Board current limits for MOSFET power

Controllers Smoothie
1

Hello—

I’m building a large printer with a 12" x 36" build platform area. I’ve got 3 bed heaters which can draw up to 10A each. I purchased the Smoothie partly because the MOSFETs on it could handle the load… but now I’m looking at wiring it up and I have my doubts because the 3 MOSFET’s are drawing current from the same input connection, and 30A would easily exceed the rating of the connection.

I could double-up and use both the soldered connection and the plug to supply power. If I did, would the board itself handle the current ? Or am I better off building external circuits to drive the bed heaters ? could I remove the MOSFETs on the board and relocate them onto their own board with dedicated power feeds ?

Thanks,
MTG

Imported from wikidot

2

Hey !

The connectors for the big mosfets are rated at 12.5A max. The board shouldn’t be able to handle more than that either.

This means that you can’t get more than 12.5A out of both mosfets together ( the input connector and the board being the limit here ).

Your proposed solution of using both the VBB and mosfet power inputs together is not adequate either, with 30A worth of heaters you would go over the limits anyway.

I think you are going about this wrong : on-board mosfets are not designed to take that much current, on -any- reprap-type board.

If you need to handle more than ~10A, what you really want is an SSR.

Both Uberclock and Robotseed ( the shops selling Smoothieboard ) sell SSRs for this exact reason.

For example get the 40A version of this : http://shop.uberclock.com/products/solid-state-relay-dc-to-dc , it’s much much more adequate for what you want to do.

If you need any help with wiring it to the Smoothieboard ( it’s very very easy ), don’t hesitate to email me at wolf.arthur@gmail.com.

Cheers !

3

Yeah, makes sense. Basically what I was planning to do back when I had an Azteeg Pro running things. I’ll probably go with separate SSR’s for each heater with the Smoothie, since I have adequate thermistor inputs…

So, do I drive the SSR’s from the MOSFET output ? or do I need to tap in upstream of that and get the MOSFET control voltage instead ?

Thanks a bunch for the help !

MTG

4

I see now … The 3D printer guide describes using the GPIO pins to run the SSR’s … cool.

Thanks,

MTG

5

You can drive an SSR with the high or low current FET outputs. It’s overkill, but it works fine and you don’t have to change anything in the configuration file. The FETs provide a switched ground, so connect the SSR + (plus) control input to the Vbb supply and connect the SSR - (minus) control input to the FET.