Bed heating weirdness

[hmanvel]

I recently had to start over with loading software, flashing drives, etc etc on my Rostock Max V2. Since then I have noticed something odd. I heat the bed up to 100, note that it is controlling closely around that setting. I then remove an insulation blanket I put on the bed, to speed heatup time. I then watch it for 10 minutes or so, the temp drops maybe 2 degrees but then recovers and again controls closely around 100. Then I start a print (abs @ 228C) and as soon as the printing begins, the bed starts dropping, eventually getting as low as 90. It then slowly recovers. Any thoughts on why this would occur only when starting a print job?

[geneb]

The hot end requires more current to reach operating temperature than it does to maintain it. Same with the heated bed.

g.

[rurwin]

The hot end will take more current when it is actively melting filament, but that should be instantaneous; even if it hogs all the current it should recover within seconds if it ever does.

Dumping hot plastic on the bed should cause it to get hotter, not cooler.

The only rationale I can think of is that the heating effect of a given PID output to the bed changes according to the total current being taken from the power supply. The implies that the power supply's voltage drops under load.

Say that the PID is outputting 50% of the full voltage to the bed*, that's 6V. The hot end starts melting plastic and takes more current. The power supply voltage drops to 10V and now 50% of that is 5V. The bed starts to cool and the PID has to wind the output up to 60% to compensate.

I would not expect the power supply to be so closely spec'd that its output would drop. Another thing that can cause this is a resistor or fuse that is common to both heaters. The more current that flows, the more is dropped across the resistor/fuse.

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* on average. The logic still works with PWM or whatever, honest.

[IMBoring25]

If this V2 has the ATX power supply it is very possible.