How much current can I source to drive PWM using MC9S12DP256 (Dragon12 board from Wytec)

[Sully81]

I am using Wytec's Dragon12 board to drive PWM for some 24V (5 Ohm, MAX 2 Amp) solenoids.  I am looking to spec out the transistor circuit to drive the coil but need to know how much current I can safely pull source from the MC9S12DP256 chip.  Can anyone point in the right direction to find this? 

[Sully81]

Jim,

Thanks for the response.  It looks like the MAX source/sink current is +/-25mA.  Due to the relatively small source current, it looks like I'll have to use a darlington pair transistor to reach the high current needs of the solenoid.

thanks

[bigmac]

Hello,

Yes, you could use a darlington transistor, or alternatively a power MOSFET with a suitable maximum current rating, provided it is capable of fully switching with an input level of less than 5 volts.

For either method, if switching operation is infrequent relative to the switching time, dissipation should not be a major issue.  Both methods will also require suppression of inductive voltage surges.  The simplest method is to use a diode in parallel with the solenoid winding.  The diode should have a repetitive peak current rating equal or greater than the solenoid operating current.

If the MOSFET solution is used, a small series resistance in the gate circuit will limit the charging current into the gate-source capacitance, to be supplied by the MCU pin.

Regards,

Mac

[JimDon]

Do you mean the MC9S12DP256 spec sheet?

You should have got a copy with the board.

Also, if you look on the schematic there should be a circuit that drives a relay, which you can copy except with a higher rated transistor.