Using MOSFET to Control Peltier

Hi -

I have a Peltier unit (http://www.cui.com/product/resource/cp60.pdf) that I am using to heat a solution. I have a temperature probe in the solution that is measured by an external thermometer and provided as a voltage value. I want to regulate the current going to the Peltier to keep the solution temperature constant.

I have all the Arduino code setup appropriately, however my circuit is the problem. The MOSFET I am using (http://www.cui.com/product/resource/cp60.pdf) gets very hot very quickly. I've put a small heatsink on it but I'm fairly certain the problem is my circuits knowledge.

I have the gate of the MOSFET connected to the PWM output from the arduino, and then going to GROUND via a 10k resistor. I have the drain of the MOSFET connected to the black terminal of my peltier. I have the source of the MOSFET connected to GROUND. The red terminal of my peltier is connected to V+ of my power supply. The GROUND of the board is connected to the V- of my power supply. This is all on a breadboard (not sure if this is ok?).

My power supply is set to 15V and a current limit of 2A. I believe the 15V is within the range required for Vgs on the data sheet. I've measured the voltage between the gate and the source and I see it transition from 0-4.9V as the Arduino's output increases - which tells me that the MOSFET is being switched on/off.

Any suggestions appreciated.

If you use the wrong MOSFET, it will not fully turn on when there is 5 volts on the gate pin. When not fully turned on... it will be a higher resistance path and that will generate excess heat and provide less current to the peltier.

You need to find a "good" logic level MOSFET. It sounds like you don't have one. Try IRLB8743

Is my understanding correct that the current MOSFET I have requires 15V on between the Gate and Source pins to be fully turned on?

If that is the case... (and it is not unusual for generic MOSFETs) you could use this scenario to fix it without buying a new MOSFET.

http://pwillard.com/files/driverm.jpg

igoy: Is my understanding correct that the current MOSFET I have requires 15V on between the Gate and Source pins to be fully turned on?

You didn't tell us which MOSFET. They are normally specified for 10V gate drive if not logic level, and typically driven at 10V or 12V.

You posted the peltier link twice, and no mosfet link.

I default the IRF3708, as it works with even 3.3v logic level signal (5v is better ofc) if I need a through-hole MOSFET - specs should be fine for driving a peltier. They're a little pricey though - like $2 each from digikey.

Sorry, the MOSFET I have is: http://pdf1.alldatasheet.com/datasheet-pdf/view/52813/FAIRCHILD/HUF75307P3.html

Compare the Current vs VGS graphs of your HUF75307P3 mosfet (figure 8 on page 4) to the IRF3708 (figure 3 on page 3). At 5V, the HUF* is just getting off the ground where the IRF3708 is able to pass its max current.

As an aside I don’t see any stock anywhere for the IRF3708.

Which peltier do you have.
The link in post#0 has a list of CP60xxx peltiers with different voltages.
Maybe you could add a small NPN transistor and two resistors, and keep your existing mosfet.
Leo…

Sorry, I meant the IRF3708PBF (identical except it has no lead in it, the leaded version is out of production).

And yeah, your MOSFET is not suitable for this load with a mere 5v on the gate (or really any significant load) - it really needs 7v minimum to acceptable performance, and it's designed for 10v.

Wawa: Which peltier do you have. The link in post#0 has a list of CP60xxx peltiers with different voltages. Maybe you could add a small NPN transistor and two resistors, and keep your existing mosfet. Leo..

Could you elaborate on what this circuit would look like and how it would work, please?

If your peltier is >8volt....

Fet: source to ground, drain to negative peltier, resistor (~470ohm) between gate and peltier supply. Now the fet is always "on".

Small NPN transistor (e.g. BC547): emitter to ground, collector to gate, 1k resistor between base and Arduino pin. The transistor can turn the fet "off" by shorting the gate to ground.

So default behaviour is "peltier on". It would need a second small transistor to make it "default off". Leo..

Could you elaborate on what this circuit would look like and how it would work, please?

If you go to the link in my post, I show you how to use an NPN as a pre-driver for a high voltage MOSFET.

Response #3

I'm just curious, why are you using an expensive Petlier device to heat anything when a simple resistive heater well work about as well and cheaper? Dwight

pwillard: If you go to the link in my post, I show you how to use an NPN as a pre-driver for a high voltage MOSFET. Response #3

Sorry, didn't see that. It is just what I explained in post#11. The 10k collector resistor is ok for on/off, but too high for PWM. I suggested ~470ohm. Leo..

I can’t get to the pdf right now but what is the capacitance of the
gate to drain and gate to source for this fet?
As for heat sinks, why not tie the transistor to the cold side of the
Peltier?
Again, why a Peltier?
Dwight

Probably because he initially wanted cooling and heating but then discovered that Peltiers are really bad at cooling and really good at heating. Now it's a heater, sourced from the junk box.