[sdiy] help with basic circuit

[Dave Manley]

Scott Gravenhorst wrote:
> 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.
>
>
>   
Are you driving 1 LED or 3 (or 14?) from this circuit.  If three, as the 
schematic shows, the transistor collector current is going to be split 
between the three LEDs.  Also the 1K collector resistor is going to 
limit the current to less than 5 mA (roughly 5V divided by 1K, assume 
the transistor is saturated  at 0V, and there's no voltage drop across 
the LEDs).  The Beta of this transistor is spec'd at a min of 300.  Ib = 
Ic/Beta = 5mA/300 = 16uA, while your circuit could provide Ib = 
(5V-0.6V)/1K = 4.3mA, which is way more than you really need.  Try 
reducing the collector resistor, or remove some of the paralleled LEDs 
so that the current isn't divided between multiple LEDs.

-Dave