I’ve been testing the Stealthboards over the last few days and have worked out most issues. The one that keeps giving me trouble is this:
When I turn on the heaters for the hotend and especially bed I get a weird sound from my power supply. It seems to make the noise (see video below) when the mosfet is switching on and off. I also see a voltage drop of about 1V from the PSU while it occurs.
The current mosfets being used are PSMN5R2-60YLX from Nexperia.
I have tried two different power supplies at different voltage ranges and two different beds and with two different boards. I am trying to figure out if it’s the mosfets not playing nice or if I have bigger issues.
Any input would greatly be appreciated.
Only when the mosfet switches on and off? Or when it’s on only?
Do you have an oscilloscope?
@Stephanie_A if it’s just heating/sitting with the gate open then no noise.
Unfortunately don’t have leads for my oscilloscope- getting them on order soon.
Running PID or bang-bang? It looks like the MOSFET LED is flickering when the bed switches on, which could suggest the PID switching frequency is in the audible range, and you’re getting a speaker effect off an inductor or something. If it’s bang-bang and you’re getting that, I would look at the MOSFET circuit schematics to see if something weird might be happening to make it oscillate.
Try adding a rather large capacitor in parallel with the heated bed, like 2200uf or so. Be sure of polarity. This should take some of the transient load off of the power supply.
@Ryan_Carlyle this was my thought as well. It’s PID. The datasheet doesn’t suggest anything that would cause a strange oscillation but I’ll swap to hang bang and see what that does.
@Stephanie_A will do. It has a 100uF sitting next to it but I know that isn’t going to do much for a 10A bed
@Stephanie_A you mean a capacitor on the heatbed supply rail upstream of the MOSFET, right? Not actually wired in parallel with the bed heater? That would cause a massive inrush current when the bed switches on and cream the PSU even worse.
Yeah you’re right, that’s what I mean. One side on gnd, other on v+ right before the bed. Not on the switched side of the mosfet.
I’m gonna throw one on the 12V rail right near the bed, but part of my wonders if it’s because I don’t have any flyback diodes which could be upsetting the whole circuit.
Shouldn’t need any diodes on a giant resistor…
Added a 1000uF and no luck. It’s a dead simple circuit. 33ohm resistor in line with the pin from the IC, and a 33k pulldown. Part of me thinks it’s the mosfet itself still.
I take back what I said… it helps a little, just not significantly. I don’t think 2000uF of capacitance should be needed for a MK2 bed anyways. Might try some different mosfets
33 ohm resistor to the gate? Thats pretty high for 3.3v try somewhere around 5 ohm.
I had a 10 ohm before. For some reason the pulldown was a 5k from the factory so I swapped it for a 33k and 33 ohm in line. I will make it a 5 or so and see if that changes anything. Sounds like a digikey order is coming very soon…
Is 3v3 enough gate drive voltage for the MOSFET you’re using and switched current/voltage?
@Ryan_Carlyle based on the datasheet, 3.3V should be enough for around 120A. I am going to grab a few different types to see if it is indeed the mosfets themselves. @raykholo uses 3.3V Vgs on his boards without issue. I think it’s a matter of finding the correct FET. If I put my ear near the PSU I can hear a fainter version of the noise while just running the hotend, which makes me think it is the FET’s themselves.
I still think it’s power supply related. While the mosfet is switching the demanded current rises rapidly, placing a strain on the supply to quickly compensate. Most power supplies list a transient rating - how quickly they can react to a changing load. The added capacitor on the 12v line reduces this transient load by offloading it to the cap.
@Stephanie_A I’ve used this power supply for almost a year, however it was previously running a Printrboard Rev F. Both boards have roughly 500uF of capacitance on the main input line. Perhaps the 32 bit LPC can switch fast enough to bother my PSU’s. I tried with my 24V as well and ran into the same issue. That one is an LED supply.
Here is what’s interesting- I ran an auto PID tune on it earlier yesterday, and all throughout that the noise didn’t occur, which could lead me to believe it’s a switching frequency issue. Immediately after, I saved the settings and set it to heat normally and the issue arose. Noise was occurring before the auto tune as well when I just set it to heat normally.
I wonder what the pid frequency is. The printrboard has a 5v, which gives a higher gate voltage.
Yup, the default pid frequency is 2khz. Marlin is around 8hz. Try changing it to 10hz.
Well @Stephanie_A looks like I really owe you one. I dropped that value down to 10hz and then 8hz and those two showed little to no deviation from eachother, however the sound is gone. All I hear is the faintest click when the FET is triggered. This is something that happened with the Printrboard if I remember correctly, as it’s only a 216W output at 12V it’s definitely feeling that bed load. I’ll do more extensive testing tomorrow, but it looks like you hit the nail right on the head.
Really really appreciate it.