# Specific 3D Printers, Scanners, & Hardware > RepRap Format Printer Forum > MakerFarm Forum >  How to Wire Mosfet Power Module to RAMPS 1.4 Board

## GBCues

Hi All,
I have a MakerFarm Prusa i3 that I wanted to decrease the bed heating times. (RAMPS 1.4)  I bought a silicone heating pad and aluminum build plate from RepRapChampion 
https://reprapchampion.com/collectio...eater-full-kit 
Installation was a no-brainer, but I kept getting "Bed Heating Failed - Printer Shut Down - Reset Printer" or something close to that.  I purchased their Heated Bed Power Module
https://reprapchampion.com/collectio...d-power-module
but I'm confused about how to wire it.  I sent them an email asking for help, but got a generic "just think of it as an extension to your RAMPS" reply - which I found inadequate.
Can someone who has installed this module to a RAMPS board give me a more detailed wiring description?
I originally thought the D8 + and - went to the DC POWER In + and - on the Power Module, but then there is no place to connect the Control Input + and -.
I am beginning to think that the RAMPS D8 + and - go to the Control Input + and -, but it bothers me that the D8 is what originally supplied 12V high amperage to the original heat bed, so don't want to blow up the power module or my RAMPS.
Also, do both sets of 12V power still connect to the RAMPS and another 12V power to the power module?
Thanks for any help,
Gary

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## printbus

Welcome to 3DPrintBoard.

The bed heating failed error was probably a problem with the temperature sensor on the bed, or the wiring to it either being open or shorted.  

I don't have first-hand knowledge with that module, but it sounds pretty typical. First, make sure the 11-amp input on RAMPS has 12V going to it. No, you're not going to use it to power the heat bed, but the RAMPS D8 Mosfet needs to have that 12V there for it to work.  Then, connect RAMPS D8 to the control input on your module.  Connect heavy gauge wiring from your power supply to the DC POWER connections on the module.  EDIT: And yes, you do need two 12V connections from the power supply to RAMPS, and an additional 12V connection for the module.  There's very little current going through the D8 circuit when you're using an external relay or switch module like this, so the 11-amp input can actually be powered by lightweight jumpers to the RAMPS 5-amp terminals if you wish.

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## GBCues

Thanks PrintBus.  That information was very useful.
Gary (also in Highlands Ranch LOL)

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## 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

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## 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.

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## GBCues

Bob,
Thanks for the reply. The thermistor is related to the silicon heater pad and I've got that under control. The mosfet power module allows me to use the 200 watt heater by bypassing or extending the onboard mosfet as KD7eir noted.  PrintBus's directions were spot on.
Thanks everyone for your help.

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## 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.

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## tugeagon

Is there an advantage to go with this MOSFET over an SSR?  Particularly with the standard 10inch makerfarm heated.

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## printbus

> Is there an advantage to go with this MOSFET over an SSR?  Particularly with the standard 10inch makerfarm heated.


The difference is in the package. If you opened up a DC SSR you'd probably find it uses a MOSFET for the switch.  The "better" one would depend on which one has the least on-resistance, since that would lead to the least voltage drop or power loss in the MOSFET/SSR.

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## Roxy

> The difference is in the package. If you opened up a DC SSR you'd probably find it uses a MOSFET for the switch.  The "better" one would depend on which one has the least on-resistance, since that would lead to the least voltage drop or power loss in the MOSFET/SSR.


Or...  stated a little bit differently...   The lower the 'Power loss', the less energy that is turning into heat and cooking the MOSFET.

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## 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

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