[sdiy] help with basic circuit

[Scott Gravenhorst]

db <dbarton at pacbell.net> wrote:
>
>
>I have basically used an existing circuit design for an LED driver (Ken 
>Stone's)
>But made a couple of changes to make it work for me..
>
>On this schematic:
>
>http://workingdistance.com/temp/led_circuit.jpg
>
>I have reduced that resistor on the base from 100k as originally designed 
>way down to 1k.
>At 100k, the base voltage just wasn't high enough to switch this 
>transistor, but at 1k, much better.
>
>I don't really understand exactly what purpose this resistor serves, but 
>eliminating it was not advised.
>Can someone shed some light on what it does in this circuit, and why I 
>really need it?
>
>(Each time I lower the value, my LEDs get a little brighter.)

The base resistor acts as a base current supply (crappy current source).  A transistor
is a current amplifier, especially in your configuration.  The collector current is a
product of the transistor's current gain (beta) and the base current.  Thus, as you
increase the base current (lower base resistor value does this) the more current
through the collector to the emitter is allowed by the transistor.  The fact that more
current is then applied to the LEDs makes them brighter.  Without the base resistor
(such as using a wire instead) the transistor could be damaged because the base-emitter
diode has it's own physical current limits.  Or you could allow too much current to
flow through the LEDs which might damage them.


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-- ScottG
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