Mosfet board vs SSR vs onboard

  • Oh, I hadn't seen that document. That really helps a lot.

    I just whipped out my multimeter, and it's showing 1.3 Ohm for the heatbed. That means I'd see at most 9.2A, but in practice, with voltage drop through the onboard mosfets and resistance from the wiring and whatnot, it'll actually be lower than that.

    Makes me wonder whether I'll be satisfied with the hotbed performance. I guess I'll find out, and if I'm not satisfied with it, I could always look for an improvement in the future by making a new heatbed using one of those silicone heat pads and mains power.

    I already have one of the cheap FOTEK-labeled SSRs that don't cost very much on Amazon, so I'll have to re-read that document and make a decision, but I could just as easily power my heatbed straight off the Duet Wifi itself. The primary reason I might still use the SSR would be just to offload some of the power demand from the board, or to isolate the Duet Wifi from the heatbed as a protective measure (the Duet Wifi being the single most expensive component of my D-bot build by far).

  • Oh, well this seals it. From the document you linked earlier, down at the bottom:

    Low voltage (12V or 24V) DC bed heater
    Use a low voltage drop DC/DC SSR such as the Auber Instruments MGR-1DD80D100 or Crydom DC100D40. The SSR may need a heatsink, depending on the current. Do not be tempted to use a cheap DC-DC SSR such as the SSR-40DD, which is basically useless for this application because of its high voltage drop.

    The cheap FOTEK-labeled SSR I have is in fact an SSR-40 DD.

  • That cheap ssr will barely let a 12vdc MK2a get above 70C.

    Now, if you go with an AC powered heater, then a DC-AC SSR will have no problems and the heat up time will be short. Mine does the full heat up of bed & hotend less than a few minutes. Of course, you do need to install a thermal fuse on the bed heater for the safety bit.

  • @T3P3Tony:

    If it is a conventional MK2a (ie the one I adapted from Prusa's original design years ago), with a resistance of 1Ohm then at 12V it will draw 12A which is fine for directly being driven from the Duet.

    That is assuming your power supply can happly provide 12A for the bed+the power for your heaters, fans etc. It may be that D-Bot power specifications work on the idea of having two power supplies one for the bed and one for the heater, in which case look at their capabilities.

    If the heated bed you have is not 1A, specifically if it is going to draw more than 15A then you should use the DC SSR (but make sure you follow the guidance here:

    Ive noticed this topic come up a few times, and the documentation referenced does not mention anything about Mosfet expansion boards like the MKS-Mos or BIQU boards ( .

    Can you comment on why this would not be an alternative to the SSR listed in the Duet3D Documentation?

  • I do not have my Wifi board yet (Also new member, Hello 🙂 ) but I already have a cheaper bed heater mosfet that I will be using.

    I also have a case fan blowing over it and should provide plenty of cooling to operate at any current draw my bed will take. Small price to pay IMO before hearing your mosfet on the duet fall off.

    I will say even without the fan on it, it stays very cool to bairly even warm in my current setup.

  • administrators


    Ive noticed this topic come up a few times, and the documentation referenced does not mention anything about Mosfet expansion boards like the MKS-Mos or BIQU boards ( .

    Can you comment on why this would not be an alternative to the SSR listed in the Duet3D Documentation?

    That listing gives no specification for that board, nor a schematic for it. Without one or the other, I can't assess its suitability.

  • Found this:
    After following the url on the circuit board.

    The only spec is 25A max and only mentions 12vdc in determining the load.

  • Some real talk here: with a 12VDC power supply, and a MK2A pcb heated bed (300x200 size) measuring in a 1.3 Ohms resistance, Ohm's Law (V=IR, or I = V/R) tells us that the maximum amperage you're going to be able to draw, excluding any voltage drop through an SSR or mosfet, or any additional resistance of the wiring between the power supply and the heated bed, is around 9.2 A.

    If the Duet Wifi is rated for 15 A safely, then the 8-9A we'll see in practice looks like it's perfectly fine for the Duet Wifi to support.

    As I mentioned above, I might still be interested in an external mosfet board just to offload that power draw from the Duet Wifi itself, just as a protective measure. I'll tell you what, though, I've been running a Duet Wifi on my Maker Select for several months now with nearly constant usage, and it's powering both the nozzle and heated bed, and it's been going like a champ.

    I'm completely satisfied that hooking up the heated bed directly to the heated bed outputs on the Duet Wifi is a reasonable and successfull combination. The only remaining question is whether offloading that power load from the Duet Wifi to an external board is going to gain me any safety or protection of the Duet Wifi to justify the expense.

  • Just for context, there's a reason those external mosfet boards are all the rage right now, and that reason is that the really cheap Melzi boards the Wanhao Di3 & clones have been shipping with have this nasty tendency to burn up their power connectors (this may be a problem with other cheap printers too, but the Di3 line is the one I'm familiar with, and it's a huge deal), because they used really crappy components that simply weren't rated for the load that the printers place on them in practice. That's why people are buying these things up: if they don't, they risk a meltdown on their controller board.

    That doesn't seem to be an issue with the Duet Wifi.

