[quote=“minitreintje”]I couldn’t find the exact specifications how much current could be drawn with the current motherboard & PSU.
I don’t think it’s been officially declared.
The MOSFETs used on the 3Drag controller (which is what Velleman is using) for the heaters are BUK6215-75C (link)
It has a max current (ID) of 57 A. By itself that figure is not very usable.
Some back of the envelope calculations for the MOSFET mounted in isolation using the datasheet:
What is the max current in a room at 25°C?
- Getting the max on resistance
Max transistor temp: 175°C
Power dissipated PD @ 25°C: 128 W
RDSon(max): P=I^2R => R = 128/57^2 = 39.4mΩ
This corresponds quite well to Fig 12, RDSon = a*RDSonmax(25°C) = 2.5 * 15 = 37.5 mΩ
- Getting maximum power dissipation at room temp
The BUK6215 comes as a SOT428 package. Its Rtheta junction to air varies depending on the pad area.
See page 12 here. Using a conservative value of 75 K/W:
Max power dissipation at room temp, P = (175-25) / 75 = 2 W when sinking to PCB.
So, having the MOSFET mounted on a pad area of around 40 mm^2 it can dissipate a maximum of 2 W in room temperature with an internal temperature of 175°C. The current flowing at those conditions would be P=I^2R => I = sqrt(P/R) = sqrt(2/39.4m) = 7.1 A
Does this mean that it’s safe to draw 7.1 A? Some factors to consider:
- The MOSFET is on a PCB with other components getting hot
- You can’t assume that the air under the printer is always 25°C
- Everything in the current path needs to be able to handle the current including wires, connectors, connections
- Actual pad and sink area might be larger or smaller
It’s however a ballpark figure that one can derate or further limit by calculating/estimating the above factors. Both alternatives should be more accurate than simply derating the datasheet’s 57 A. I’ve had no problems with my setup with the heater cartridge drawing 3.1 A (30W@12V => 47W@15V) for instance. I’d be hesitant go higher without modifications - the board connector is rated for 3 A (BTWS1X2 and BTWM10).