Hi All.
I'm trying to control a 12v RGB led strip with my pro mini 3.3v version.
I've done the same in the past with the 5v version, and i used BC337 transistors with 1K resistor on base terminal to control each of the color channels.
Now i moved to the 3.3v version...
So my question is.....is BC377 still suitable for my purpose? are 3.3 v enough to control it with PWM technique?
I read about using mosfets but i'm totally new to these kind of transistors....
So my question is.....is BC377 still suitable for my purpose?
Change the base resistor from 1K to 680 to 560 ohms.
the important thing here is the transistor must turn all the way on.
With a HIGH on the output pin, the Vce voltage should be ~.5 volts, less is better.
Mosfets are generally superior to bjt's for use as switches - less voltage drop, capable of handling a higher load in the same package.
They're slightly more complicated to use though, since you need a pull down on the gate, but the bigger problem is that mosfets that can be turned on with a 3.3v signal are hard to find in through hole packages. The irf3708 works well, but it's kinda pricy.
I would say that if your setup is small enough to use those transistors with, keep using them.
(Conveniently enough, I happen to sell breakout boards with and mosfets mounted on them 
see tindie link in sig)
LarryD:
the important thing here is the transistor must turn all the way on.
With a HIGH on the output pin, the Vce voltage should be ~.5 volts, less is better.
Thanks Larry.
reading the datasheet ( " Technical Documentation - Design | onsemi " ) and in particular the graph of Vce/ Ib i realized that :
3.3v / 560 ohm = 6 mA Ib
at 6mA ,with a Ic current of approximately 300 mA ( i'm just around it ) , the graph reports that Vce should stay below 0.2 .My strips won't be so long to draw more current than this.
3.3v -.7(Vbe) / 560 = ~4.6mA
Did you get it working?
A 5volt Arduino and a 1k base resistor gave you ~4mA base current. Good for ~80mA collector current.
If you want the same base current from a 3.3volt Arduino, you have to lower the base resistor to 560ohm.
But ~80mA collector current is not a lot of LED strip. Maybe 30cm.
You could lower the base resistor to 100ohm (3.3v Arduino only!).
Good for maybe a meter of strip.
Above that, you have to think "logic mosfet".
Leo..
LarryD:
3.3v -.7(Vbe) / 560 = ~4.6mADid you get it working?
Definitely missed that....maybe it's time for me to rest, it quite late night in my country 
I'm going to try tomorrow and test it all.
A 5volt Arduino and a 1k base resistor gave you ~4mA base current. Good for ~80mA collector current.
If you want the same base current from a 3.3volt Arduino, you have to lower the base resistor to 560ohm.But ~80mA collector current is not a lot of LED strip. Maybe 30cm.
You could lower the base resistor to 100ohm (3.3v Arduino only!).
Good for maybe a meter of strip.
With the 5v version i used to have strips from 1m to 2m . Maybe because i'm using 3528 strips that are really eco-friendly and do not draw a lot of current...
funksoulbrother:
With the 5v version i used to have strips from 1m to 2m . Maybe because i'm using 3528 strips that are really eco-friendly and do not draw a lot of current...
What's important is the current draw for a given length of strip (datasheet).
Transistors like that need at least 1/20 base current for saturation (full LED brightness/ cool transistor).
You quickly run into trouble with a limited base current.
Leo..