24v thru Conn3

Hey all.
I finished building a Gen7 1.5 and was building a sevenswitch for running 24v's when I got to looking at the schematic for the Gen7 board and it hit me: Is it possible to connect a 24v power supply thru the Conn3 on the board?

It looks like it should work. There's only a couple of issues that to me look like could be a problem:
1. Ground. If I connect another psu to this connector the ground for each supply would end up connected. Is that okay? If not, should I isolate by cutting a small slit thru the pcb ground circuit at some point past the mosfets, like between Q2 mosfet and the X_Min connector?
2. Would my hotends' heater resistor be able to handle 24v's? If not, then that would be a stopping point I would suppose.
3. Any other issues that I might have missed?

The rest of the hardware seems to be the same in this power "block" on the gen7 board as it would be on a sevenswitch, minus swapping out resistors for the led's of course.

Thanks for any and all help. =D

Ground connection shouldn't be a problem, unless it's at a different level to the other grounds on the board. I've isolated the 12V heater grounds from the rest of the board on my tweak of Gen7, but only so I could ensure a short, high current ground path.

Your resistor will have a power rating, so long as you don't exceed that you'll be fine

Hey thanks majic.

Well, it worked, a little too well lol.

The hotend resistor was rated for ~113 volts but at 24 volts it would go from 0 to ThermoNuclear in about 10 seconds flat lol.

So, that was the end of that.

Wired the hotbed to a sevenswitch and called it a day, no use crying over spilled milk I guess lol

Anyways, thanks again!

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Is it possible to connect a 24v power supply thru the Conn3 on the board?

Yes, it's designed this way.

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1. Ground. If I connect another psu to this connector the ground for each supply would end up connected. Is that okay?

Also yes. If you don't connect GNDs, currents will find a way somewhere through the MOSFET. Unlike a mechanical relay or a transformer, MOSFETs don't isolate galvanic.

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2. Would my hotends' heater resistor be able to handle 24v's?

Either use a resistor with four times the resistance of the 12V one or limit the maximum PWM. The latter is a kludge, but if PWM is always lower than 50%, you also get half the heating power, only. Marlin has some MAX_PWM #define for this, Teacup has BANG_BANG, BANG_BANG_ON and BANG_BANG_OFF.

The problem of very quick heating is less the fact there's plenty of heating power, more the fact the generated heat has to reach the thermistor quickly enough to make the firmware recognize it.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY