# Biasing a transistor to drive LEDs

Discussion in 'Electronic Basics' started by Wong, May 27, 2004.

1. ### WongGuest

Hi,
I would like to bias a 2N3904 transistor as a switch to drive 4 LEDs
in parallel. So I have this configuration but I am not sure whether
the cct will work.
The connection is from 5V source to 47 ohms resistor and then 4 LEDs
in parallel, then to Collector. Emitter is directly to ground.
For the Base, it is connect by a 1K current limiting resistor and
have a 5V, 0V ON/OFF control. I assume LED will consume 20 mA and
require 1.5 V.
Is this cct going to work ? If yes, how to justify by calculation ?

2. ### Tim WescottGuest

Everything looks more or less OK, except that PN junctions tend to vary
from one to another.

So with the four diodes in parallel you may see quite a bit of
brightness variation -- if I were doing this I'd use four resistors, one
for each diode. You may also want to consider using the diodes in
series pairs, with one resistor for each pair. Assuming a regulated 5V
supply and a fairly constant diode temperature this will leave you with
about 1.8V for the current limit resistor.

3. ### Tam/WB2TTGuest

Better to use a separate resistor for each LED.

Tam

4. ### Watson A.Name \Watt Sun - the Dark Remover\Guest

It works but each LED should have its own individual resistor. Each
resistors should be about 150 ohms. Use a 2N4401 instead if the
collector voltage is above 0.2V when it's turned on.

5. ### WongGuest

I am concerning more to the transistor actually. I think the
calculation on the LEDs should be OK for me but I just not so sure
with the biasing of transistor. In another words, is the Base current
setup correctly to operate ?

7. ### Tim WescottGuest

Oh, _that_.

You're set up for about 70-100mA on the collector, and you're giving the
base a hair more than 4mA. This means that you need an hFE of about 25
for it to work.

Now, I'm too damn lazy to go look, but 100mA and hFE of 25 sounds
plausible for a 3904, but it may be asking for too much collector
current. It's your design, so _you_ should get on someone's website,
get a data sheet and check to make sure.

8. ### glen herrmannsfeldtGuest

My choice would be 12V power supply, and the LEDs in series.

One of my college dorm projects was to make a big counter/display
with four LED's for each segment. A 7447 will directly drive
such four LED's, I think to 30ma. A 12V power supply and appropriate
resistor are needed.

-- glen

9. ### WongGuest

Yes, I was designed this circuit based on the following:
absolute maximum of Ic is 200mA, min hFE of 30 at Ic=100mA, Vce=1V
maximum dissipation=625mW
So I think it should be OK, isn't it ?

10. ### Watson A.Name - \Watt Sun, the Dark Remover\Guest

You want to saturate the transistor, by forcing the beta to be 10. THe
Ic is 10 times the Ib. The voltage at the collector should be only a
few tenths of a volt when it's on. If it's higher, then you need more
base current.