Quote
NoobMan
// If your bed has low resistance e.g. .6 ohm and throws the fuse you can duty cycle it to reduce the
// average current. The value should be an integer and the heat bed will be turned on for 1 interval of
// HEATER_BED_DUTY_CYCLE_DIVIDER intervals.
//#define HEATER_BED_DUTY_CYCLE_DIVIDER 4
Dunno how this is computed but looks like something i would stay away from, instead why not use a use higher rating fuse or a slow-blow fuse, i am not sure i understand the motivation of this.
Well, a little of google triggered this:
http://forums.reprap.org/read.php?1,147557,147557
Which basically explains that the variable HEATER_BED_DUTY_CYCLE_DIVIDER is a tweak made by Dabble and, in his own words:
You are correct that it is a software PWM, it is crude but appears to be effective. The rate seems to be very slow.
The logic is that bed_is_on is incremented from 0 to HEATER_BED_DUTY_CYCLE_DIVIDER repeatedly. When bed_is_on is 0 i.e. for 1 out of HEATER_BED_DUTY_CYCLE_DIVIDER times, the heat bed is turned on.
I found that a value of 4 i.e. turn on for 25% of the time, worked for me.
Anyway, I also had it deactivated...
In my case the most likely origin of failure were the PSU vs MOSFET specifications. I had the "official" P55NF06L:
[attachment 28267 MOSFET.png]
From the curve one can see that, at the 16A and 12V supplied by my PSU, it is out of the RDS safe limits for periods larger than 10ms (or frequencies of 100Hz)...Nevertheless, this MOSFET is supposed to work fine according to some other sources:
http://www.soliforum.com/topic/2080/magic-smoke-on-ramps/
Still, I would choose a IRL3803 (lower chart, to the right):
[attachment 28269 IRL3803.png]
Gives more confidence to me...with 12V it can still manage 30A...although is about 1€ more expensive..