A surplus Mean Well PSP-600-48 48 V 12.5 A power supply just arrived, I dialed it back to 40 V, and swapped it with the 36 V brick I’d been using to drive the M2’s improved heated build platform.
The improved platform was designed for a 30 V supply that would run it at about 150 W, which took slightly less than forever to reach operating temperature.
With the 36 V supply set to 38.6 V, the platform drew 6.2 A at room temperature, which worked out to 6.2 Ω and 240 W. It was a tad pokey getting up to temperature
At 40 V, the platform starts at 6.3 A / 6.3 Ω / 250 W from a bit over room temperature and drops to 5.8 A / 6.9 Ω / 232 W at 70 °C.
At about 250 W, the platform takes about three times longer to reach operating temperature than the extruder, but it doesn’t require calling down to the engine room for more coal before maneuvering. I must run some numbers on it, now that I have a power supply with a useful range.
There’s obviously an upper limit to the peak power the PCB traces under the glass can handle, but it runs at the same average power (to produce the same average temperature) and, at least so far, hasn’t shown any signs of distress. The few additional watts at 40 V won’t make any difference.
Note that you must use an external DC-to-DC solid state relay, because the Rambo controller board can’t handle anything over 24 VDC and high current loads tend to melt its Phoenix-style connectors. When you add the SSR, replace the HBP connectors with Anderson Powerpoles, use fat wires, and be done with it.
The M2’s Marlin firmware uses bang-bang control and tends to overshoot the setpoint; I’m not sure a few degrees makes all that much difference, particularly because it’s not measuring the temperature at the top of the glass plate.
The Smell of Molten Projects in the Morning 
Flame on !
Ed!
If the M2 is using a version of Marlin it is possible Rick et al did not remove the PWM/PID control code. Have you verified, enabled and re-compile/uploaded?
The Marlin version comes straight from the Github source, about a year ago, and lacks all of the updates since then.
However, that SSR has a fairly sluggish response time and probably can’t keep up with even a few hundred Hertz. I don’t believe the hardware matches any of its alleged datasheets, so I should measure its response before leaping to any conclusions.
PWM-ing half a dozen amps through those long cables may not be a Good Idea, come to think of it.
And I’m not sure PWM will make any difference at the top of the platform, because the thermal mass of that glass plate is so high and its conductivity is so low.
Yet Another Project! [grin]
Ed, you CAN turn on PID bed control, at least in the latest version of Marlin.
Aye, but … see my musings in response to Brian.