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24v for 3D printer MK8 Extruder?

Posted by KM6VV 
24v for 3D printer MK8 Extruder?
October 12, 2017 11:12PM
I finally received my extruder, and it says its for 12/24V (I didn't notice that before!). I want to operate it on 24v, but I don't see any way to select the voltage. I understand the cautious on running the RAMPS board on 24V.
Re: 24v for 3D printer MK8 Extruder?
October 13, 2017 12:25AM
Im guessing the 12/24V is referencing the heater cartridge. You can see what was sent to you by testing the resistance. Most common cartridges are 40W so;

40w 24v heater is (24*24/40) = 14.4ohms
40w 12v heater is (12*12/40) = 3.6ohms

You want the cartridge to match the supply voltage. A 24V run off 12V will take ages to heat up.
Re: 24v for 3D printer MK8 Extruder?
October 13, 2017 04:26AM
Yeah, I suspect they are being a bit disingenuous about the description. Its either designed for 24v or 12v the only way it would work perfectly on either voltage would be if it has two heater coils within it to offer different overall resistances, and then it would be 3 wire not 2 (I haven't ever seen such a heater for a hotend).

They are possibly suggesting that it is a 12v heater that can be run on 25% PWM at 24v but this is not good practice since the overall power over time might be equivalent on 25% PWM @ 24v but the instantaneous heating will be 4 times higher i.e. 160W instantaneously but only switched on 25% of the time. This is a fire risk, if the mosfet fails permanently connecting 24v instead of pulsing it, then 160W will melt your aluminium heater block fairly rapidly, and start a fire. Technically 40w can also do this but you will have a few more minutes to notice and do something about it.

As scottybfg said measure the resistance then combine that with the voltage using a calculator like this [www.rapidtables.com] to get the power output. They are often underrated in that most 40w heaters are only really giving you about 34w as they have a resistance of 4.2ohms not 3.6. Be aware that common or garden multimeters struggle to measure low resistances, you need to measure the resistance of your leads first and subtract that from the indicated resistance of the heater cartridge.

I recently started stocking a 50w 24v heater cartridge in my shop (see my sig), yet despite the manufacturer saying it would be 11.5ohms it was 13.5ohms when it arrived, so only produces 42W, but thats still 8W more than a typical 40w heater cartridge. As long as your firmware has good thermal safety features which can protect against a decoupled thermistor, or a heater cartridge that has fallen out of the hotend, you are unlikely to encounter any problems.

Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Re: 24v for 3D printer MK8 Extruder?
October 13, 2017 12:53PM
Thanks guys for the information!

Knowing the hot end is 40w sheds some light on the problem.
I read 3.6 ohms on my Fluke, so it wants 12v. Disappointing.

I want to avoid a dual (12/24v) power supply, but I can initially use two supplies, and later look for a 24v hot end (I see you have one).
Too bad they don't do a dual element hot end). I intended to run 24v for heater bed, extruder, NEMA #23 steppers (Gecko driven), fans and the hot end. Oh well.

After discovering that the hot end might be 12v and posting, I started wondering if the hot end (and heater bed) were PWM driven, as they mention setting up PID controls for the RAMPS board under Marlin. However, a heater does not have inductance like a stepper which would be further current limiting. Little fan is 12v as well. It may be better just to keep the stepper power supply and driver boards separate from the heater supply. I can cable up the RAMPS (sans stepper driver modules) to interface to my old parallel port driven stepper boxes, and make up my new new PCB router/ 3D printer/ laser with a DB25 pin connector.

I am a little puzzled about the mounting of the MK3 extruder. NO mounting holes? I examined the stepper (disassembled it), and It occurred to me that I could possibly mount the unit by the stepper holes; make up a bracket out of thin sheet metal. A quick search on the internet found a 'Metal-MK8-Extruder-mount-motor-plate-for-Reprap-Prusa-I3-3D-Printer'. However the screws aren't that long! Cheap enough 'tho.
Re: 24v for 3D printer MK8 Extruder?
October 13, 2017 01:06PM
12v/24v is a bit of a pain heaters and steppers benefit most from 24v with smaller wires and less effect from motor inductance repsectively. Obviously with fans it doesn't matter worth a damn but 12v/5v fans are more commonplace and often cheaper.

I have one machine where I've gone fully 24v and it is neater and one other where the bed is 24v and everything else 12v and I have 2 PSUs but it's a big, hefty machine so they don't really show.

Sorry I don't know anything about that extruder.

Edited 2 time(s). Last edit at 10/13/2017 01:08PM by DjDemonD.

Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Re: 24v for 3D printer MK8 Extruder?
October 13, 2017 09:19PM
Pure 24v would be good and clean in design.
My CNC PCB Router is about 12" x 12" in footprint, and I'm considering building its power supply into a 5" high 16" x 16" rack mount. the router should mount nicely on top. A small enclosure will hold the RAMPS board, processor and LCD.

Initially I'd planned to drive the PCB router with a PC. That may not be necessary if Marlin can do the job. However, no G83's (drill cycles).

So a complete machine w/Centronics-type interface to connect up with a PC, and a RAMPS/Marlin box that can also plug into it.

With the RAMPS in a separate box, I can also try 3D printing or Laser on my CNC'd Sherline mill.

Re: 24v for 3D printer MK8 Extruder?
October 17, 2017 12:34AM
Yeah, I never did like the MK8 design or mounting options. Most of the time you mount it to a thin bracket that has no support and will flex during moment due to the weight of the stepper resulting in shit quality prints. Then there is the hotend that ships with it. No real heat sink on the throat and just a bad design overall. (reminds me of the early home built hotends).

If you want to run off 24v just buy a new heat cartage, they can be had for less than a few bucks, just means a few more weeks downtime while you wait
Re: 24v for 3D printer MK8 Extruder?
October 17, 2017 12:34PM
I'm still looking at the MK8 extruder I bought. The brackets I've seen are indeed too flimsy. As I have to design a mounting bracket for my PCB router anyway, I might as well make a split-ring type of mount.

I've also just found heat cartridges advertised as 12/24V, Which is obviously wrong. Probably just more 12v elements. No mention of the resistance of the units.

So I continue to get all the parts working together. Got the NEMA #23 steppers fired up with a Gecko driver, fed by the RAMPS board. The axis doesn't move that fast, I've started to play with the max axis parameters. The feed parameter 'F' doesn't work per the RS274/NGC Gcode specs I'm accustomed to. I note that there are MAX's per axis, so the feed command is different. G1 X F or something? I need some examples. The 'cheat sheets' I've read don't cover it. Axis movement is pretty slow right now!
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