12->24V conversion, some questions and sanity check



  • Hi all. As I plan my conversion from 12V to 24V I'm looking at this power supply:
    https://www.amazon.com/gp/product/B005T6NNKO/ref=ox_sc_act_title_1?smid=A1T2NZWYM1SZ9A&psc=1

    It's a 450W 24V, which means 18.8 amps max. My current 12V Meanwell PSU is rated for 350 W and has been fine so far, but the heated bed I want to use is power hungry, and for a reason.

    The heated bed I was looking at was this 350 W, 24V silicone pad from Keenovo:
    https://www.amazon.com/approx-thermistor-KEENOVO-Silicone-Printer/dp/B073SZ3SFB/ref=sr_1_1?s=electronics&ie=UTF8&qid=1500948887&sr=8-1&keywords=200x300mm%2B24V%2Bheated%2Bbed%2Bkeenovo&th=1

    Here's my problem. Doing the math, the 350W pad would use 14.58 amps. I just ordered some 40W 24V E3D heater cartridges. That's another 1.67 amps. We're already up over 16.25 amps. My steppers are currently set to around 1200mA (the extruder's a little lower), and there are five in total (2 for X/Y, 1 for Z, 2 extruders. That's over 6 amps required right there. This PSU couldn't support it. This also doesn't even include whatever power is required for the fans, the short LED strip I use, and the Duet Wifi electronics itself.

    If I went for the 300W version of that pad, the numbers would be:
    pad: 12.5 amps
    heater cartridge: 1.67 amps
    steppers: approximately 6.2 amps.

    Again, I'm still over budget by a good amp and a half, not including the Duet Wifi electronics, fans, and LEDs.

    My current mk2 heater PCB must be weak is crap if my printer is currently running fine with a 350 W power supply, and the 450 W supply I want to use at 24V simply can't do it with even the weaker of my 24V silicone heater options.

    As a check here, am I figuring this right?

    I want to avoid going with an A/C-powered bed heater. I've been trying to keep my electronics simple, so for example I've been running my heated bed straight off the Duet Wifi's internal bed heater mosfet. The Duet Wifi is fully capable of the voltage and supporting the current requirements of my selected heater pad, but it's dawning on me that even a 450 W psu simply isn't enough.

    Since I'm not really all that experienced in electronics other than some simple hobby stuff I've done in the past, can someone confirm or deny that I'm thinking about this correctly?

    Going with even higher wattage PSU will get spendy if I insist on going with good quality. I'm worried it'll also get big, and that might require that some things be relocated that I'd rather not have to deal with. I also don't really want to go with a computer PSU, though I suppose I might end up having to.



  • Oops, my power requirements calculations are off too low by another 1.67 amps because I forgot to include two 40W heater cartridges, not just one.



  • The next step up from the 450W seems to be a 600W, which could support 25 amps. The 300W pad, five steppers at 1.2A apiece, and two 40W heater cartridges adds up to 21.8 amps. I'll assume the short LED strip, four fans, and the Duet Wifi electronics themselves won't add up to the remaining 3 amps. So, the 600W psu could do it.

    This Meanwell 600W 24V psu can be had off Amazon for $100 with no shipping, or $76 plus some shipping from a place like Mouser.
    https://www.amazon.com/gp/product/B00DEDB2U2/ref=s9_dcacsd_dcoop_bw_c_x_3_w

    This conversion is starting to get really spendy. Is it really worth spending this kind of coin just to avoid using an A/C powered bed? The bed itself would be a similar cost to the one I'm looking at, and I could just keep my present 12V power supply. The additional SSR and whatever other circuitry I'd need to go with A/C safely would surely add up to far less than an addition $100 for a new PSU, plus the cost of replacing my fans.

    I need a new heater one way or another, to install under the Mega Slab of Doom 1/2" MIC6 aluminum plate I bought to use as the build plate. My existing mk2 PCB takes forever to get up to 105 C and won't even go over 106 or so during printing, so with the Mega Slab I'll definitely need more power. It's either expensive 24V conversion or going to A/C, and I'd just rather have the relative safety of 24V versus creating what is in fact a controlled short circuit of my home's 115V A/C system.

    Anyhow, I'm open to opinions.



  • Those are peak amps pulled that is if you are starting the heated bed, both hotends and running all steppers at the exact same moment. Sustaining the temps should take less. I currently run one 350W heated bed with a single 40W heater and steppers at .8, 3 fans and a 7in on a 500W 24V and it works great. If Im wrong please correct me.

    https://www.amazon.com/gp/product/B00UWCD22O/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1



  • Yeah that sounds about right. Once I get the second extruder running and am actively doing dual extrusion, I'll need to have a psu that's able to sustain that second stepper and heater. I also run my steppers a little hotter than you do, and that adds up a surprising amount of power.

    I'm glad to hear you're able to run the 350 W heated bed though. I may go with the 600W psu and the 350 W bed and then hope that this works well. While heating up the heated bed the steppers won't be running, and hopefully if I insulate my new Mega Slab print bed well enough it won't draw max power for long during sustaining heats.

