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Topic: Simple Transistor Question (Read 736 times) previous topic - next topic

aKiwi

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

jackrae

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 ?

markbee

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

aKiwi

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. ;)

Thanks.



James C4S

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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.
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aKiwi

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.

Grumpy_Mike

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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.

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