RAMPS 1.4 Replacement Mosfets?

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RAMPS 1.4 Replacement Mosfets?

Postby vgordin » Sun Jul 14, 2013 12:57 pm

Hi all, I burned out the heated bed mosfet on my ReprapDiscount RAMPS 1.4 board. The wiki specifies these from digikey (http://www.digikey.ca/scripts/DKSearch/ ... 0034248468), but I only need one and would end up paying more for shipping than the part. Would any of the options available from Radioshack locally suffice?

http://www.radioshack.com/search/index. ... osfet&sr=1
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Re: RAMPS 1.4 Replacement Mosfets?

Postby cvoinescu » Mon Jul 15, 2013 10:27 am

As a general comment, you want a MOSFET that, at 4..5 V gate voltage (VGS) will have a low enough on resistance (RDS(on)) that the dissipated power at the current required by your heated bed is low enough not to burn it out. It must also be able to carry that current (IDS, or ID(on)), but that's usually not a problem. The dissipated power is P = I^2 * R, so if you're using a standard 12 V heated bed, that needs some 10-12 A -- say 11 A. If the RDS(on) is 20 mohm (at VGS = 5 V), that works out to P = (11 A)^2 * 20 mohm = 2.42 W. As a ballpark, a few watts is acceptable, but more than 2..3 W needs airflow or a small heatsink.

Be careful that some MOSFETs aren't of the "low RDS(on)" type, and many are not "logic-level compatible" (they're specified for 10..12 V control voltage, not the low 5 V available from digital logic). You'll need to check the datasheet and select one with RDS(on) below about 20..30 mohm, ideally lower, at VGS = 5 V. The NTE2986 seems adequate, but you can probably do better.
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