    The guys with the 600W A/C heated beds report hitting 100C in like a couple of minutes or so, so hopefully a 350 W bed will take just a few minutes longer. That will be infinitely better than my current mk2 PCB, which takes something like 20 minutes to hit that temperature, and then only if I lay a cork board on top of the bed while it's heating up.



  • You can never go wrong by oversizing the PSU. Less fan noise and cooler electronics. As always, if budget allows go for the big one.

    But I do not see why an AC heater and SSR is much of an complication. Pretty simple wiring and you should know how to handle higher AC voltage. But if you look at it another way. It is more important to be careful when making connections where there is high currents. High currents needs thicker cables and more solid connectors. Using soft wires with many thin strands it's important to use crimping ferrules in any screw terminal.

    I will be replacing my 24V PCB-heater one day with an 230V silicone pad. Just so I do not have to buy a new PSU and draw high currents through the Duet.



  • +1 for mains heater. Seems scary and unecessary at first but if you wanted to make some toast you wouldn't use a 12/24v DC toaster, or kettle to boil water, mains AC is excellent for heaters, the wiring and switching is tiny by comparison as the current is very low (2-3A at 240v). you just need to ensure you follow basic safety that you already do when handling the AC side of your PSU.

    Have you considered dual DC PSU? Run one PSU for everything except the bed here you only need around 150-200w, so laptop charger type PSU's are often very suitable, and cheap. Then run one larger PSU for the bed ie your 450w unit you mention above. Consider an external mosfet if your bed is quite large and running off DC. It is never a bad thing to take the bed current off the Duet board. Whilst its rated for 18 amps, its £150, and an external mosfet is £5, so as a means of ensuring you don't melt your duet I'd consider it money well spent.


  • administrators

    The steppers don't draw anything like their full rated current from 24V. See https://duet3d.com/wiki/Choosing_the_power_supply#Total_power_needed. If you choose the 300W heater then the 450W PSU may be sufficient.

    How large is the bed? If it's 20x30cm the same as the heater, then 240W would be sufficient IMO (0.4W/cm^2).



  • Thanks for the replies everyone. I'm not afraid of A/C power per se; I just have a healthy respect for it. I'm no electrician or pro electronics guy, but I've used A/C on stuff before, like the tube (valve for you guys over the pond) guitar amplifier I built from scratch. A 24V mishap probably wouldn't kill me, but screwing up and shocking myself with 115V A/C could. Yes, there are safety precautions that can be taken, but if my needs can be satisfied with 24V DC instead I consider that the most effective safety precaution there is. Any safety precaution I install has to rely on me having done it correctly, me having correctly identified all the precautions necessary, and the precautions themselves not failing over time.

    My current bed is nominally 200x300, but it's actually a little larger than that. I bought a 9x14" MIC6 aluminum plate that's 1/2" thick to use as my new Mega Slab heat bed. That's 229x356mm. I probably won't be able to print over that whole surface but I'm hoping to get around 220x340 or so print area, depending on how well I do at "slimming out" my print carriage. With this much metal it's going to soak up a lot of heat coming up to temperature.

    Given the 600W power supply I can get from Mouser for only about $20 or $25 more than the 450W one, I'm thinking I'll go with that 600W Meanwell and the 350 W heater pad. It's good to know that the steppers don't pull their full rated current at 24V.

    I'll double check my numbers once more before buying the PSU and heater pad, but it looks like this is the way to go. I've already ordered some 24V fans. Arg, I need to order some 24V LED strips now too to replace my 12V ones.

    I appreciate the comments and the suggestions everyone. Thanks!



  • You could just double up on those 12V LEDs and wire them in series… Don't know if thats doable but...



  • My 2 cents/bullet points:

    • While I'm very pro-24V for stepper motors, there is a much wider range of fan (and LED, etc.) choices at 12V. So I have a fanless 24V supply for my steppers and hotend heaters, and use an inexpensive switching regulator (I used one of these: https://www.aliexpress.com/item/LM2596-LM2596S-ADJ-Power-supply-module-DC-DC-Step-down-module-5V-12V-24V-adjustable-Voltage/32271465162.html) to supply 12V for fans and any other 12V accessories (Duet Wifi conveniently and simply supports a different voltage for fans).

    • While I'm also a proponent of using mains power for bed heating for the same reasons DjDemonD listed, if you insist on 24V then a separate supply for the bed isn't a bad idea (apparently I'm just copying DjDemonD's notes for this bullet). While the average power usage will be much lower than bed+hotend1+hotend2+peak stepper current, etc., you still have to size your PSU for the peak, because these switching power supplies don't have much energy storage to handle transients above their rating.

    That toaster analogy is a good one, DjDemonD - the inefficiency of converting hundreds of watts of mains power to 24V to warm up a resistor makes the efficiency expert in me cringe! 🙂


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