How to Wire Mosfet Power Module to RAMPS 1.4 Board

[uncle_bob]

Hi

Keep in mind that there are a whole bunch of different thermistor types made. In general they are not interchangeable. You will need to match whatever thermistor is in (or on) the silicone bed to your firmware in order to get things to work.

Another basic issue: The FET on the Ramps is only good to 11A. If you are running 12V, that is a 120W power level. It does not matter what sort of heater you put on there. With 12V, 120W is the best you can do. If you put a 600W heater on there (with a 12V supply), all that will happen is that the thermal fuse will cut out and you will get a "heating error". The answer to that one is to run a higher voltage (which has it's own issues) or to run an outboard solid state relay.

Bob

[kd7eir]

Hi

Keep in mind that there are a whole bunch of different thermistor types made. In general they are not interchangeable. You will need to match whatever thermistor is in (or on) the silicone bed to your firmware in order to get things to work.

Another basic issue: The FET on the Ramps is only good to 11A. If you are running 12V, that is a 120W power level. It does not matter what sort of heater you put on there. With 12V, 120W is the best you can do. If you put a 600W heater on there (with a 12V supply), all that will happen is that the thermal fuse will cut out and you will get a "heating error". The answer to that one is to run a higher voltage (which has it's own issues) or to run an outboard solid state relay.

Bob

He is bypassing the FET on the RAMPS board with the MOSFET he ordered. The silicone heating pad is 200 watts, and the MOSFET he ordered is rated at 25 amps, so that is fine. The NTC3950 thermistor that is shipped with the 200 watt silicone heater is a type 11. The five volt control signal from the RAMPS connects to the MOSFET he ordered, the 12 volts for the silicone heater connects straight to the MOSFET, as does the silicone heater, so he is only using the 5 volt control voltage from the RAMPS. This is actually a very nice setup - similar to my setup except I am using an SSR.

[Roxy]

He is bypassing the FET on the RAMPS board with the MOSFET he ordered. The silicone heating pad is 200 watts, and the MOSFET he ordered is rated at 25 amps, so that is fine. The NTC3950 thermistor that is shipped with the 200 watt silicone heater is a type 11. The five volt control signal from the RAMPS connects to the MOSFET he ordered, the 12 volts for the silicone heater connects straight to the MOSFET, as does the silicone heater, so he is only using the 5 volt control voltage from the RAMPS. This is actually a very nice setup - similar to my setup except I am using an SSR.

Incidently... I killed my RAMPS board on my Folger Tech i3-2020 printer. I'm not sure how it happened, but I might have shorted something on the Bed Heater circuit. I had a previously killed RAMPS board from another printer in my spare parts pile. I de-soldered two of the MOS-FET's from the other RAMPS board and transplanted them on the freshly killed board. I put one MOS-FET where it normally goes. And then the other MOS-FET got soldered on the bottom of the board to the same pads. So now I have two MOS-FET's switching the bed current.

I know some people are cringing. But I did look at the current / voltage curves for those MOS-FET's. And when the bed is heating up, I can feel each of the doubled up MOS-FET's getting warm. But they are much less warm than they used to get. Just doubling up the MOS-FET's does seem to work and not stress them as hard.

[longjohn119]

Incidently... I killed my RAMPS board on my Folger Tech i3-2020 printer. I'm not sure how it happened, but I might have shorted something on the Bed Heater circuit. I had a previously killed RAMPS board from another printer in my spare parts pile. I de-soldered two of the MOS-FET's from the other RAMPS board and transplanted them on the freshly killed board. I put one MOS-FET where it normally goes. And then the other MOS-FET got soldered on the bottom of the board to the same pads. So now I have two MOS-FET's switching the bed current.

I know some people are cringing. But I did look at the current / voltage curves for those MOS-FET's. And when the bed is heating up, I can feel each of the doubled up MOS-FET's getting warm. But they are much less warm than they used to get. Just doubling up the MOS-FET's does seem to work and not stress them as hard.

Putting MOSFETs in parallel like this doubles the current/watt handling and also drops the ON resistance in half. The only real drawback is you really need to match the pair for gate turn-on voltage which are notoriously sloppy in FETs of all types. In this case it's probably not that big of a deal unless they are wildly different it should work OK. If you have an IR temperature 'gun' a quick way to get a decent idea if they are handling the current equally which means the gates are fairly well matched is to measure the MOSFET's body temps and the closer they are to being the same the closer your gates match. Unless there is a large difference in temperature for this simple application (as compared to say a high performance, high efficiency motor speed driver) you'll likely have no problem.

Paralleling FETs and bi-polar transistors is an old trick used to make low noise audio and radio preamps because paralleling them also drops your Noise Figure in half which can be significant in the first stage of a microphone or weak signal RF preamp. If you look at the internal schematics of many analog ICs you'll find they use parallel transistors a lot. It's also used a lot in power amps for both audio and RF although in those applications matching is critical since they need to run linear for low distortion, something that doesn't matter at all for turning on a bed heater