12v Pump - Will this transistor work?

Hello :)

I plan on purchasing a pump for a garden project. It's rated at 12v and 2amps.

I have a few TIP102 NPN Darlington Transistors laying around. The transistor's sheet says their max power is 2W and the pump I gather will require 24W.

Does this mean the transistor will not work or am I reading the sheet incorrectly?

Because they wouldn't let me post links in my first post here is the pump link: http://tinyurl.com/27ey8au

and the digikey transistor page: http://tinyurl.com/28netfd

In a word, no Electric motors draw far more current to get going than their rated running current. Your transistor will probably not survive the stress.

If you want to run what is effectively a high current device I'd suggest a Mosfet, which has an extremely low on resistance.

I know it's the wrong thing to say but why not use a simply relay to switch the pump circuit. Low tech, reliable, and takes loads of punishment.

jack

Hey Jacrae,

Thanks for the quick reply.

I suppose I could look into a relay. If I found a correct MOSFET, would it be the best choice for turning the pump on and off from an arduino? If the answer is yes, what would I have to look for? What math would I have to do?

While the transistor is too small don't make the mistake that a 24W load needs a 24W transistor to switch it. The power in the load is something totally different from the power in the switch. See:-

http://www.thebox.myzen.co.uk/Tutorial/Power.html

don’t make the mistake that a 24W load needs a 24W transistor to switch it.

That seems to be exactly what I’ve done.

The power in the load is something totally different from the power in the switch.

This is where I’m really confused now. What does totally different mean? Does this mean the power from the pump can be higher than the power rating for the transistor and that I should get a transistor with a lot of wiggle room? Like this MOSFET with a 40.5W power-max rating? http://tinyurl.com/2e3vsxq

What does totally different mean?

It means that the two are not related to each other by very many factors.

Power is current times voltage. In the load this is the energy that does useful work. In the switch (transistor or FET) this is the energy that is wasted in performing the switching action. They are totally different things and so the results you get are totally different.

Example if you have a light taking 1A with 24 volts then you have 24 X 1 = 24 Watts of power in the lamp.

Now suppose the switch is an FET with an on resistance of 0.5 ohms, then the voltage across the FET is 0.5 X 1 = 0.5 volts. This gives a power absorbed by the FET of 0.5 X 1 = 0.5 Watts.

So the power in the switch is totally different from the power in the load.

ohhhhh. I get it!

When I posted my first post I was hoping grumpy mike would comment on my thread.

Thanks mike :)

ohhhhh. I get it!

:) Thanks, made my day :)