Power supply

  • hiya,
    I am planning to run two different power supplies, the first is the original one that came with the tronxy p802ma, the other is one from ooznest i.e. the 24V 350W Power Supply, the one from ooznest would be used for most of the hardware including any extruders, the other power supply would be dedicated towards the heatbed through a mossfet.

    The question i have is, when i connect the two at the same time both supplies work without an issue, however with the extruder drawing power from ooznest psu, the extruder starts failing or getting a fualt. please help, as the when everything was connected to the original psu it was all good, the amps of the ooznest psu is 14amps maxed and the original as acording to the ebay page is 40amps

    along the same lines, the temperature after a print continues to decrease, and takes a long time to before it builds back up for the next print.

    Please Help,

    Kind Regards,

  • administrators

    Please provide a diagram showing how you have connected the power supplies to the Duet and the other components.

  • ![](<a href=)" />

    Here's a diagram of how i am connecting it so far, the heat bed heats up without an issue but when the extruder heats up, it starts faulting once it reached the required temp

  • administrators

    What do the boxes labelled M represent?

  • From his description above, I'd imagine M = mosfet. H = heated bed.

  • Sorry i forgot to provide a key,
    DuetWifi(that awesome creation)

    The mosfet is this type:

  • I get why you have the mosfet between the second PSU and the bed I have the same arrangement. Its nice to keep the bed current off your controller board and it enables you to utilise your second PSU. Those mosfet boards work really well, they barely get hot and can handle big currents.

    However I would swap the PSU's over and use the larger one for the bed. Unless you plan to have 8 stepper motors and 4 extruders, you wont draw more than 150w for 3 axis motors+1 extruder motor, 1 hotend heater, so the 200 watt PSU would be more suitable (and sounds like the more reliable unit).

    The 350w supply from oozenest sounds suspect. Do you have an ammeter to measure its output current? If its a chinese LED brick style unit they often don't put out the power they claim. When purchased from a chinese vendor for not a lot you cant really complain but oozenest are a reputable UK company charging more for it than the chinese guys they bought it from, if it doesnt actually produce 350w send it back.

    Is the mosfet between the 350w PSU and Duet to enable you to have PS_ON and PS_OFF functionality? Are you planning to have supplementary 5v power to the duet (via USB/USB wall adaptor?) without it you wont be able to turn your PSU ON.

  • Heres the PSU

  • Hiya
    @DjDemonD, thnks for your response, everything is useful up to the point where you describe the PS_ON/PS_OFF options; Would the PSU's 14amps be letting it down., what if i run the everything of the 350w PSU? instead,
    When I did run the 350w psu the hotend heated up really fast then fualted since it went out of range, if theres no other issues with this PSU would it be powerfull enough to run everything including an eventual bigger heatbed/ 24v heatbed currently 220*220 12V
    Sadly with ooznest I would need to pay for return package, so this option is definetely a no go.
    How would i get this to work "Is the mosfet between the 350w PSU and Duet to enable you to have PS_ON and PS_OFF functionality? Are you planning to have supplementary 5v power to the duet (via USB/USB wall adaptor?) without it you wont be able to turn your PSU ON."
    Should I just decrease the 350w PSU by the dial of the PSU

    Kind Regards

  • When buying a PSU whats more important Amps or the Wattage or Volts, excuse my arrogance as I am only used to buying PC PSU which are in laimens terms 'out of the box ready to use'

  • Think that the thing missing here is the Voltage of the 200W Psu I am guessing it is 12V and the 350 is 24 V also you won't need the mosfet for the extruder the inbuilt one on the DUET is more than adequate for that role. I suspect that the mosfet that you have for the extruder may well be the problem here. You are better of with the higher capacity one for the heatbed as everyone has said however the Duet will be better run of 24V.

    I also suspect that the issue you have with the extruder may be that it is probably a 12V Heater in it and running it at 24V can be dangerous (Instead of being 40W for example it would now be 160W)

    Your question about which is more important Amps/Watts/Volts they all interact with each other so you can't treat them in isolation.

    Formula you need for this is V=IxR Pwr is P=IxV where I= Amps V=Volts P=Watts and R=Resistance



  • this would be the PSU that came with the Printer,

    Once i get the original E3d V6 Hotend, the version i am planning to get is the 24v version, would the 12v PSU work well with this.

    Or would I need to get a 12v Heater Cartridge

    Worse case scenerio I'll use my my old cartridge which is 12V

  • IMHO You would be better converting it all to 24V and the 350W 24V psu may well be enough depending on what you Heatbed resistance across the 24V Connectors is?

