can someone explain rambo mosfet circuit?

hello,

Im trying to learn how to use mosfets as a switch. I want to use one to control current sent to a TEC for temperature control.

Ive tried some different circuits that I've found online, but none seem to be in full 'on' saturation with a +5v digital on from an arduino uno. The best I've been able to get is about 0.25A to come on. I want to switch on closer to 1A

I know the heat bed powered through my rambo pulls much more than 1A, so I wanted to just try and mimick this circuit and see what happens. Unfortunately I dont really understand the attached schematic. I can't figure out where X38 is connected to and same with X11-9, X11-10.Also, I know it should connect to pwm pin 3, or I guess pin 7 on the mega2560, but I dont see where the connection to that is either.

The rest of the circuit diagram is here.

If anyone could explain how this circuit works or how to wire one up just by itself and switch it on or off with an arduino PWM pin that would be amazing.

thanks for any help

The arduino pin is on the left of each circuit, and labeled BED-HEAT on the Q5 circuit. X38, and X11-10 and X11-9 are the connectors. I think X11-10 and X11-9 are pins on the large connector at the top of the board, and X38 is the set of holes just below it which allow for the user to install a different connector is desired. The indicator LED and its current limiting resistor are wired in parallel to the connectors, all of which are between the 12V rail and GND.

The two resistors on the left can be omitted if you really want to. The 10 ohm one is a current limiting resistor between the processor pin and the mosfet gate. Both the pin and the gate are, in effect, tiny capacitors, and without that resistor the current flow between them can be huge. But it'll work without that resistor. The 100k ohm is a pulldown resistor. Without it, the gate floats until the firmware is booted and driving the pin one way or the other. This can cause the mosfet to be acitvated briefly when the device is powered on.

The important thing is to use a suitable mosfet. You need to use a logic level mosfet, or they need more than 5V to fully activate, which will result in the mosfet having a high resistance, limiting the current you can pass through without destroying the device.

Awesome, thanks for that explanation!

now I see this is the same as the mosfet low side switching I tried before. I think maybe my current limiting resistor between the PWM pin and the gate was too high before (around 6k) because I wasn't sure what value to put in there. switching to the resistor values from the rambo board I easily pushed 4 amps through the TEC and mosfet without much heating of the mosfet.

One weird thing though, I just use analogwrite() to set some duty cycle on the PWM pin. The current supplied to the TEC changes as expected, but there is an audible buzz coming from...somewhere!? This seems scary. the pitch and volume of the buzz changes when I change the PWM duty cycle. lower values seem to produce a louder buzz. analogwrite 0 or 255 and the buzzing goes away.

It seems like the buzzing is coming from somewhere on the arduino, which seems scary...

the mosfet or the TEC wouldn't buzz....would they?!

The arduino is powered through USB and the TEC is powered from an external 12v supply. the arduino ground is connected to the low side of the external power supply.

actually...the buzzing is coming from the power supply

haha. thanks for the help!