I'm going to make a circuit using a mosfet to run my 450W 24V

3D Printing
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21

@Panayiotis_Savva you could go to a dc solidstate relais. But that is the same as the mosfet :wink:

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I am a fan of the logic-level MOSFETs that you can now buy, e.g. the RFP12N10L from fairchild semi, that will switch upto 100V 12A from a logic level (3-5v) input. https://www.fairchildsemi.com/products/discretes/fets/mosfets/RFP12N10L.html

23

Here’s the circuit that I’m using for my 20A Heated Bed for over 30 days of machine-on time. This circuit is nice because you get 12V gate drive for two MOSFETS (which hardly heat up at all even under full load) and optoisolation. However I’m not a big fan of pulling the Power MOSFET gates up to 12V. If the 2N7000 fails open, the FETs will be permanently on.
You might be able to rework the circuit to use a P-channel gate drive FET
https://plus.google.com/photos/…

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@Ishaan_Gov ​, please can you post up the photo again, or check the link.
I am not able to access it.

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@Panayiotis_Savva , Definitely, hopefully it works:

missing/deleted image from Google+

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Don’t forget the thermal fuse, 450w is enough to be a problem if the MOSFET sticks on.

27

U rather want to use a relay. Mosfets are desigbed to stand switching power supplies. If you used it as a CC it will burn in seconds with 400w

28

Ok, if you’re planning to run this out of the D8 screw connector on the RAMPS board, the original posted circuit won’t work.

RAMPS has a low side switched FET, so:

  1. One side is always +12V (or whatever the RAMPS is being run on).
  2. The other side shorts to ground through the on-board FET.

Either:
a. Use an opto coupler and then invert it using a FET like @Ishaan_Gov is using in his circuit
b. Invert the signal (eg: using a FET) as it’ll be the opposite what you need.

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@Stuart_Young ​, if I remove the existing mosfet on the ramps, and run the gate directly from the ramps, would that be a better option?

Otherwise I’m better off implementing @Ishaan_Gov ​ schematic…

30

@Stuart_Young ​, if I remove the existing mosfet on the ramps, and run the gate directly from the ramps, would that be a better option?

Otherwise I’m better off implementing @Ishaan_Gov ​ schematic…

31

@Panayiotis_Savva , if you plan on using my schematic, use a MOSFET like the IRFI4410ZPBF that has a Vgs Max > 24V (instead of the IRL8748s). The optoisolator and “inverter MOSFET” should also be able to withstand 24V at their collector and gate respectively.
Alternatively, clamp the MOSFET Gates with a Zener diode with a voltage < Vgs Max (20V for the IRL8748s)

32

I’ve got 2x IRFZ44N at hand, which have a rating of ±20V on the Vgs.
I’ll give those a try with a nice big heatsink :). See what happens.

I know they’re not the best fit this application, but I’m waiting for some very nice VISHAY SQM120N04 to arrive via post…

33

@Panayiotis_Savva , the circuit I posted exposes the gates of the MOSFETs directly to the supply voltage (which was 12V in my case). Using a 24V supply will exceed the Vgs and cause the MOSFETs to fail, so I would not advise it

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@Panayiotis_Savva , Do you happen to have a +12V rail on your printer that you could use as a gate drive?

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Yes, the D8/D9 pins from the ramps 1.4 board.

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@Panayiotis_Savva , As long as you use 12V to drive the gates of the MOSFETs, you should be fine

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I just received 2pcs of SQM120N04 from Vishay :smiley:
Datasheet:

These are very nice spec mosfets, which should be able to handle the amps fine.

I’ll be using an optoisolator circuit as designed by @Ishaan_Gov to ensure there is no chance of burning my ramps board.

Thank you all for your suggestions.

38

Ishaan Gov, one trick with the Mosfet gate is to use a zener, that way you can isolated the 2 supplies.

Below is what I used for my Custom driver. And here is a link to a pic of one of the prototypes. Those are 30A rated terminals.

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@Stephen_Beeson ​,
I really like your implementation with the zener.

@Ishaan_Gov ​
I’m looking to run 24V 450W heated bed

Can you suggest the schematic with the components below, with your setup for 24v?

Components I have so far at hand:

0.25W 1K Resistors

0.25W 10K Resistors

1/2W 4.7K Resistors

0.25W 100R (Ohms) Resistors

5.3KV 30V Optocoupler 4N25

0.5W 24V Zener Dioded (0W5-24VX4)

2pcs SQM120N04 Mosfets

25A fuse.

24V power supply 600W

#Ramps 1.4 which will send a PID control signal @12V

I also have 3pcs of IRFZ44N Mosfets, but i quickly realised they will burn out in minutes if not second at a 20A load

40

@Panayiotis_Savva , if you have a 12V rail to work with, I would follow @Stephen_Beeson 's schematic pretty much exactly, but instead of a zener diode, you can tie the voltage rail straight for 12V