Transistor heating up

Question

Hi Everyone!

I am new to this site and have just gotten into electronics recently. I aspire to work as an embedded software engineer one day. I am currently a web developer and graduated with a Computer Science degree so I've gotten the software down.

Recently I have picked up a few books and started out with the Arduino. I am still getting used to hardware side which takes me back to my question.

I have been playing with transistors but its given me different results each time. Sometimes it will work, sometimes it will not. Can someone clear it up for me.

Below is the simple set up. The other day it worked just fine but today without the base receiving current from the base the motor still moved, slowly and transistor started heating up. I understand the fan isn't valued for 16 V but

it seemed like the transistor can handle that current.

Link to transistor datasheet : http://www.farnell.com/datasheets/1845309.pdf

Thanks in advance!

johnbeetem · Accepted Answer

Three comments: 1. A single 2222 may be a little weak for this application. With 1K base resistor you're getting 4 mA current from base to emitter, which the transistor amplifies by a factor of 40 or so resulting in 160mA, which is less than the 200mA required by the fan. So you might want to reduce 1K down to 750 or so to get more margin. I don't know how much current the Arduino can source: see what voltage you are actually getting at the Arduino pin. If it can't source much current, try a Darlington pair instead of a single NPN transistor: http://en.wikipedia.org/wiki/Darlington_transistor A Darlington pair has two stages of current boost, so you can use a very small current to switch a very large current. 2. You need a Flyback Diode: http://en.wikipedia.org/wiki/Flyback_diode Let's say you've successfully turned on the fan with the transistor. The fan is an inductor with 200 mA or so running through it. Inductors cannot change current flow instantaneously. When you turn off the transistor, the current has to go somewhere so it charges up the stray capacitance at the transistor's collector to a really high voltage, which shorts out your transistor. If you've been burning through transistors, this is why. The flyback diode goes across the motor from NPN collector to your +16V supply. When the transistor shuts off and the collector voltage starts to rise, the flyback diode switches on and provides a safe place for the motor's current to go. 3. IMO you don't need a collector resistor. The fan motor provides enough of a voltage drop.

clem57 · Answer

This is a NPN transistor which needs a resistor on the base which you have and a resistor on the collector which is missing. This limits the current on the collector and would be the reason for the heat on the transistor. It is not the voltage, but the current flowing causing the heat issue. Please review NPN Transistor Tutorial - The Bipolar NPN Transistor carefully to understand how the electrical flows work.

Clem

clem57 · Answer

Yes the fan has a load but not the same as a dc motor in the example. This is like saying a beetle and a truck have the same affect in an accident. Work in this field has many details to understand just like the web. I am sure subtle differences in browsers can affect web code. Study Ohm's law first and you can see that math is the key here.

Clem

Transistor heating up

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