[sdiy] help with basic circuit

db dbarton at pacbell.net
Mon May 18 10:49:16 CEST 2009

>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.

I don't quite follow all that you said, but the LEDs are actually a cheaply 
purchased flashlight that has 16 LEDs in it, and is wired just as shown.. I 
also used on with 24 LEDs. In each case one resistor is used in it to drive 
all the LEDs.

Does that have some effect on the resistor used on the base of the transistor?


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