Heatbed questions

  • Hello!

    So for the life of me I cannot find the power information for the Prusa MK52 24V heatbed. I've created a clone of sorts but using a Duet WiFi. I will be testing the Duet Maestro later as well.

    I know the Duet WiFi supports up to 18A. My MK52 seems to heat up slowly compared to my Prusa i3 MK2.5 that's only on a 12V PSU. If I use an SSR (like this one https://www.auberins.com/index.php?main_page=product_info&cPath=2_30&products_id=674) will it heat up any faster? How many amps will the heatbed draw? I only ask because I don't know what is limiting the current (either the heatbed or the Duet WiFi.) If the bed is connected to the SSR, will it draw more than 18A to heat up quicker?

    Basically I'm just trying to see if it's even worth hooking up an SSR to my heatbed. What I find is odd is that the SSR I linked shows running the ground wire from the PSU to the SSR, then from the SSR to the load (heatbed), and then positive wire from the PSU to the heatbed and the VCC terminal on the SSR; I guess it's just because of this type of SSR.

    If it is more beneficial to use that SSR, I'm hoping to put it in the same enclosure as my Duet WiFi. I'm not sure how much heat it's going to produce, so maybe an external fan blowing into the case directly over the SSR would help.

    Thanks for the help everyone!

  • measure the resistance of the heated bed then calculate the power using
    P = V*V/R

  • Did you measure the resistance of your heatbed? If so, you can estimate its current as 24 divided by resistance.

    If it's conveniently within the Duet's spec, you can skip the SSR (it will not speed up heating time).

  • @zapta said in Heatbed questions:

    ll not speed up heating time

    Gotcha. So, I apologize, but I'm not used to measuring resistance. Do I just run my multimeter leads on the + and - terminals of the heatbed while it's at a certain temp? (setting it to Ohms)

  • Yeah, measure when bed is cold, that is the worst case scenario afaik. Disconnect (at least) one lead from the board to ensure the circuit doesn't affect your measurement.

  • @bearer thank you. I'll try that soon. Currently printing and trying to tune the stepper motors (with no luck ...posted another topic for that one.)

  • @ringo1508 said in Heatbed questions:

    Do I just run my multimeter leads on the + and - terminals of the heatbed while it's at a certain temp? (setting it to Ohms)

    Yes. I don't know what multi meter you have but also verify that the multi-meter is on the lowest ohms range (e.g. showing fractions of ohms) and that when you short the two leads, without the heated bed, you get 0.0.

  • @zapta thank you. It's a Southwire 10030S multimeter. The lowest I can select is 200 Ohm on the multimeter. Shorting the leads shows 0.1 Ohms. Disconnecting the positive and negative leads from the Duet WiFi heatbed input and measuring the resistance, I show 4.0 Ohm.

    So 4.0 Ohm - 0.1 Ohm (from when I tested the leads) = 3.9 Ohm. Then I take 24V / 3.9 Ohm nets me an estimate current of 6.2A (rounded up to the tenths.)

    This means that the bed heater is roughly 148.8W.

    So no need for the SSR, correct?

    Thanks again (hopefully others find this when searching the forums so that others don't have to ask the same questions. 🙂 )

  • yes 6.2A is no problem for the duet.

  • @veti said in Heatbed questions:

    yes 6.2A is no problem for the duet.

    +1. I run about twice that current with no SSR and works just fine.

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