I want to switch a Thermoelectric Peltier Cooler (TEC1-12705) (12V - 77 Watts) using an Uno. I am an engineer but not an EE so I could use some guidance on circuit setup. There was a very good discussion on the page below but I need a bit more assistance on R values etc..
How to drive a MOSFET with an optocoupler?
I have the following:
Here is my circuit:
I don't see any reason why your schematic wouldn't work as is. I've never used an optocoupler like yours, but here's a breakdown of what I looked at.
Is the input current safe for the optocoupler and the arduino?...
$$I\_diode = \frac{Arduino\_V{pin}-Diode\_V{on}}{R1}$$ $$I\_diode = \frac{5V-1.2V}{330\Omega} = 11.5mA$$ $$Diode\_Ion < I\_diode < Diode\_Imax ... Good$$ $$I\_diode < Arduino\_Imax ... (Good)$$
Is the current through the optocoupler BJT a safe value?...
$$Ice = \frac{V_{supply}}{R2+R3}$$ $$Ice = \frac{12V}{2k\Omega+2k\Omega} = 3mA$$ $$Ice < Ice_{max} ... (Good)$$
Will the voltage divider formed by R2 & R3 turn the FET ON?...
$$Vgs = V{supply} \cdot \frac{R2}{R1+R2}$$ $$Vgs = 12V*(2kohm/(2kohm+2kohm)) = 6V$$ $$Vgs > Von ... (Good)$$
Is the final output safe for the FET, peltier, and power supply?...
$$I{supply} = \frac{V{supply}}{Peltier\_R} $$ $$I{supply} = \frac{12V}{2.4ohm} = 5A $$ $$I{supply} < Supply\_I{max} ... (Good) $$ $$I{supply} < FET\_ID{max} ... (Good) $$ $$I{supply} < Peltier\_I{max} ... (Close) $$
The current through the peltier is close to its max limit. Is that a problem? I don't know. The max rating could already be derated and there could be variation in the peltier resistance or the supply voltage. Personally, I would probably add a 1-2ohm power resistor, but that would waste power to give myself piece of mind. Another option would be to increase Ron, by decreasing R3 or using a less efficient FET.
The resistor values have the following requirements:
R1 limits the current into the optoisolater. Look at the datasheet for that part. Find the forward voltage of the LED (Vf) and an appropriate current to turn it on (Ion). If your arduino IO voltage is Vio then you want R1 such that:
(Vio-Vf)/R1 = Ion
Make sure the arduino IO can handle that current, as well.
R2 and R3 form a divider to drive the gate of the FET. The gate voltage when the FET is on (assuming small Rdson of the FET) is 12V*R3/(R3+R2). This voltage should be enough to turn on the FET but not so much as to over-voltage it's gate. It seems to me that you could do without R2 since the FET is rated for > 12V...
