# Turning on a motor using a transistor (question)

I'm building a fire-fighting robot for a science expo and everything is fine so far (at least the electronic part is!) but I`m stuck with the fan motor. I`ve already fried a N3906 (don`t know what I expected).

I want to turn on/off a dc motor that uses 1.5A at 3V. The best transistor (2N5550 transistors) I could find at home can only deliver 0.600A. What can I do apart from buying another transistor (I'm broke lol)? Can I sort of connect 3 of those in parallel and have them share the same Base or add a huge heatsink? By the way do I need to limit the current flowing towards Base? How do I calculate the resistor values if I need to limit the current flowing to base?

Thanks a lot LMR!

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I realize this post is getting old now, but I don't see an actual answer to your questions. Yes, you can paralell three power transistors to allow more collector-emitter current.

As to how to figure the base current, for most power switching, the rule of thumb is the base current should be 1/10th the collector current, but some power transistors are about 1/8th. Anyway, I looked up the pdf on the 2N5550 and it showed base current at 1/10th.

So for 1.5 amps, that means 150 mA to the base(s)

(If each transistor supplies .5 amps, they need 50 mA to each base, which again adds up to 150 mA total.

Looks like the transistors will drop about 1.2 volts across collector-emitter, so the supply voltage needs to be the 3 volts needed for the motor, plus the 1.2 volts for the (paralell) transistors.  You will need a battery that supplies 4.2 volts or so.

To calculate the resistor value, take the supply voltage you use and divide it by the current. (Ohm's law.)

Well, if you own a soldering iron, I say go scrounging in old garbage, I have filled my parts bins many times over from old thrown away stereo equiptment, printers, computers, tv's(be careful with those!), vcr's. You can always get lots of goodies, be it steppers, dc motors, pullies, resistors, transistors, ic sockets etc.

I've just picked up an old computer, but that was just luck as I was walking down a street and randomly found it. But where do you go for finding electronic devices?

Garage sales are a good source.

Also, lots of people throw away older electronics. Contact your town and see if they have an electronics collection policy. You might be able to work something out with them.

I know that your broke, but you might save up and buy a IRFZ44N or any other MOSFET transistor. This one can handle 49A @ 55V and its only \$ 0.99

http://www.ebay.com/itm/IRFZ44N-IRFZ44-Transistor-MOSFET-N-Channel-/140599493470?pt=LH_DefaultDomain_0&hash=item20bc62035e

Well if the seller really means the shipping to Canada is free then 0.99 \$ won't ruin anyone!

I have ordered lots of stuff from that seller and had no issue what so ever.