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Fry-a-Part then Fix-a-Part, Small Upgrades Begin, & Misc Notes

A project log for Monoprice Select Mini Electro-Mechanical Upgrades

Endeavoring to build upon the existing fanbase work of this $200-ish printer.

michael-obrienMichael O'Brien 07/25/2016 at 12:167 Comments

Remember this: the A2SHB MOSFET for the hotend fan Google's out to the H&M Semi HM2302A which is possibly a knockoff of the Vishay Si2302DS. While trying to probe the fan leads/pin header, I invoked the powers that be and I saw glowing red and Magic Smoke.

Discussions

john wrote 09/11/2016 at 18:47 point

Thank you so much for this documentation.  I've discovered that I also fried my fan MOSFET.  I have absolutely no experience with surface mount soldering.  Someone suggested just finding a place on the board to get 12V and make an 'always on' fan.  Is this a reasonable solution?  Any tips for the best place to grab 12V?

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razorboy wrote 09/07/2016 at 15:29 point

Did the MOSFET with the A2SHB top marking fail shorted or open? I was reading about failure modes for MOSFETs, and it sounds like they can fail either way. I would think a dead short that causes physical damage would cause it to fail fully open. 

http://electronics.stackexchange.com/questions/27124/do-mosfets-usually-burn-open-or-closed

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Michael O'Brien wrote 09/09/2016 at 21:43 point

It failed open because I shorted it for a long enough time frame that it glowed a brilliant red. Normally it would fail as a short.

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Matt Bennett wrote 08/10/2016 at 11:40 point

When you say the crystal doesn't have capacitors, and the datasheet mentions 24-30pF- do you mean the STM32 datasheet or the crystal datasheet? Usually a crystal won't oscillate at all without any capacitance, so they may be built in to the crystal. The load capacitance on a crystal is determined by the crystal itself, not the device it is attached to (though the device will have some parasitics that add to the overall Cl on the crystal).

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Michael O'Brien wrote 08/10/2016 at 16:43 point

The crystal's datasheet. There is enough parasitic capacitance to make it oscillate, but it's no sine wave

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K.C. Lee wrote 08/10/2016 at 23:00 point

The external load capacitors cause a phase shift that's needed for the oscillation.  Likely that the Chinese removed these caps to cut cost of $0.01 as there is just enough parasitics.  Not sure about the stability and the accuracy of the crystal as a result.

The crystal would need to have 3rd terminal to connect the load capacitors to ground (like some of those ceramic resonator).  Don't remember seeing that on the HaD review.

They probably have a buffer (or schmitt trigger?) inside the ARM to clip it into a square wave assuming the waveform is monotonic.  The PLL would fix the duty cycle and remove some of the jitters.

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Michael O'Brien wrote 08/11/2016 at 10:17 point

No one has specifically noted any problem with the diver board unless the thermal warmup with no fans causes enough to drift that the printer crashes. In the next week or so, I'll pick up a few caps to solder in to stabilize the crystals output to see if this affects operation al all.

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