Mixed 24v/12v supply voltages
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You could do a 24v to 12v converter or even use two separate supplies, provided the outputs of both supplies are isolated such that their grounds can be tied together.
The Duet switches the low side, so you could provide the Duet with +24v and GND, and power the fan with +12v and run the fan's negative to the negative terminal on the Duet - the Duet will still handle PWM and on/off.
Again - you have to be sure your power supplies can be tied together at common for this approach to work.
There is one small issue with this, which is that the extruder LED will get 12V of reverse bias if you use a 24V VIN supply and a 12V extruder supply. This might cause the extruder LED to fail due to avalanche breakdown. It's not a big deal if it does fail because the current is limited by the series resistor, it just won't work any more.
In the Duet WiFi design we anticipated that some people might want to use a different power supply voltage for the bed heater, so we added a protection diode for the bed heater LED. But we didn't expect anyone to run the extruder from a different supply, because 24V heater cartridges are readily available and inexpensive.
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I have been observing my kossel mini which draws only 85W from the mains when the heated bed is off, its in motion printing, with the hot end heated, hot end fan, print cooling fans, electronics fan and LED's on. So I am leaning towards a 120w 24vdc laptop style PSU with USB (additional 5vdc rail might be handy). Whilst this is not very powerful compared to the 300-400w brick style PSU's that are quite commonplace they are running heated beds as well as everything else. I have a corexy which uses a laptop style psu for everything except the heated bed and seems to work fine. Any thoughts?
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Yes, 24V 120W should be sufficient if it's not running the bed heater. That's what my Delta uses.
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There is one small issue with this, which is that the extruder LED will get 12V of reverse bias if you use a 24V VIN supply and a 12V extruder supply. This might cause the extruder LED to fail due to avalanche breakdown. It's not a big deal if it does fail because the current is limited by the series resistor, it just won't work any more.
Only when the extruder is off
What's the part number for the LEDs on the 0.8.5/Wifi? Most LEDs I've seen have breakdown ratings of 20v or so.
EDIT: Actually now I'm a little confused.
With the extruder on, the FET's drain and the E0- terminal are just slightly above 0v. - so this should be OK regardless of Vin/E0+
With the extruder off and 24v Vin and 12v E0+, wouldn't E0- be at +12v, resulting in a 12v forward bias voltage? So the LED would stay lit all the time.
With the extruder off and 12v Vin and 24v E0+, E0- would be at +24v and then you'd get 12v of reverse bias. But I don't think that's what he's looking to do.Or am I just mixed up?
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Tim, you are right. I was thinking of the case where the bed heater is run from 24V and everything else from 12V. So the only issue will be that the E0 led will be on so the time (but a little brighter when the heater is on). It will also fed a few mA into the 12V supply, but that is unlikely to matter.
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The 24v to 12v converters are basically pwm devices, so how do they handle their input being pulsed, eg when used for a hot end heater? Or would you use one to supply the positive side of the heater and modulate the ground through the duet?
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The Duet switches the ground of the heater, so the input to the voltage converter wouldn't care as it is driven by steady voltage.
That's what I was explaining when I said the Duet switches the low side.
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Thanks so run the positive heater output from the duet to the converter positive input, grounding the converter negative input to the psu, then the converter output to the heater cartridge then back through the duet and bobs your mother's brother?Same for a pwm controlled fan (with a smaller converter).
Do I presume I do nothing with the converter output ground wire?
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The converter negative output is probably connected directly to the negative input in the converter, but best check with a multimeter.
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It is and the wiring scheme above works perfectly. Luckily I used the duetwifi wiring diagram as it appears the silkscreen is the wrong way around for e0 heater.
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Yes that silk screen is the wrong way round.
