BANG_MAX equivalent

Hi,
whats the best way using a 12v hotendheater with 24v input an the duet
on ramps with Marlin i used the BANG_MAX Setting.Thx a lot
Christian 
I know this doesnt answer your question but I got better results with changing bed heater to PWM

I think the answer is it's not ideal to run a 12v heater with 50% duty cycle pwm at 24v.you can by setting S0.5 in M307 definition for that heater.
EDIT I'd go with David (see below) make that S0.25.
Or buy 24v heater cartridges.
Or get a buck converter 24v to 12v needs to be at least 5A capable. Wire the input positive to heater positive on duet, the input negative to the psu negative, then send the output to the heater cartridge and ground it via the heater negative on duet. This way you get pwm control

can i use
M301 H1 S0.50 ; Set heater 1 output scale factor to 50%
or is this something different? 
Like he said probably not a good idea, Thats why i said it does not answer your question. I was just letting you know that i get better results on PWM … but I am all 12v

A 12V heater run at 24V will produce four times its normal power, not two times. So you would need to use M307 S0.25 (not M301, that was for older firmware) to limit this power. But I don't advise it because of the meltdown risk if anything goes wrong.

Just to reiterate, don't risk the hotend turning into a puddle of molten plastic metal and flames..
But if I'm not mistaken, a PWM output is pretty well molded as a duty cycle controlled voltage source, so halfing the duty cycle, which halves the average voltage, actually reduces the average power by the factor of 4 required. Power has a quadratic relation with PWM duty cycle.

Halving the duty cycle doesn't
halvequarter the power.When the PWM is on, it is producing 4x rated power. When the PWM is off, 0x. If PWM is 50% on, that results in 2x rated power.
You'd need 25% PWM to be at rated power. Using a 12v heater on 24v is unsafe in the event of a controller failure.

I'm confused, you said halving the duty cycle doesn't halve the power, then said that when the PWM halves (from 100% to 50%) the power halves (4xPrated to 2xPrated)… I'm definitely not trying to be difficult I just think the math says otherwise.
Let's establish some rated conditions. At Vr=12v, the heater draws Ir=Vr/R where R is the (more or less) fixed resistance of the heater. The power Pr=Ir*Vr=(Vr^2)/R.
If I have a switch driven by Vin and pulse the switch with a 50% Duty Cycle, the average output voltage is 12v. Because we only care about the long term effects (relative to the PWM frequency) we can use the average value in our calculations.
So at 50% DC (duty cycle) with Vin=24v, Vpwm is DCVin=12v and because the heater resistance is broadly fixed, the heater Power is (VpwmVpwm)/R, which is also (DC^2*Vin^2)/R.
So doubling Vin means DC^2 needs to reduce by a factor of 4, which means the duty cycle is only reduced by a factor of 2.
Let me know if I got something wrong, but that's why I think you'd limit the PWM to 50%. The nonintuitive part comes from squaring both voltage and duty cycle in the power equation.

Sorry  I meant halving the duty cycle doesn't quarter the power (which is what you were stating).
You can't use the average value of the PWM'ed voltage because power varies quadratically not linearly with respect to voltage. Again, while the PWM is on the heater is producing 4x rated power, while when the heater is off it is producing 0x rated power. That results in 2x rated power on average.
Assuming a 12v 30w heater, I think you'd agree it would produce 120 watts at 24v. If it is at 24v half the time, then the average power must be 60w.
I am certain that a 12v 30w heater will produce 60w at 24v 50% PWM. The only way your equations work is if the PWM's output is low pass filtered to achieve a constant 12v.

Sorry  I meant halving the duty cycle doesn't quarter the power (which is what you were stating).
You can't use the average value of the PWM'ed voltage because power varies quadratically not linearly with respect to voltage. Again, while the PWM is on the heater is producing 4x rated power, while when the heater is off it is producing 0x rated power. That results in 2x rated power on average.
Assuming a 12v 30w heater, I think you'd agree it would produce 120 watts at 24v. If it is at 24v half the time, then the average power must be 60w.
I am certain that a 12v 30w heater will produce 60w at 24v 50% PWM. The only way your equations work is if the PWM's output is low pass filtered to achieve a constant 12v.
Are you and electrical engineer? Otherwise where did you learn this stuff i am very interested in learning

I'm a mechanical engineer. Most of my electrical engineering skills are self taught, though I did have a Circuits class in undergrad.