Propeller VIN
Chouzar
Posts: 3
Hello,
I'm working on a project with some high torque servos where high current draws are expected, and was wondering if the pins that get the power from an external supply (VIN) on the Propeller Activity Board have a maximum current cap. Did some research and found that those servo rails are connected to this -> MOSFET <- , but I'm some kind of a newbie and don't know if that Current Continous Drain rated at 4.2A is actually the maximum current cap that can be supported through those rails.
Thanks in advance!
I'm working on a project with some high torque servos where high current draws are expected, and was wondering if the pins that get the power from an external supply (VIN) on the Propeller Activity Board have a maximum current cap. Did some research and found that those servo rails are connected to this -> MOSFET <- , but I'm some kind of a newbie and don't know if that Current Continous Drain rated at 4.2A is actually the maximum current cap that can be supported through those rails.
Thanks in advance!
Comments
The simple answer is no! That -4.2A rating for the DMP2305U is listed in the Maximum Ratings section running at 25°Cjun junction temperature.
Furthermore, the gate drive voltage be -4.5V which the Prop can't deliver.
Even the -3.4A with a junction temp of 70°Camb can't be used.
This spec is a bit cryptic in it states "Steady State" current. But it also states a pulse width of ≤ 10s.
Presumably, at the end of the test the junction temperature will be 150°C.
Yes the MOSFET can do this but only once in a while and with a -4.5V gate.
OK, what can this MOSFET do?
70°Camb ambient, -3.0V gate drive, 90mΩ, 150°Cmax, 90°C/W thermal resistance.
150°Cmax - 70°Camb = 80°Crise temperature rise.
80°Crise / 90°C/W = .9W power dissipation.
(.9W / 90mΩ).5 = -3.1A
But clearly this is the limit.
Prudent design would limit the current to 75% of this:
-3.1A * 75% = -2.4A
I would stay below -2.4A for safe operation.
Duane J
Then I guess I'll have to stick out of the convenient servo pins, and have the servo connected directly to the battery; 10A of peak current is expected from these so anyway I guess the rail width itself wouldn't be able to handle that much.
Thanks!
I can't figure it out, but I've had already done some tests with another ActBoard having the servo connected directly to the battery, and the Board to the PC USB and somehow when high loads of force where acting upon the servo, the Propeller resets... but I don't know why is that supposed to happen having the servo power independently from the Board power. The only thing connected from the Board to the servo is the common ground with the battery and the signal pin.
When you say "Servo" what exactly do you mean?
Give us specs and part numbers please.
Duane J
I'm working with these Digital Servo Motors connected directly to this Battery (this Battery can provide 100 Amps max. of continous current), instead of having those Servo-Motors connected to the Board.
These are the pins that I'm no longer going to use since the current drawn by these high torque Servo-Motors are much greater than the supported by the board.
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