  • i run 1 power supply (12v pc rated 600watt) but have 115vac 750 heated bed and SSR. a lot less wiring to do and crazy powerful. bed heats to 70c in less than a minute. you can tweek some power supplies to output out to 14vdc (be careful!!!) for a little more power but with load from bed on AC, DC power supply can more than handle everything else without hitting limits of board or power supply. If you have really big steppers maybe 24 vdc for more power and or speed might be good (FT5 solid aluminum build plate\ dual z motors in series on lead screws at 1.5 amps appears to be plenty) working on z mod to reduce to 1 motor now… and yes, 24 volts as mentioned above is always best, 1 day...

  • So I can't mix and match on voltage


    Thnks for all the feedback fellow prusa's

    would this PSU work well for the heatbed?


    Now ideally, Would two PSU allow the heatbed and the hotend to heat up quickly?
    @CaLviNx Thnks for that I will implement that and leave the second mosfet as a spare

  • One of my machines is a 12v for the duet and 24v for the heatbed setup. To be honest we both would be better off using 24v for everything (so buy 24v heater cartridge for the hotend and 24v fans) as mentioned as motors generally run better at 24v (theres more to it than this but its a reasonable rule of thumb), you dont need to buy 24v motors they're very tolerant of a wide range of voltages. I only stuck with 12v PSU as I have loads of 12v fans and heaters and would like to use them, when they're gone Ill switch my 12v PSU for a 24v one and get the right heaters/fans..

    What power is your heated bed capable of? To find out measure the resistance across the main power terminals of the bed, either the 24v terminals if you plan to use 24v or the 12v terminals if you plan to use 12v (PCB heaters have two sets of connectors, if your bed heater only has one set of connectors then it is designed for one specific voltage only eg 12v or 24v but not either, although you can use slightly higher voltage like 10-20% but not double the voltage by running a 12v bed on 24v - see footnote below though.

    Once you have the resistance use this formula Power (watts) = Voltage 2/resistance(ohms) typical values for 12 beds designed to be 200w are around 0.7ohms as a rough guide. The power of your bed determines the PSU you are going to use. If its 200W then you need at least a 200w PSU running it if using a dedicated unit, although having some overhead i.e. 250w or 300w is never a bad idea - see footnote below. If you use one PSU for everything then abnk on 150w for motors/fans/hot end and whatever your bed power has been calculated to be plus some overhead.

    Footnote, I have a 300x200mm PCB heater, it has the 12v and 24v connectors, when I first wired it up using its own 24v psu, and wired it "correctly" to the 24v connectors, it barely got warm, its resistance was too high something like 4ohms, which works out at 144w not enough for a bed this size. So I wired the 24v PSU to the 12v connectors - what! I hear them all cry - that's dangerous, but it isn't in my case, as my 24v psu was only 250w, so when the bed is on it maxes out the 250w psu but cannot go any higher as the PSU can't give it anymore, as such it heats up very nicely and within 4-5minutes. The PSU got a little warm too, so I wired a 24v fan to the spare terminals and plonked it on top of the psu near to the heatsink where the mosfets are at one end. Problem solved, been working like that for over a year, probably more. If you're worried get a thermal fuse rated around 150 deg C and wire it in series (in line) with the power to the bed and tape it, or situate it directly in contact with the bed heater or aluminium plate above it or whatever, if anything goes wrong it blows permanently and the bed cools down.

    As for PS ON and PS OFF, sorry to confuse I just couldnt work out why you're using a mosfet for the hotend as it only draws 5A max which is small fry. If using a PC power supply (or xbox PSU on smaller machines) they switch on their 12v rails when given a signal to do so. But the 5v rail stays on. So you can power just the duet board's electronics (not heaters) from this 5v rail, and the duet can switch a mosfet/relay/SSR whatever to activate the power supply by including M80 in your start gcode, and M81 to turn it off after printing (or use it for emergency stop, or whatever) its a nice extra safety feature but one that isn't in use that much. I have it on one machine which uses an xbox PSU.

  • how would i converted the bed to 24v ?
    or would that hictop do the job

  • The bed is just a length of wire, whether it is a pcb, a silicone heater or a kapton heater. The wire has resistance, as the current flows it encounters this resistance and generates heat. If the bed resistance is 1 ohm then at 12v you get 144w of heat. If you stick 24v through it instead it generates 576 watts! These assume your psu is ample for the job. Use this calculator to see how these values relate http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm

    So you can see that a heater designed for 12v has a resistance that gives the power output it is designed for at 12v. At much higher voltage you can be taking big risks with fire etc. I got away with it by limiting the current supplied by undersizing the psu.

    Measure your bed's resistance it's important. If it only has one set of connections (a pcb heater often has 2 sets so you can switch it to 24v from 12v) then you are limited to that resistance and therefore using the calculator it should be obvious if its wattage sounds really high. If you're not sure post the values you measure and well make sure you don't burn your house down (or melt your printer).


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