# What value resistor do I need for a transistor base? Answered

Hello, I am trying to work out what resistor I need to use on the base of a 2n3904 base. I am using a 555 timer to flash some leds all powered by a 12v power supply. There are 4 leds in series each with a forward voltage of 2.1v and they require 50ma. I have found that I need a 82 ohm resistor to protect the leds using a online calculator.

Pin 3 is driving the transistor base and the transistor controls the leds but I do not know what resistor I need on the base.

Any help is appreciated!

Edit: here is the finished circuit.

http://vimeo.com/33513619

Tags:

## Comments

The forums are retiring in 2021 and are now closed for new topics and comments.

those are pretty beefy LEDS at 50mA each...most you usually see is 20mA, 30 max...

Anyhoo, the BE current on a transistor is miniscule; you just need to ensure it gets to saturation voltage, which is usually (iirc) .7 volts.

Math:

http://www.kpsec.freeuk.com/trancirc.htm

halfway down the page:
"Connecting to an IC"

"A resistor RB is required to limit the current flowing into the base of the transistor and prevent it being damaged. However, RB must be sufficiently low to ensure that the transistor is thoroughly saturated to prevent it overheating, this is particularly important if the transistor is switching a large current (> 100mA). A safe rule is to make the base current IB about five times larger than the value which should just saturate the transistor. "

Example
The output from a 4000 series CMOS IC is required to operate a relay with a 100ohm coil.
The supply voltage is 6V for both the IC and load. The IC can supply a maximum current of 5mA.

Load current = Vs/RL = 6/100 = 0.06A = 60mA, so transistor must have Ic(max) > 60mA.
The maximum current from the IC is 5mA, so transistor must have hFE(min) > 60 (5 × 60mA/5mA).
Choose general purpose low power transistor BC182 with Ic(max) = 100mA and hFE(min) = 100.
RB = 0.2 × RL × hFE = 0.2 × 100 × 100 = 2000ohm. so choose RB = 1k8 or 2k2.
The relay coil requires a protection diode.

SO, in your case you needto know the gain of your 2n3904
@50mA hFE = 60
Ic = 200mA (safe)
hFE required by this application seems to be about '2' since you're not amplifying anything, the 555 can already source more current than the led string requires.

you may prefer to use: RB = 0.2 × RL × hFE

.2 * 68 * 60 = 984, so a 1k should do fine.

but don't just trust me, do the research and check my math.

Thanks for the reply, I know the 555 could happily supply the current needed but I am mainly using this circuit to learn about transistors and experimenting with using them.

Wow, that is way higher than the 100 ohm resistor I have there at the moment!

I was never any good at maths at school so I am also using electronics calculations as a way of getting better at that too.

lol I have some 240mA 2v leds around here somewhere!

I have a one more question if that is ok.

What does "RL" stand for?

Thanks.