[sdiy] help with basic circuit

[ASSI]

On Monday 18 May 2009, db wrote:
> I have basically used an existing circuit design for an LED driver
> (Ken Stone's)

I haven't seen the circuit you're referring to, but it is abundantly 
clear that it was designed to work with much higher voltages.

> But made a couple of changes to make it work for me..
>
> On this schematic:
>
> http://workingdistance.com/temp/led_circuit.jpg

It is not advisable to simply parallel the LED like you've shown.  If 
they aren't matched rather well, some of them will get much more 
current than the others.  Put a resistor in series to each LED 
individually so that at the maximum current the voltage drop is VE-
Vf(LED)-Vce,sat(Transistor).  This works out to something in the 
100...1k Ohm range depending on the type of LED and the current you 
design for.  The transistor should get the sum of all LED currents 
divided by its beta as the base current.  If you really want to drive 
14 LED in parallel (why?), then you should also check that you don't 
exceed the maximum power dissipation for the transistor.

> I have reduced that resistor on the base from 100k as originally
> designed way down to 1k.

This may or may not be what you intended to do, what current are you 
trying to drive the LEDs with?  If you have this 1k collector resistor 
for all the LED, then you will drive them with less than 5mA even if 
you just short the CE of the transistor and this current will be 
divided among the LED too.

> At 100k, the base voltage just wasn't high enough to switch this
> transistor, but at 1k, much better.

At 1k you have over 4mA of base current, which doesn't really buy you 
anything if the rest of the circuit is as you've drawn (see above).


Achim.
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