Simple Transistor Question

Using Arduino General Electronics
1

Hello fellow peeps!

When i use -> digital(pin, HIGH) for the base pin on my little transistor (2N3904) i get everything working.
But if i use the 5v pin from Ardunio it doesn't..

What basic principle am i missing here?

/slightly confused

2

Do you have a base current limiting resistor (say 4k7) in the transistor base connection. If not then the base junction is causing a (almost) dead short across the supply line. Why would you want to connect the base to the 5volt line ?

3

in fact you almost always need a base resistor. for the common npn-transistors like 2n3904 or bc547 etc. it's quite easy to calculate if you want to use the transistor as a switch:

Rbase = Uvcc-Ube/ Ibase => Rbase = (5 volt-0,7volt)/0,002A = 2150 ohms.

So if you want to drive the transistors base with 2 mA you need a resistor to the base of about 2k2 ohms.

markbee

4

Thanks for the replies.

My basic newb thinking was that iirc, digitalwrite at high was = to 5v, so with some quick proof of concept testing i thought it should do the same thing.

Clearly i have much to learn. :wink:

Thanks.

5

My basic newb thinking was that iirc, digitalwrite at high was = to 5v, so with some quick proof of concept testing i thought it should do the same thing.

The difference wasn't explicitly stated: the I/O pin limited the current to the transistor, the 5V supply did not. Neither is a good thing, because the I/O pins are not designed to be current limiters. As already stated, you should always have a current limiting resistor connected to the base of the transistor (in cases where you are using it as a switch.)

With a current limiting resistor in series with the base, the I/O pin and 5V supply will work the same for turning on the transistor.

6

Thanks for that James.

Have used this transistor with another 9v circuit, running pwm with no resistor.

Duly noted, if using as a switch - need resistor. (With calc used above)

Used a 1R2 (instead of 2150 ohms) resistor for a test and it held it open and then faded out. So i think i need to do some googling to further my knowledge.

Thanks for the help.

7

Used a 1R2 (instead of 2150 ohms)

In this context a 1R2 resistor is just like no resistor at all. Unless you mean a 1K2.