Michael O'BrienRemember 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.
- That MOSFET only likes 1 W of power dissipation at 25 °C and it downrates sharply from there. An improved part, SQ2310ES, and itts RDSon is about half or better, the max junction temp is better, and the max power dissipation is about 1.4 W @ 70 °C vs 0.8 W @ 70°C.
- An even better MOSFET that's less expensive and have less Rdson is the PMV16XNR by NXP, fwiw.
- The stock 30 mm fan on the hotend is a 1 W fan while the Noctua I mentioned draws about nearly 50% the current.
- The power resistors and and replacement caps showed up.
- The 12.3 V from the brick isn't as clean as it could be and that is dirtying ground with up to 500 mV of noise. There is a spike I wish to remove.
- The thermistors have no less than 500 mV of noise on them; partially conducted through the chassis. I twisted the hotend thermistor's wires yesterday and that seemed to halve this noise to +/- 0.25 ˚C @ 190 ˚C. There is an un-populated resistor at the base of each thermistor hookup, but I didn't have a chance to check to see how it's traced on the PCB. I've not checked what the caps, C3 & C13 are valued at.
- I sampled the response of the x-axis switch, easiest to toggle with a finger, and determined that the time constant is about ~3.56 ms. or about ~0.14 mm @ the rated 40 mm/s travel speed. The caps in-line for debouncing are C25, C27, & C11. I didn't check their values at this time.
- The 8 MHz crystal isn't populated with capacitors for the STM32. This was the case for Benchoof's Select Mini and mine as well. The datasheet calls for 24-30 pF, which I don't have in hand at 0603 imperial sizing.
- Noticed this off of my UTC171C's frequency counter & duty cycle mode and verified it with my scope, but when both the heated bed and the hotend are powered up, the max frequency for the MOSFETs is ~147 Hz, about half of the previously noted 300 Hz. I've made some preliminary PID settings that undershoot quite a bit, but remain within 1-1.5 ˚C within 30 seconds of touching their setpoint.
- Despite the hotend being rated up to 250 ˚C, I'm noticing control issues at and beyond 230 ˚C. I need to trace back how the thermistors are hooked up the the ADC pins on the driver board, but I believe this is a limitation unintentionally induced by the hardware design for reading the thermistors.
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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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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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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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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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The crystal's datasheet. There is enough parasitic capacitance to make it oscillate, but it's no sine wave
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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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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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