# I grabbed the wrong kind of transistor at Radio Shack : a little help? **URL:** \ **Category:** Factual Questions\ **Created:** [July 20, 2015, 3:46am UTC](https://boards.straightdope.com/t/i-grabbed-the-wrong-kind-of-transistor-at-radio-shack-a-little-help/725663 "2015-07-20T03:46:46Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [July 20, 2015, 3:46am UTC](https://boards.straightdope.com/t/i-grabbed-the-wrong-kind-of-transistor-at-radio-shack-a-little-help/725663/1 "2015-07-20T03:46:46Z")
Ok, I’m building a test circuit that uses a power MOSFET. The microcontroller I’m using emits 3.3 volts on a pin. When I dug around the “parts bin” aisle at the local Fry’s Electronics, I found a “logic level” MOSFET that needs 5 volts instead of 3.3. I could have ordered a 3.3 volt mosfet, or a gate driver online, but let’s do it the fun way. Just need a simple circuit to let the 3.3 volt microcontroller output control the MOSFET. So a quick perusal of youtube found [this nifty video](https://www.youtube.com/watch?v=Csvw7RVc7QA&spfreload=1). Looks about right. So I went to radio shack looking for the 2N3904 shown in the schematic. Ended up with a 2N3906. Figured that was close enough, bought it and went home. Well, turns out, the 2N3906 is a PNP type transistor. The kind I was supposed to buy is an NPN transistor. The last digit is actually a big deal. The state school I attended did not think that power transistors was a needed part of the curriculum for the computer engineering program, so while they taught me all about various filters and the software side of things, I don’t know how to switch power. Can I just flip the diode portion of the transistor (the emitter and collector) and use the same circuit? Or is a more serious overhaul needed to use the BJT to drive the mosfet. A little visual aid : [https://dl.dropboxusercontent.com/u/16656673/IMG\_0248.JPG](https://dl.dropboxusercontent.com/u/16656673/IMG_0248.JPG) State schools also don’t cover how to use perfboard. Turns out this is a really great, easy to do way to make a test circuit. Beats the heck out of breadboarding - permanent, cheap, easy, good solid electrical connections. Wish I had known about perfboard when doing senior project… ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [July 20, 2015, 3:59am UTC](https://boards.straightdope.com/t/i-grabbed-the-wrong-kind-of-transistor-at-radio-shack-a-little-help/725663/2 "2015-07-20T03:59:34Z")
I think you’re out of luck, personally. You need the NPN transistor. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [July 20, 2015, 4:13am UTC](https://boards.straightdope.com/t/i-grabbed-the-wrong-kind-of-transistor-at-radio-shack-a-little-help/725663/3 "2015-07-20T04:13:38Z")
The driver in the video is for a P-Channel MOSFET. Is that what you have? If not, you need to eliminate the “lower” resistor to get the off-state voltage down to something reasonable. ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [July 20, 2015, 4:24am UTC](https://boards.straightdope.com/t/i-grabbed-the-wrong-kind-of-transistor-at-radio-shack-a-little-help/725663/4 "2015-07-20T04:24:09Z")
> [@beowulff](#): > > The driver in the video is for a P-Channel MOSFET. > Is that what you have? > If not, you need to eliminate the “lower” resistor to get the off-state voltage down to something reasonable. Sharp eye. It’s an N-channel.