Running a 12v heated bed from 24v
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Declan
Probably not but you could use a 12 V Dedicated supply for the Bed and use the negative rail of the Duet Mosfet I think to switch it there are some threads on here about doing this (maybe the other way round re the volts but technique is the same)
Doug
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I'd use a 12v psu for the bed running from an external mosfet board controlled by the bed heater output on the duet. Then you only need a modest laptop psu for your 24v supply and no potentially troublesome issues with the two supplies being tied together as they're isolated used in this scheme. A converter can work but it'll need to be capable of around 15A at 12v.
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I'm using a similar setup - an T3DP3D Mini Kossel converted to 24V. My heated bed and fans still use the original 12V PSU, the rest of the printer runs on 24V. Duet MOSFETs switch low side, so you should wire it as follows - positive lead from 12V PSU to the first wire on the heated bed, the second heated bed wire connects to negative bed output on the board. Positive bed output on the Duet should be left unconnected. Then you have to connect the negative output of the PSU to the negative power input on the Duet. It's a bit of a squeeze to make sure that you've got a secure connection for the negative power in since you have to get both 24V and 12V PSU leads in there. This also relies on both of your supplies being floating, which most of them are anyway.
You can also route a lead from 12V PSU to Fan Vin pin so that you don't have to switch from commonly available 12V fans to way more expensive and rare 24V ones.
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It's an option and i appreciate the suggestions but if possible i'd like to avoid two discrete power supplies as that's a bit of hassle, the small laptop psu is good idea and i'll look into that further but i'd like to continue researching and hearing other peoples opinions on DC-DC step down converters for now
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You'll find that most people are steering you away from them, including me. Maybe because they aren't going to work well, or maybe because we just haven't tried it and for some reason don't like the idea. I have used them for hotends and 5A ones do that job nicely, but for the inefficiency of first going 240v to 24v then more losses in the 24v to 12v seems to me like the wrong approach. Twin PSUs is less simple sure, but offers advantages. Why not get a 24v bed, prusa are using 24v on the mk3 so they will be available at some point?
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Tom S. on youturbe did an interview with Prusa at TCT and he said that the sale of his beds wouldn't be any time soon due to the low amount they can produce for their own machines.
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Fair enough, other 24v bed heaters are available.
Decyg the laptop psu is only sufficient if the bed is powered separately, either mains/line or second psu they don't have enough output for beds generally.
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Yeah, the trouble with a DC/DC converter is that you need a lot of power for the heated bed. So it's going to be mains voltage -> DC24V -> DC12V. You'll end up heating a lot more parts than you need to. Plus, generally you can easily source pretty efficient low-power converters. But at the 10-ish amps your bed will consume… Not so sure.
As for the hassle... Really not that much. I've wired a double switch to control both PSUs from one button, and not running the bed from 24V one has let me keep power requirements for it way down. This way I could just pick up a genuine Meanwell 24V for the main PSU without breaking the bank.
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Thank you all for the great advice and insight, after looking around and frowning at my current build a lot i've finally decided to just downgrade the whole build back to 12v, it's really not ideal but running two power supplies is less ideal and all the dc-dc converters i found were awfully skimpy looking.
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I am running two supplies with this http://amzn.to/2yfb19c to send power from the largest PSU to the bed. Been working great. I bought two more and installed on another printer I have for poor manufacturing reasons.
