I’m going to make a circuit using a mosfet to run my 450W 24V heated bed.
Any feedback?
Looks good to me.
Is it an inductive load? You way wish to research the switching of them. At the very least you want some capacitance on the input to the fet.
@Adrian_Chapmanlaw , this is the specs of the PCB heatbed
Voltage: 24V DC
Current: ~20A
Resistance ~1ohm
Heat output: ~480W
Do you have any rating suggestions for the capacitor?
What size,model, or type?
@Panayiotis_Savva it’s been a while since I looked at it but if you Google “switching inductive loads” that should get you most of the way there.
The thing is that what you switch it off the electricity wants to keep flowing so you get a voltage spike this needs to be soaked up. Normally with a capacitor or a zenor diode
It’s a heating bed only. It’s pretty much a resistive load and its inductance is negligible.
This is a resistive load after a little googling, and see a confirmation from @Gato1 .
I’ll make a small circuit board tonight or tomorrow morning.
I’ll post up the results
@Adrian_Chapmanlaw resistor works better. The input (gate) is a cap… Inductivity would not be that high on a hetbed. Ive powered some teslacoils with mosfets, dont worry, be happy
Look at the datasheet of the mosfet. Thresholdvoltage needs to be very low ro switch this monster… Id rather go with a fet-driver (like icl7667)
You still need some surge protection ideally.
@Adrian_Chapmanlaw two 15V z-diodes are all you need for that case… Also the fet has a surge diode inside, which will fck off all overvoltage from the bed…
@VolksTrieb I’m rusty on this stuff just remember it’s not “good practice” to rely on the fet protection 
@Adrian_Chapmanlaw yeah for a 500DC motor maybe. And even there most companys dont give a fck… But this has like 25nH inductivity 
How is the signal from the RAMPS generated?
The diagram suggests that it is 12V which is fine for switching a MOSFET but if that signal is generated from a pullup resistor to +12V, which is pulled down with another MOSFET, the 10k resistor will form a potential divider with the RAMPS pullup and reduce the gate drive voltage.
I would check if this is the case before implementing this circuit.
Hanging additional components on the gate of the MOSFET for protection is not necessary but I would certainly check the source of the drive signal from the RAMPS.
Worse case is if the RAMPS output is an open-drain (which might be the case if it was for driving a heated bed) which would result in the MOSFET not switching on due to the 10k pulldown resistor keeping the gate-source voltage at zero.
@Neil_Darlow yeah thats why i suggested a driver. Think its only TTL signal…
Hi. I would consider a overtemp protection so you dont cook yourself. Our heated blankets sometimes get very warm. Br
have you thought about using a pi zero running node red?
that would give you temperature monitoring and control from your phone.
i have a couple of pi zeros running my central heating in the house. it takes some time to get it setup but it works really well.
Oh tempora, oh mores.
Things you need to check:
Inductive load- do you want to splurge for flyback diodes
Gate source voltage- is it enough to drain that current with your mosfet
Might want to put a resistor in line with the gate source voltage
That’s a pretty serious bed. At that wattage, I switched from D.C. To Ac and used a solid state relay.
Reason I’m sticking with the DC is because I already have the power supply and bed.
Unless I manage to sell the setup for 80 Euro or so, it’s probably not worth me switching to AC now…