A temperature regulator for a small fridge
Some details: - the regulator has to control a peltier element which will cool the air in the fridge - the peltier element functions with 12 V and 5 A - first I need a circuit which measures the temperature, then its result will be compared to a fixed value, and depending on the difference between the 2 values, the peltier element will be controlled
What I did:
- the picture shows a hand-drawn circuit which I tested on a test board the results of the test are written in red
- I measured the temperature through a pn junction (a bipolar transistor connected as a diode)
- since the voltage variation with the temperature is very small (aprox. 2.5mV/deg), I amplified it through an operational amp, realized so that an offset voltage it obtained in order to amplify only the voltage variation of a few mV. - then there is a comparator which compares the measured voltage with a given voltage (through a potentiometer)
- at the output there is a power MOSFET which controls the Peltier element
After realizing this circuit, I understood that in order to control the Peltier element, I need 5A at the output of the comparator. What is more, the group MOSFET-Peltier has to be powered with 20 V, so that the Peltier element can suck 12 V and it still remains 8 V for the MOSFET. I simulated the circuit in Spice 4 (using some similar components to those used in reality and simulating the Peltier through a resistance of 2.4 ohm), so that I could then correctly dimension the components. But already after the amp, the voltage is not equal to the one measured on the test board and later on, the differences rise. I tried to vary the resistances of the comparator so that I obtain 5A at its output, but no matter how much I vary them, the result remains approximately the same.
Another thing I don’t understand: I was told I have to control the Peltier using PWM, but first of all I read on this forum that this is not a good idea and second, I don’t understand why I should need PWM, since the circuit I already realized controls the Peltier. Anyway, I drew a circuit with a 555 timer under the original circuit.
Please help me modify my circuit so that it works! And also, if you could, help me undersstand how and why I should use PWM.
Thanks a lot!
I just modified everything you suggested and also added the circuit for PWM. Do you think it's ok?