Figuring out the correct transistor for the job

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi,

I want to control a 12V light bulb. Powersource for the bulb is is a 12v 1.6A wall adapter.

Now, I'd like to figure out which transistor, switch i can use to control the power to the bulb through the arduino. But my questions is a little more generic, is there some sort of a tool where i can plugin all the values of the inputs/outputs and it will point me to the part number that i could use?

On top of that, if anyone can point me to a descent tutorial on how to use a transistor or a MOSFET, that would be great. (I've tried wikipedia, and it's not the greatest place to learn from)

Thank you.

2

You need a FET with a voltage rating (BVDSS) greater than 12V, a current
rating (IDmax) greater than the current required from the lamp and a logic
level input.

There are a lot of FETs that can do this. One that was discussed in another
thread was the FDP8878 -- TO220, 30V, 40A, logic level input. It is apx $1 from Digikey.

(* jcl *)

www: http://www.wiblocks.com
twitter: http://twitter.com/wiblocks
blog: http://luciani.org

3

Gotcha, i'll look for that MOSFET, i was using IRF520A for a smaller LED setup, i'm wondering if that would work for the lamp.

The way it worked for me for USB LEDs went like this

Gate: Arduino Digital out
Source: Negative cable to LED/small motor mount
Drain: Negative lead from USB
Positive cable connected on its own.

When i tried to repeat the same setup, but using the 12v power adapter, the light was always on, it would not turn off when Arduino told it to.

I skimmed through the data sheet, and it seems to be alright, but i guess i misread something somewhere.

Thanks for the help :slight_smile:

4

The IRF520 may work but you are better off with a logic level FET. The logic
level FET is designed to be "on" at a logic levels. The IRF520 looks like
it has a 10V gate drive.

Also the IRF520 is a 100V device which is much higher than you need.
Using a higher voltage device will give you a higher on-resistance. Choosing
a 30V device would be better.

I would add a pull-down resistor on the gate -- 10K to 100K from gate to source.
This will insure the FET is off until the Arduino output goes high.

(* jcl *)