So if i want to drive the LED at 100ma and above, i should use a resistor
of like 1K or so?

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On Tue, 23 Feb 1999, Gerhard Fiedler wrote:

> At 15:14 02/23/99 -0800, Lynx {Glenn Jones} wrote:
> >I need to calculate a bias resitor? i was just going to use 10K, is this
> >not a good value?
>
> that's a "base resistor." you have a current through the led, which is
> (Vcc-V_led)/R_led, which is the current the transistor has to switch (Ic).
> something between 1 and 20mA usually, depending on the led and how bright
> you want it. to switch that current reliably, the base current must be at
> least Ic/hfe (hfe or beta is the current amplification Ic/Ib). the voltage
> at the base is around 0.6V (when the emitter is on ground), so your base
> current is Ib=(Vcc-0.6V)/Rb. 10k gives you 0.4mA, with a plain vanilla
> transistor (hfe=50) you get a max. Ic of 20mA, which should be ok.
>
> if you use your first circuit, the base current would be considerably
> lower, because the voltage across Rb would be reduced by the led voltage:
> Ib=(Vcc-V_led-0.6V)/Rb, which could be as low as 0.2mA. is probably still
> enough to drive your led, but you see the difference.
>
> ge
>