[OT: Electronic] Size of transistor

Jonte · Guest

Hi!

Sorry for this off-topic question. I normally do just coding but need, for testing, switch a dc motor. The question is what size of transistor is needed to drive a 12 volt dc, 2.7W motor. It's pulsing it about 50-60Hz.

I'm not an electronic guy at all! Does the dissipation effect need to handle the 2.7W the motor takes or less. It should be able to be on pulsing 24/7 without burning up but at the same time not be over sized! I was hoping to be able to use a SOT-23 sized transistor!

Thanks for any help and sorry for the OT.

Thanks,
Jonte

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

I would chose a MOSFET (don't forget the parallel protection diode, some FETs have this built-in), MOSFETs have very low on-resistance, and they are voltage controlled rather than the traditional (BJT) transistors (which are current controlled devices). Some newer types are also available with TTL level gate voltages so you can connect them directly to a microcontroller (in low power applications when the relatively slow turn-on/off time is not causing dissipation problems)

Do you need bidirectional motor control ? Then you'll need a H bridge (4 mosfets).
What do you mean by "It's pulsing it about 50-60Hz." Is it a PWM speed control running at 50-60 Hz ? That's too low frequency for the carrier I think.

Guest · Posted: Fri Aug 01, 2008 4:19 am

Thanks!

As for now I pulse with a delay loop as it's a 12F508 chip used for this tiny motor driver. The motor is a asynchronous one where i pulse the two "-" poles while the 3rd wire is always on "+"

RLScott · Joined: 10 Jul 2007 Posts: 465

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

OK. thanks for the correction. I though these diodes protect from the same thing: reverse polarity getting on the FET's poles.

RLScott · Joined: 10 Jul 2007 Posts: 465

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

You might start with a Fairchild FDV303N, SOT23 25V 0.68A about $0.08 each from Digi-Key. It is about double the voltage and double the current you theoretically need, a decent cushion for small design errors, but still small and fairly cheap.
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