Duet 3 Mini 5+ OUT0 practical current limit
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Hello,
I am in the process of implementing final touches to my heavily modified CR-10S Pro running on Duet 3 Mini 5+. One of the remaining issues is the bed heater PWM frequency that I would like to increase beyond visual flicker recognition range, effectively above approximately 100 Hz. Based on the empirical results I am seeing, I believe the stock Creality SSR is switching too slow to support this. I was about to purchase a (very expensive) Crydom SSR when I realised that the Mini OUT0 is rated at 18A (TBC).
The printer is equipped with a 400W bed heater that results in approximately 16.7A nominal DC current at 24V. I reviewed the data sheet of the Duet OUT0 LSS MOSFET and it appears to be perfectly capable of handling this level of current. It also looks like the worst case LSS power dissipation would not be substantially above ~1W. One marginal issue could be the 15A fuse that might trip when exposed to 16.7A for prolonged amount of time.
Does Duet3D have any long term use experience with this magnitude of DC current? The fuse, if it's an issue, could potentially be stepped up to 20A but I would then have concerns with the integrity of board traces under fault conditions. I am not concerned with the thermals as I have a well designed forced cooling of the board back side and I also use one of the ADC inputs for thermistor based board temperature monitoring. Currently the component side PCB temperature between the most heavily loaded stepper drivers (X&Y) never exceeds 55C.
Thank you,
Peter. -
Upping the fuse should be ok, the board and mosfet should be good for that heater load. You've identified all the potential issues and constraints.
For the PWM freq 5 to 10hz is recommended, but that's usually meant for zero switching AC SSRs. But 5 - 10 hz should still reduce flicker.
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Thanks very much for your input. It's been running at 10 Hz, but unfortunately this makes the room unusable with lights on. Most replacement LED bulbs are very sensitive to line loading (voltage) and flicker like mad. House wiring is all fine, it's just the LED bulbs. I am also in Canada which makes the line voltage drop worse.
I might go for the Crydom SSR in the end, will ponder it over.
Thanks again,
Peter. -
@rext3d if you need a good DC-DC SSR, take a look at the ones from Auber Instruments, https://www.auberins.com/index.php?main_page=index&cPath=2. The 80A one is specifically designed for fast PWM and is not expensive.
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Try 5hz or lower. I had some light flicker from my AC heat bed when I had it in a different room with halogen pot lights. Reducing to 1hz made it stop.
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Thank you both very much for your suggestions!
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Just a quick follow-up that can perhaps help someone else...
I ended-up purchasing a Crydom DC100D40, primarily because I was able to get it quickly and with free shipping, as opposed to the Auber relay that would unfortunately take a while to arrive and was very expensive to ship to my location.
I am now able to run up to 1kHz PWM frequency on the bed heater, although all light flicker disappears just above 100 Hz which is where I keep it. As another HUGE bonus, I am thrilled to have eliminated a potential fire hazard of the stock SSR.
The printer shipped with a DELIXI CDG1-1DD/40A SSR that was installed on two spacer posts with no heat sink of any kind. This SSR was running so hot in use that it was impossible to touch. For kicks, I measured the voltage drop on the SSR when it was on and my jaw dropped even more than the voltage when the multimeter registered 1.65V! This is equivalent to approximately 0.1 Ohm internal resistance, resulting in 27W of power being dissipated by the SSR while driving a 400W load. Unbelievable!
The Crydom DC100D40 under identical conditions measures 0.084 V drop, which is equivalent to just over 0.005 Ohm and 1.4W of power being dissipated. I mounted the Crydom on a small heat sink, per data sheet requirements, which in retrospect was totally unnecessary. The SSR stays cool to touch even under extended full load operation. Beautiful!
