What value resistor do I need for a transistor base?

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.


Picture of What value resistor do I need for a transistor base?
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frollard5 years ago
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.



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. "

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.
Alex1M6 (author)  frollard5 years ago
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?

Just remember, V=IR;

Your 3904 trans needs about 5mA Ibe to saturate...

12v @ 100ohm

V = IR
I = V/R
I = 12/100 = .05 Amps = 50mA
10x the current needed to saturate the BE
Alex1M6 (author)  frollard5 years ago
I just measured the voltage across the base and emitter whilst the leds were running and it reads 0.4v. Is that too low?
you want .6min to .9max for that transistor.
Alex1M6 (author)  frollard5 years ago
Also I just checked the voltage between base and emitter with the 555 frequency set high and this time it read around 0.8v.

I feel like such a novice haha.
Alex1M6 (author)  frollard5 years ago
Also the circuit only draws 30mA max according to my multimeter, strange...
small currents are notoriously poorly calculated as a series multimeter ammeter...

that said if it was a BE voltage of .3 then it would be choking off the current to the leds.
Alex1M6 (author)  frollard5 years ago
Dude I found out why, I had the leds flashing at a low-ish frequency (around 10hz) and when I turned the frequency up so the leds were pulsing too fast for my eye to see the multimeter read around 50mA.

I guess my multimeter just has a slow sampling rate.
resistanceLoad the resistor on the string of LEDs
Alex1M6 (author)  frollard5 years ago
Oh so shouldn't that be 82 ohms?
yes, typo
that said; that number seems really low for all the other circuits I see using 'around' 10k. ymmv.