Question about transistors.

Hello,

I’ve been using arduinos in very small electronic projects for a while and wanted to add a larger motor. I followed a youtube video and essentially came up with the circuit in the attatched picture. The problem is that the transistor doesnt seem to be making the connection when the base goes high. It works okay if I use the 5v pin on the arduino as the source for the motor, but i’d like to use the external 12v so i can get more current into it. What am I doing wrong?

bjt transistors are current controlled devices, in order for that to work you would probably need whats called a darlington pair

Make that 10K resistor something like 330R, to get more current into the base.

The 2N2907 is PNP - for low-side switching you need an NPN switching transistor.

Your motor doesn't have a free-wheel diode connected across it - this is not optional, it stops the transistor being destroyed.

Your diagram doesn't distinguish emitter from collector - this matters (the arrow on the emitter shows which way the current flows, its really helpful for sanity-checking too!)

Ah, okay, so the only NPN I had lying around was a TIP3055 (http://www.radioshack.com/product/index.jsp?productId=2062611) and it worked! The only problem is it worked a little too well and when the Arduino pin is low, there is still some current flowing through the transistor. Do I need to couple the thing with a PNP to get the ground?

Thanks,
Pete

P.S. Attached is the new schematic, I added an LED because I didn’t have any normal diodes!

The 3055 is overkill! Think you need a lower base resistor value, try 220 ohms - may help it turn it fully off as well as fully on. 2N2222 is the counterpart to 2N2907 BTW.

You've drawn an IGBT symbol, not a bipolar transistor. Which reminds me an n-channel logic-level MOSFET is another possibility.

Ah perfect! Worked like a dream. I also had to make sure everything had a common ground :-p silly me! Thanks MarkT!