[Fermé] Heating bed with 24 volts?  

Re: Heating bed with 24 volts?

The Mk42 is a 12Volt only board.

It probably does have some overhead, but I very much doubt that would appreciate being fed with 20+ Amps of current!

Peter

Re: Heating bed with 24 volts?

I very much doubt that would appreciate being fed with 20+ Amps of current!
So do I.

David, common heatbeds have two options for connecting input power - 12V pads and 24V pads, they usually contain two circuits which are connected either in series (24V) or in parallel (12V). MK42 heatbed got only 12V pads and is designed for 12V input. With doubled voltage there will be quadruple power delivered to the heatbed, you will most likely severely overload the headbed.

I think it should be OK to put up to 15-18V on bed with extreme caution, but 24V ... is too much.

Re: Heating bed with 24 volts?

Not sure I follow your logic here.

The risk of higher voltage is subsequent higher power and possibly burning the resistor of the heatbed. However, since the heated bed is temperature controlled, that will not happen (except when the current is so high that the copper traces burn before the heat gets propagated).

The reason why heatbeds have two connections is that, if you would put 24V on a 12V bed, you would need a 24V power supply rated for 4 times the power rating of the 12V power supply or risk frying your power supply, so that would typically require a very beefy and expensive 1000W power supply.

It is, however, possible to limit the power to the heatbed in Marlin via a PWM/duty cycle setting, so if you would use that mechanism, you can see what the effect is of adding some power (starting with e.g. 30%).

Re: Heating bed with 24 volts?

The risk of higher voltage is subsequent higher power and possibly burning the resistor of the heatbed. However, since the heated bed is temperature controlled, that will not happen (except when the current is so high that the copper traces burn before the heat gets propagated).

The reason why heatbeds have two connections is that, if you would put 24V on a 12V bed, you would need a 24V power supply rated for 4 times the power rating of the 12V power supply or risk frying your power supply, so that would typically require a very beefy and expensive 1000W power supply.

It is, however, possible to limit the power to the heatbed in Marlin via a PWM/duty cycle setting, so if you would use that mechanism, you can see what the effect is of adding some power (starting with e.g. 30%).

Joris

The Mk42 heat bed is rated at 12V and has a nominal resistance of 1 Ohm. The copper tracks of the heated bed are the resistor.

Put 12V though and it will draw 12 Amps and generate 144W of heat. Put 24V through (even momentarily via PWM) and it will draw 24 Amps instantly and burn out - it is a resistor, not a capacitor nor inductor.

It is also very likely to burn out if you exceed 15V (maybe less). The reason why this heated bed has 2 connectors is one for 12V rail and one for 0V. There is not a third connector for a 24V rail.

And finally, as mentioned earlier, being a resistor, PWM will not reduce instantaneous current draw.

Peter

Re: Heating bed with 24 volts?

Peter,

Think about it ...

It takes a leisurely 10 minutes for the heated bed to reach a very modest 100 degrees.

And you are saying that quadrupling the power (which will still only reduce the heating time to 100 degrees to 2.5 minutes), will instantaneously (or at least in 125 ms (Marlin PWM period)) burn the copper tracks?

As I said before, the 24V connectors are there to limit the current demand to the power supply and NOT the current supply to the heated bed,but Marlin provides other ways of doing that.

Re: Heating bed with 24 volts?

Joris

Not much point in me trying to explain further if you are simply not reading what I write, so I will just suggest that you try applying 24V to the bed heater and let me know the results.

Peter

Re: Heating bed with 24 volts?

Peter,

I don't know what you base your comment that I'm not reading on, it's just that I don't agree with your conclusion. Apart from 'it won't work' and a few calculations, based on Ohm's law, I have not seen any scientific arguments (e.g. the amount of energy to be dissipated).

But hey, obviously your ego can't handle the discussion, so it won't happen again.

On your second comment: I do have a set-up like this (on a different printer, not a mk2) and it does work fine. Thanks for asking. And I do have a degree in electronics engineering, but I'm sure you have superior qualifications to that.

Signing off from this forum.

RE: Heating bed with 24 volts?

For those wondering as I was, your bed won't burn up instantly, even at full current, I tried it, but read on... Resistors and inductors, like heated beds, are rated for constant current/power, not PWM or pulsed current. So if you PWM your bed, you are effectively lowering the power, in this sense, Watts dissipated as heat through the bed.

HOWEVER, each pulse still does actually draw twice the current and thus 4 times the power per pulse from your PSU, which, if you're lucky, will just brown out and reboot your control board, and at the very least, cause your steppers to pulse and act erratically, unless you take steps... a GIANT bypass capacitor(+10mF) for example (I tried this too)... While the power MOSFET switching your bed can likely handle this (the FET on my SKR 1.3 is rated 220A), the traces on your board certainly can't handle that, so I wouldn't recommend anyone try this at home.  Worst case is, at full current, your board/PSU doesn't brown out, and you burn out your bed and/or board before the thermistor can register the temp change. 

What I WOULD recommend is simply getting a 24V-12V, 20A DC-DC converter from Amazon for $18, and an external heat bed MOSFET too for $5. This will get you going smoothly and without any risk of damage. This is tested and is working great. Happy printing!