12V bed running from 24V PSU

  • Hi All,

    I'm in the process of upgrading from 12V to 24V.
    The heated bed (260mm dia) is 12V only and shows 0.8 Ohm resistance => 180W
    If I were to power this from 24V it would produce 30A current and 720W which would clearly not work.

    So, Is it possible to use PWM control to limit the power for example:
    M307 H0 B0 A120 F50 S0.25 ;Set BED to PWM, 120Deg, 50Hz, max 25% duty cycle

    Or the 30A peak current would damage my duet wifi either way?

    Also if this could work, which pwm frequency would you recommend to not harm either the bord or the meanwell psu?

  • I would upgrade to the right bed heater. Is it worth taking a chance wrecking a $160 duet board?

  • administrators

    I agree, running a 12V bed heater from 24V is dangerous. If the existing bed heater is not dual voltage, get a new one.

  • Okay, thanks, I'll do that

  • The bed should have a problem with that. But I full agree that the board and the PSU could take damage even with PWM limit. PWM only limits the time when the heater is running, but since a heater have a really big inertia the power output from a heater gets averages.

    Short story: get a dual voltage bed unless you have an powerful external mosfet/DC SSR and a ~1kW PSU.

  • I just wanted to spare the ~1 month wait until a new bed arrives, I could not find any round 24V heated beds in my country.

    My initial tought process was the following:

    From the viewpoint of the bed:
    It has a large thermal mass, so if I send pwm with high enough freauency (probably anything above 5hz would suffice) then it should not have a problem with it.

    From the viewpoint of the PSU:
    It was not rated anywhere close to this power, so I should use small impulses to steal small amounts of energy from the output capacitors, so it would represent a tolerable load, however if I use frequencues too high I might start some rezonance effect somewhere.

    From the viewpoint of the panel:
    The switching element generates most heat during switching on or off, because that is the point where both current and voltage are present, so I should use a relatively low the pwm freq, which is high enough to not fluctuate the switching element temperature.

    It does not metter now, I threw the idea away because of the previous advices, so I'll just have to wait.

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