Local source for RAMPS Q3 MOSFET replacement? (upgraded to 24v, now no power)

[james.r...@internode.on.net]

I guess my real question is, Which one of Jaycar's MOSFET's would be (most) suitable. If none, then another local supplier preferably.

I've recently purchased a 150W 24v SMPS, as my 360W 12v SMPS was only able to reach ~80deg.

I have wired it up and discovered that now i'm not getting any power to my HBP.  I'm seeing 24v on both legs of the 11A polyfuse, but no HBP activity led, and 24v only on one leg of the Q3 MOSFET, so it appears a replacement is in order

(I wired the HBP directly to the 24v SMPS and managed to get to over 100deg without any under-plate insulation, so with any luck the 150W PSU will do)

Regards,

James Robertson

[Stuart Young]

Don't bother replacing the FET with an identical one, as it'll most likely fail in the same way. The default STP55NF06L's are not the best choice, and there are better components available.

Drop in replacements with better characteristics (eg: lower Rds(on) value, suitable Vgs, etc) that shouldn't fail with the higher voltages/more current you're possibly pulling:

 IRLB8743
 IRLB3813

 FDP8870
 AOT240L
 CSD18502KCS
 

Many of these you can pick up from Element 14, RS or Digikey.

Note: IRLB models also come in a similar package called IRLU. Same sort of mounting but not as good for mounting a heatsink. Unless you're drawing some serious current however, both the above will stay cool without a heatsink, though you should make sure there is moving air around them (eg: a fan).

PS: If you use a 12V heatbed at 24V, you will pull twice the current (so >20A), and put out 4 times the power (bed will heat too fast, which can cause weird oscillations in temp as it constantly overshoots the temperature). The tracks and connectors on RAMPS are not rated for 20+A (try about 12A absolute max, and less on cheaper versions), If you're having issues with the heated bed reaching max temp, try some marginal voltage increase instead like 13.8V or 15V, rather than 24V.

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Stuart Young (aka Cefiar)

[james.r...@internode.on.net]

 Cef,

Sorry - i should have mentioned, its the HBP similar to this one http://reprap.org/wiki/File:Reprap_heated_bed.jpg

so designed to run at either 12v or 24v depending on wiring

Jaycar's preferable as i can stop by on the way home, however they only list part numbers for their MOSFETS, no detailed specifications.

Regards,

James Robertson

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Re: [CCHS] Local source for RAMPS Q3 MOSFET replacement? (upgraded to 24v, now no power)

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