Mosfet board vs SSR vs onboard
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I agree, it's probably caused by the voltage at the VIN terminals of the Duet dropping when the bed heater cycles. Some suggestions:
1. Make sure that the screws in the power input terminal block on the Duet are tight.
2. Use a multimeter to measure the voltage at the power supply output terminals with the bed heater both on and off, and also the voltage at the VIN terminal block. If the voltage at the PSU output drops significantly, get a better power supply (cheap ATX power supplies are not very good in this respect). If the voltage at the PSU output remains stable but the voltage at the VIN terminal block drops significantly more, use shorter and/or thicker wires between the PSU and the VIN terminal block.
If you don't have a multimeter, then as Ian mentioned you can use M122 to read the voltage at the VIN terminal block.
3. Switch the bed heater to PID mode.
You've just given me an idea: I could adjust the extruder PWM automatically to compensate for VIN voltage changes.
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I have the power supply at the base of the tool and the Duet in the top (converted Rostock Max V2 to V3) I measured the voltage at a terminal strip near the Duet (12 in wire between Duet and terminal strip) during bed heating and I did see a variation 12.01 to 12.05. The heater is in bang bang mode.
I have a meanwell 350W supply
From M122:
Supply voltage: min 11.8, current 12.1, max 12.2, under voltage events: 0, over voltage events: 0
=== Heat ===
Bed heater = 0, chamber heater = -1
Heater 0 is on, I-accum = 0.0
Heater 1 is on, I-accum = 0.4I have 12 gauge wire from the PS to the terminal block and to the Duet
With the heater just turned on voltage at the terminal strip, Vin and Bed out all read the same at 12.05
I have 12.66 amps running to the bed through the Mosfet when it is on. There does not seem to be any current running on the control lines from the Duet to the Mosfet. I have 22 gauge wire for the control wires. -
OK I misunderstood, I didn't get that you were using an external mosfet to control the bed heater. If the voltage at the Duet really only varies between 12.01 and 12.05 with the bed heater off and on, then I am surprised that it has that much effect on extrusion. OTOH the Duet's own voltage monitor is recording a much bigger variation. If that's the first time you have run M122 since starting the Duet, then the 11.8V figure could be the voltage just after startup. The maximum and minimum recorded voltages are reset whenever you run M122.
Anyway, if the extrusion variation really is caused by power voltage fluctuations affecting the hot end temperature, then switching the bed to PWM should solve it.
If the issue only happens or the first layer or two, then another explanation is that the variation in bed temperature is affecting how the filament flows when it is extruded. Again, switching the bed to PID will solve this.
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So switching to PID did solve the problem.
You don't have to read the rest.
I am curious why this happens. On the SeeMeCnc forum one person attributed it to the hot end temperature changing, but my hot end temperature was steady. This is why I assumed there was a changing speed on the extruder.
I am also using a Bondtech extruder, so I wonder if it is more noticeable on a geared extruder.
Does it only happen when you run an SSR or Mosfet? Using these the current on the Duet for the bed heater is very small. Does it happen when running the heater through the Duet?
Anyway, I am finally getting decent prints (I introduced and fixed several issues when converting a Rostock V2 to V3)
I am loving the Duet Wifi. -
I'd guess it isn't the extruder speed changing, it is something flexing due to heating of the heated bed, pushing the heated bed up/down which looks like over/under extrusion.
PID holds the temperature more steady as compared to bang/bang.
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