Transistor Help

I want to use a transistor to switch on and off a 6 volt dc motor (7 amps max) using a microcontroller. Thta's it - nice and simple... On and off. Will this transistor do the job? The current ratings are fine, but I am not familiar with all the other stats. Will a microcontroller be able to "turn on" (sorry, don't know the actual term) the transistor, and will it be ok with running 6 volts trhough it?

MJE2955T

http://www.farnell.com/datasheets/64761.pdf

Yes that transistor will do but you will have to keep an eye on the power dissipation. The collector emitter voltage when on is 1.1V at 4A and a massive 8.8 V at 10A. This means that it will get quite hot and you will have to look at serious heat sinks. In general you would be better off using a power FET because these tend to dissipate less power for a given current switching due to the low On resistance. Have a look at:- http://www.thebox.myzen.co.uk/Tutorial/Power.html for information about power calculations and:- http://www.thebox.myzen.co.uk/Workshop/Motors_1.html for information about motors.

The collector emitter voltage when on is 1.1V at 4A and a massive 8.8 V at 10A

Not sure Grumpy_Mike? i think that you miss read the specs! Vce of 8.8V at 10A? :o looks more 0.8A to me! (this is when saturated i think) but i could be wrong also :-/ 0.8V x 6A = 4.8W witch should have a small heat sink attached to it. BTW bfrye if you supply the motor transistor to 6V only 4.2V when the transistor is on will go to the motor, if you realy need 6V for your motor, you should have a higher voltage power supply!

I think that you might be pushing the max current rating of that transistor too close. Heat sinking would most likely be required depending of the duty cycle you run the motor at.

I would recommend you search around for logic level N- channel MOSFET switching transistors. They are much easier for the Arduino to drive and have less voltage drop when switched fully on.

Lefty

IRF530 is a cheap, generic power-NMOS that works great for what you need. IRF510 is sold at radioshack and is it's 'little brother' (still plenty for what you need to do).

There are several other types of NMOS power FETs as well, those seem to be the 2n2222s of the power FET world though.

If it's just a simple on / off a relay would cheaply give you the amperage & voltage you need on the cheap and you wouldn't have to worry about heat.

How quickly & often do you need to switch this on and off?

Both the IRF530 & IRF510 are not logic level MOSFETS and require a full 10vdc gate to source voltage to fully reach their rated maximum current.

A logic level MOSFET will have a lower gate/source turn on threshold such that the Arduino's logic high voltage will fully turn the device on and lower the heat disspation of the MOSFET. A BUK-555 is a nice choice:

http://www.datasheetcatalog.org/datasheet/philips/BUK555-100A.pdf

Lefty

The collector emitter voltage when on is 1.1V at 4A and a massive 8.8 V at 10A

Not sure Grumpy_Mike? i think that you miss read the specs! Vce of 8.8V at 10A? Shocked looks more 0.8A to me! (this is when saturated i think)

Well it looks like 8V to me. Extract from the data sheet.

It is the saturated collector emitter voltage that multiplied by the current gives you the power dissipation in the transistor.