# 2n 3904 NPN Info Please???

Discussion in 'The Projects Forum' started by dyeraaron, Jan 20, 2010.

1. ### dyeraaron Thread Starter Active Member

Oct 27, 2008
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0
Good morning....I was wondering if somebody could help me out with a little info..I'd appreciate it.....so I working on a circuit at home that deals with a lighting sequence and I'm using 3904 NPN transistors to make the LEDs brighter....I've noticed that all 3904's are not the same....The only one I had that actually worked how I wanted it to ( which I only had one of) the writing on it said 2N 3904 730 below it.......what does this 730 mean? I've seen others that say like 2n 3904 with a 415 or something like that under the 3904 as well.

Jan 18, 2008
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3. ### Jack_K Senior Member

May 13, 2009
140
0
There are two different lead configurations for the 2N3904. Maybe you have some of each?

Jack

4. ### SgtWookie Expert

Jul 17, 2007
22,198
1,800
Actually, all 2N3904 transistors in a TO-92 plastic case should have the same lead configuration.
With the flat face towards you, pins down, from left to right:
1: Emitter
2: Base
3: Collector

I would connect them like this:

Just two shown to keep the schematic simple.

R1-R3 calculated for Vcc/Vdd=5v, Vf(LED) = 2v @ 20mA
Since I'm using the transistors as saturated switches, Ib=Ic/10; since desired Ic=20mA, Ib=2mA
Rbase = (Vcc-Vbe)/2mA = (5v-0.7v)/0.002 = 4.3/0.002 = 2150 Ohms. You could use 2.1k or 2.2k.
Rled >= (Vcc-Vf(LED))/20mA = (5v-2v)/0.02 = 3/0.02 = 150 Ohms.

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5. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
They should be EBC for new ones, but older manufactured ones are probably BCE. There was some real confusion until the TO-92 pinout got standardized. Just wait until you realize you have both pinouts in the same drawer.

6. ### Audioguru Expert

Dec 20, 2007
10,789
1,212

The output high voltage of a CD4xxx Cmos logic IC is 5V when the supply is 5V and the output current is zero. Its output voltage is typically only 4.0V when its load current is 2mA. So the transistor here has a base current less than 2mA.

You might not be able to have a high enough current in your multiplexed display to be bright. The solution is to have a separate LM3915 for each frequency so the LEDs will be continuously turned on so they will be bright.