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Marlin states that PID_MAX limits current to the hotend and the bed, does lowering this effectively protect the RAMbo from the full 18.5 amps of a 24v PS running the onyx?

No it wont because the Rambo just switches the bed on and off, so every time its switched on, it will draw the full 18A at 24V.

magicmushroom666 wrote:No it wont because the Rambo just switches the bed on and off, so every time its switched on, it will draw the full 18A at 24V.

I thought (way outside my realm of expertise here) that the whole point of PWM was to limit current via really fast switching. The current never has a chance to get to max becuase the switch gets shut off. Or am I a dofus?

foshon wrote:

magicmushroom666 wrote:No it wont because the Rambo just switches the bed on and off, so every time its switched on, it will draw the full 18A at 24V.

I thought (way outside my realm of expertise here) that the whole point of PWM was to limit current via really fast switching. The current never has a chance to get to max becuase the switch gets shut off. Or am I a dofus?

It does not limit the current, it limits how long the current is applied. Think of it like flipping a light switch on and off at a variable rate. The light comes on as bright as ever every time the switch goes on but the relative light output
is less because the time it is on is varied. I hope that makes sense. You are not a dofus and we need these type of questions so that we may all understand. Never stop asking questions, we all benefit.
Carl

You could put a large capacitor across the PID output and it would then limit the voltage and as a result the current.
You'll get some of the same effect from the existing mosfet circuits, because they don't respond instantaneously and the pulse widths are relatively small, but I wouldn't trust that without checking it with a scope first.

With my Rostock running marlin on an original Rambo, the bed isn't controlled with PWM, but just bang bang on off control. At 100 degrees the bed is on for around 5 seconds at a time. I use a mechanical relay to drive the bed from a separate 18V supply by the way.

Got it. Thanks for the education!