Rambo 15A fuse limits heated bed power

Hi RepRap,

I am wondering what determines the F4 fuse of the rambo board to be 15A. When I look at the board schematics, the power MOSFET driving the heated bed output is a PSMN7R0-60YS which Digikey lists with 89A max current. It has 6.4 mOhm Ron. I plan to run the heated bed with 24V @ 25A thus the MOSFET would generate about 5Watts of heat. Thus, my question is, can I simply replace F4 with a 30A fuse or is there some other limiting factor?

Thanks for your help.

... I've "successfully" melted MOSFET's rated for 70 Amps with currents of 12 to 15 Amps ... and they were better cooled, than the ones of the shields

On the other side, the most current limiting factors are the track width and connector type -- according to data-sheets, most connectors are recommended for 3-5Amps max!

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Viktor
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Quote
stefanharjes
Hi RepRap,

I am wondering what determines the F4 fuse of the rambo board to be 15A. When I look at the board schematics, the power MOSFET driving the heated bed output is a PSMN7R0-60YS which Digikey lists with 89A max current. It has 6.4 mOhm Ron. I plan to run the heated bed with 24V @ 25A thus the MOSFET would generate about 5Watts of heat. Thus, my question is, can I simply replace F4 with a 30A fuse or is there some other limiting factor?

Thanks for your help.

A few things:

1. Whoever designed the RAMBO should never have chosen that mosfet. The 6.4mohm Rds(on) is at 10V gate voltage and there is no Rds(on) specification for 5V gate voltage. The gate threshold voltage is 4V maximum, so 5V gate drive is nothing like enough in the worst case.

2. It's difficult to make PCB traces wide enough to carry large currents. The 15A fuse rating may be related to the PCB trace width.

3. The terminal blocks (or whatever connectors the RAMBO uses) will also have a current rating.

If your bed needs 25A @ 24V, that equates to 600W of power. For that amount of power, I would use an AC mains heater controlled by an SSR instead, just as I do for the 350W heater in my delta printer (see [miscsolutions.wordpress.com]). Of course, using AC mains adds additional safety requirements, especially if the printer has a moving bed.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].