Duet 3.3V supply quality

Hi Treth, which spike are you talking about? If you mean the spike on the mosfet drain, I really don't think it's a problem as regards reliability and operation of the board. If you mean the spike on the +12V rail, then I agree that this is a problem.

Some possible fixes are:

1. Add capacitors on the +12V input, as close to the Duet board as possible. This is my least-preferred solution, and on its own, probably the least effective. It's more effective if you add the capacitor on the back of the Duet board, between bed heater +ve and the -ve side of the 12V input. I have added a 1uF ceramic capacitor here.

2. Add a resistor in series with the mosfet gate. This is a tricky mod to do and I don't recommend it. I added 100 ohms here, but I think around 200 ohms would be better.

3. Add a capacitor on the back of the Duet board, between bed heater -ve and the negative side (upper terminal) of the +12V input. This is an easy mod to do. I would not normally recommend connecting a capacitor directly across a mosfet, but the mitigating factor here is that the mosfet turns on slowly, so the discharge current is modest. Radian used 1uF, but I would choose 0.47uF because I am less worried by ringing on the mosfet drain than he is.

Fixes (2) and (3) also get rid of the spike on the mosfet drain, or at least reduce its amplitude and sharpness. You could also get rid of the spike on the mosfet drain by adding a Schottky flyback diode, but I think this is pointless unless you tackle the spike on the 12V supply.

Edited 5 time(s). Last edit at 01/04/2014 05:59AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Dmould

Whilst not disagreeing with you. There is a post somwhere here that I have replicated Radians work using a battery scope and Duet powered from Laptop USB ie no mains at all, not even laptop charger. There was a suggestion about using two scope probes in difference mode AC coupled with the two earth clips joined but not connected. I have yet to try this but I believe Radian has.

Rory

Edited 1 time(s). Last edit at 01/04/2014 06:01AM by Rory166.

Just to add that I've started a dedicated thread to discuss the Heater switching transients issue.Unfortunately this particular thread about the quality of the 3.3V supply contains an awful lot of discussion about the heater switching transients. The two are indeed related, but there are parallel issues to do with the "not so LDO" regulator and noise immunity that might be better continued here. I hope we can pick up the topic of heater switching transients in the new thread.

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RS Components Reprap Ormerod No. 481

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Rory166
Dmould

Whilst not disagreeing with you. There is a post somwhere here that I have replicated Radians work using a battery scope and Duet powered from Laptop USB ie no mains at all, not even laptop charger. There was a suggestion about using two scope probes in difference mode AC coupled with the two earth clips joined but not connected. I have yet to try this but I believe Radian has.

Rory

I tried loads of tricks Rory, but didn't really make any breakthoughs. I was investigating the transient response of both the switching and linear regulators and got some pretty wild looking waveforms, but I freely admit the results were hampered by measuring difficulties. As far as I could progress things, it remained the case in my opinion that there were very fast transients happening on the 3.3V rail and that the layout of inductors/capacitors around the microcontroller was not helping but hindering things. However I switched my attention to the cause of the transients when I found that reducing their severity increased printing reliability. Call me pragmatic - or anything you like

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RS Components Reprap Ormerod No. 481