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Power requirements of a hotend

Posted by smartroad 
Power requirements of a hotend
June 25, 2013 03:54AM
I am currently building a printer which will (eventually) have at least 2 hotends. At the moment I am looking at electronics to drive it. My favorite at the moment is the RUMBA. However I am a little worried about power. It has two power inputs, one at 10A for the heated bed and another at 5A for everything else. Hotend wise I am thinking of the E3D variant which supposedly comes with a 40W heater. Now 40W (@ 12V) is 3.33A which would leave nothing for another hotend (once the motors and electronics have taken their slice of power).

The RUMBA can run 3 hotends but all this comes from the 5A side and I am not sure how it would be able to supply that.

My only other thought is that the hotend would only take full power while going through its initial heating, once running it should consume much less. So the key would be to heat each hotend in turn (with multiple hotends) initially so as not to blow the 5A fuse.

Or am I missing something? I don't want to buy the electronics and then find out it can't run my setup smiling smiley

Thanks for any assistance offered!
Re: Power requirements of a hotend
June 25, 2013 04:17AM
Usually only one hot end at a time is actively beeing heated.
Re: Power requirements of a hotend
June 26, 2013 08:19AM
Srek, but that would mean waiting several minutes between each output, while the secondary hot-end warms up to temp. It would be more effective for maintaining print speed to idle the second hotend at or near the target temperature, which would imply a need for twice the maintenance current.

One could theoretically heat up the first extruder, then heat up the second, to reduce the transient demand, but unless I'm mistaken, that would require re-writing portions of the firmware, since I don't think it does this now.
Re: Power requirements of a hotend
June 26, 2013 09:14AM
This is just off the cuff an apologies if not fully thought out but ....
It seems to me that the advantage of using a 40W heat cartridge is that it gives a faster warm up time than would be the case with a traditional wire wound resistor. Are there other values available which will reduce the current demand? The resistance of the heat cartidge, 40W at 12V works out a 3R6 - quite a bit lower than the typical RepRap hot end resistor of 5R6 or 6R8.
Re: Power requirements of a hotend
June 26, 2013 09:51AM
There is another way. I am using this way to run the heated bed, but it could also be used for hotends as well.
I use the mosfet output to drive a solidstate relay, and that can then switch on another power source that has enough power to do what you want.
In my case I am using mains power to heat the heated beds, so I can use a DC-AC solidstate relay. If you are to use DC power then you would need a DC-DC solidstate relay, however it is possible to use AC for the hotends and just a cheap low voltage transformer that has enough amperage to cover the hotends that you will be using. The DC-Ac solidstate relay is also cheaper and easier to find.

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