Electrical – Control LED strip with MOSFET and PWM Arduino

If you pick the right N channel MOSFET for the load and you have the load in the drain up to V+ and the source connected to 0V then applying PWM inputs to the gate (with respect to source) will effectively cause the FET to act pretty much like a switch opening and closing. This is an approximation but for low switching frequencies it's not a bad one.

It's never as clean-cut as a switch of course but it can be reasonably approximated to one. When the "contact" closes it has "on-resistance" which can be as low as 1 milli-ohm on some FETs. When the "contact" opens there will be a little bit of leakage current but probably not much more than 10uA.

When it switches, it doesn't do so instantaneously and this is where there can be a significant amount of power loss. The "contact", over a few micro-seconds or in some cases a few tens of nano-seconds gradually changes from high impedance to low impedance. Parasitic capacitances make this worse generally and you need to "drive" the gate quite hard to achieve decent efficiency.

The space-mark ratio that you drive the gate with multiplied by the power voltage (V+) roughly tells you the average voltage applied to the load. Thus, if your supply is 12V and you drive 40:60, then the average voltage on the load will be approximately 7.2V i.e. the FET in "on" for 60% of the time.

This will work. You need to select a MOSFET that has low Rds(on) at the drive voltage from your arduino and to limit the PWM frequency to a relatively low frequency (normally no problem with thermal systems). If you don't do either one of those things, the MOSFET can get quite hot as a result of: \$P = I^2 * R\$ , heating from Rds(on) or from the dissipation during the switching.

You could even use a 1 second PWM cycle if you like.

I would suggest adding a small series resistor to the gate (perhaps 50 ohms) and a pull down resistor (100K is normally okay) so if the gate becomes floating because the micro output is disabled then you won't have the gate floating up into the zone where the transistor starts to conduct, gets hot and damages itself.

If your heating supply is (say) 12V then your current with a 3.9 ohm heater will be about 3A (36W). To avoid having a heatsink on a TO-220 or similar larger package you might want an Rds(on) of less than 35m\$\Omega\$ (about 470mW when hot). One possibility for 5V drive might be the A&O AOI514- 11.9m\$\Omega\$ and rated at 30V, which will run cool if switched infrequently and with 5V drive.