Any electrician/electronics guys here?
I am trying to understand how Circuit 1, below, works. I got this design off the Internet.
http://www.heatsink-guide.com/content.php?content=control.shtml
It is supposed to allow temperature control of a CPU cooling fan. I have crudely re-drawn the circuit diagram and added my understanding of how the components behave in text boxes, and for convenience I added three reference points A, B, and C.
First, I'm confused by the role of the potentiometer part of the circuit.
Suppose the potentiometer part of the circuit were eliminated. Then my understanding is that, when temperature at the thermistor is high, resistance across the thermistor is low, so voltage at point A is high. Then voltage at point B, the MOSFET gate, would also be high. Once the voltage at the gate exceeds the MOSFET's threshold voltage, the MOSFET switches to "on-state" and resistance from source to drain goes to a low value. So the MOSFET passes its max current and the fan runs at high speed.
The website where I took the design says the potentiometer allows you to prevent the cooling fan from running all the time.
"Before you start using the temperature control, the temperature at which the CPU (or case, etc...) should be kept must be set. This is done by adjusting the spindle trimming potentiometer. A good strategy to do this is: Start with a cool CPU (or case). Turn potentiometer until the fan doesn't spin at all. Then, watch the temperature rise. As soon as the temperature gets close to the maximum it should reach, adjust the potentiometer so that the fan just starts to spin."
Okay, suppose at a given temperature the thermistor resistance is low enough that voltage at the MOSFET gate would normally be above the threshold voltage. But you don't want the fan to be running. So the potentiometer somehow lets you control voltage at the gate?
I am confused. Why wouldn't you instead place the potentiometer
in series between the thermistor and the gate, instead of as a parallel bypass from gate to source?
Second, is it correct that in this circuit, if the temperature is high enough that the fan is not running, current is nonetheless flowing through the potentiometer so that the whole circuit is consuming power?
I want the circuit to consume no (or very little) power when temperature is high. This is to preserve the life of the battery that will be driving all this.
Third, I'm actually trying to control not just a fan, but also a Peltier cooling element - this combined load that can draw several amps.
The cheapo ($3) MOSFETs I find at Radio Shack are rated for only 3 amps current from source to drain.
So I am thinking, instead of connecting the MOSFET directly to the load, I should connect the MOSFET to the control circuit of a relay, and connect the fan/Peltier to the relay's load circuit?
Kind of like Circuit 2, below?
Sorry if I'm not being clear about all this.