Couple questions on Transitors and Mosfets

Husker

I have tried using both a 2N3904 and a 2N3906 transistor to turn on a 9.0V PMDC. With a 9V battery for power and the basic stamp at the base I am able to turn on the LED that is in series with the motor to ensure the switch is transistor is working. However, the motor will not turn on even though current is flowing through it. Is the voltage emitted directly related to how voltage I have at the base? How much voltage does the transitor eat up? Is there an advantage of using a pnp over a npn transistor without worrying about direction?

Also, what are the advantages/disadvantages of using a mosfet versus a transitor?

I know I asked a lot of questions and I appreciate the help.

steve_b

Husker,

can you draw out your schematic in 'paint' or something, and post it?

What's "PMDC"?

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<FONT>Steve



What's the best thing to do in a lightning storm? "take a one iron out the bag and hold it straight up above your head, even God cant hit a one iron!"
Lee Travino after the second time being hit by lightning!

Phil Pilgrim (PhiPi)

LEDs are typically rated for a maximum of 20mA or so. If you've limited the current to keep from destroying the LED, it's probably not enough current to drive the motor. What is your motor's current requirement? Also, your 2N3906 is a PNP transistor. It should not be used alone to drive the motor, since the BASIC Stamp pin will be subject to current from about 8.4V on the transistor's base. Finally, if you haven't done so already, you need to add a resistor in series between the Stamp pin and the 2N3904 base to limit the base current. However, if this resistor has too high a value, there may not be enough base current, combined with the transistor's gain, to dirve the motor.

Repeating Steve's exhortation, please show us your schematic.

Regarding your MOSFET vs. bipolar question: MOSFETs are voltage-driven devices; bipolars, current-driven. A voltage on a MOSFETs gate, with almost zero current draw, will cause it to conduct in such a way that it resembles a low-valued resistor. A MOSFET's disadvantage is that the gate voltage required for maximum turn-on may be higher than 5V. There are "logic" MOSFETs that are designed with lower turn-on voltages. However, in 3.3V systems, even these may be inadequate. Also, a MOSFET's gate can exhibit a high capacitance w.r.t. its source, especially if it's capable of carrying high currents. This limits the turn-on and turn-off speed, unless the gate is driven with specially-designed driver circuitry capable of high transient currents.

-Phil

Husker

PMDC is permanent magnetic direct current motor. I attached the schematic below that I have been using. I know that the motor is 7.2V, but I don't know the amp rating because it was pulled out of another application that I am trying to control with the BS2. I have also used several different resistors with no success.

Phil Pilgrim (PhiPi)

Attached is a typical circuit for driving a motor with an NPN transistor. The diode is there to snub any reverse voltages generated by the motor winding and should not be omitted. The 2N3904 is rated for a maximum continuous collector current of 200mA, but it's not characterized above 100mA, which makes that the practical maximum. If the coil resistance of your motor is greater than 90 ohms, you should be able to drive it with a 2N3904 in this circuit, using a 1K base resistor. If the coil resistance is less than that, you'll need a transistor with a higher current rating.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 12/5/2007 5:15:23 AM GMT

Desy2820

I think you're running into a source current problem.· Your 9 volt battery can't deliver enough current to turn the motor.· I think you need a heftier current source.

Since you mentioned 7.2 volts, my guess is that this motor is from an R/C car or possibly a cordless drill.· In either case, the motor will draw a lot of current.

Using Phil's schematic above, try powering the stamp from the 9 volt battery and use another power source for the motor.· If you have a spare wall-wart (try one rated around 6-9 volts at 900-1000mA) , you can try using that for power.· You can also try a 6 volt lantern battery for a motor power source.· Make sure you connect the negative wires from the two power supplies together.·

I hope this helps.

Husker

You guys were right. It was a current problem. I ended up using a darlington transistor to control a relay that powers the motor. Thanks for the help. After messing around with all those different transistors, I understand them all better now.