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

Sully81 — Mon, 03 Mar 2008 23:30:13 GMT

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?

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

JimDon — Tue, 04 Mar 2008 01:25:37 GMT

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.

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

Sully81 — Tue, 04 Mar 2008 03:21:06 GMT

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

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

bigmac — Tue, 04 Mar 2008 11:07:42 GMT

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