  • administrators

    So you are right that any high current will stress any electronics. We had thermal testing conducted on the prototype boards which the results are written up here :

    That is where we get the 15A limit from. You will be interested to see the limit is due to the board copper traces rather than the connectors (which are rated to 35A).

    We have commissioned further testing to continue to check on our production boards so will publish an update. We may be able to raise the stepper driver software limit from 2A to 2.5A but let's see how the testing goes. We are also looking at the bed traces.

  • Why do the external mosfet boards and stepper driver boards and whatnot have these massive heatsinks on them, and yet the controllers and chips on the Duet Wifi board don't have any heatsinks, and do just fine?

    Is it a quality thing, where the controllers and mosfets and whatnot used by the Duet Wifi just aren't dissipating that much heat?

  • administrators

    Its not one single factor, for example some of the external boards can handle a lot more current than the DuetWifi.

    Stepper drivers and FETs come with a wide variety of specifications, including on resistance and switching time these effect how much power is dissipated while on, or while turning on/off. There are important interrelating factors - you can get a FET that turns on/off very very quickly (so dissipates less power in the process) but then that very quick on/off switch generates high frequency EMI.

    You will see from the thermal testing that we have done we know how much current the drivers and FETs (and other components) can take before they reach certain temperatures. We also use a thick copper PCB to help transfer the heat away from the FETs/drivers - the PCB is acting as a heatsink. That's why we recommend a fan blowing on the back of the board for cooling if you are using the drivers/FETs at the top of their power range.

  • I'm getting some heatsink adhesive for a different purpose. Would it be worth it to buy some of those tiny copper heatsinks that people put on ram chips on video cards or whatever and gluing them on to the stepper drivers and the mosfet?

  • administrators


    I'm getting some heatsink adhesive for a different purpose. Would it be worth it to buy some of those tiny copper heatsinks that people put on ram chips on video cards or whatever and gluing them on to the stepper drivers and the mosfet?

    Modern surface mount chips have thermal pads underneath them so that heat is transferred to the PCB. The top of the chip is made of plastic and doesn't conduct heat well. So if you are running at high currents close to the limits of the board, use a fan to blow cool air over both sides of the board. Putting heatsinks on top of the chips might actually make things worse if they restrict the flow of air over the PCB.

  • Thanks! As we speak I'm working through either a new mount box or a remix of an existing mount for my D-bot. There's one box I like a lot that has good fan provisions, and another box I like better except that the airflow/fan considerations aren't nearly as good. I may have to roll my own and try to have the best of both worlds.

  • My mount box is here which takes an internal fan blowing on to the back of the board or this one which takes both the duet wifi and the expansion board. I included the OpenScad files so you can hack then about to suit. I stuck thermistors on one of the stepper chips on each board and have the fans set thermostatically to come on at 45deg C which makes it silent when idle. HTH

  • I hope this is the right place for this. I currently have one of the bigger mosfets delivering power to my heated bed controlled by a Duet Wifi. The bed heats great and everything works well. But I found when the heater cycles it does something that causes my extruder to run slower (?) I get a fairly regular thick and then thin extrusion pattern. When I turn off the heated bed, the effect goes away. Is there something I can fix here? I have verified all the wiring and connections.

  • Voltage drop? Running M122 will show if that is the case or not.

  • administrators

    I agree, it's probably caused by the voltage at the VIN terminals of the Duet dropping when the bed heater cycles. Some suggestions:

    1. Make sure that the screws in the power input terminal block on the Duet are tight.

    2. Use a multimeter to measure the voltage at the power supply output terminals with the bed heater both on and off, and also the voltage at the VIN terminal block. If the voltage at the PSU output drops significantly, get a better power supply (cheap ATX power supplies are not very good in this respect). If the voltage at the PSU output remains stable but the voltage at the VIN terminal block drops significantly more, use shorter and/or thicker wires between the PSU and the VIN terminal block.

    If you don't have a multimeter, then as Ian mentioned you can use M122 to read the voltage at the VIN terminal block.

    3. Switch the bed heater to PID mode.

    You've just given me an idea: I could adjust the extruder PWM automatically to compensate for VIN voltage changes.

  • I have the power supply at the base of the tool and the Duet in the top (converted Rostock Max V2 to V3) I measured the voltage at a terminal strip near the Duet (12 in wire between Duet and terminal strip) during bed heating and I did see a variation 12.01 to 12.05. The heater is in bang bang mode.
    I have a meanwell 350W supply
    From M122:
    Supply voltage: min 11.8, current 12.1, max 12.2, under voltage events: 0, over voltage events: 0
    === Heat ===
    Bed heater = 0, chamber heater = -1
    Heater 0 is on, I-accum = 0.0
    Heater 1 is on, I-accum = 0.4

    I have 12 gauge wire from the PS to the terminal block and to the Duet

    With the heater just turned on voltage at the terminal strip, Vin and Bed out all read the same at 12.05
    I have 12.66 amps running to the bed through the Mosfet when it is on. There does not seem to be any current running on the control lines from the Duet to the Mosfet. I have 22 gauge wire for the control wires.


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