LED Dimmer Circuit: Questions & Answers

[ranger]

I was given this circuit and it was supposed to be an LED dimmer circuit that changes its brightness (gradually) when the switch is closed/open. I was told that the darlington pair was supposed to "smooth-out" the changing brightness of the LEDs. But here are my questions about the circuit:

1)Why the need for the darlington pair? Cant one transistor do the job?

2)Regardless of the transistor, won't the LEDs come on any way? I mean the way I see it the anode is currently more positive than the cathode - shouldn't this cause the LEDs to light up wihtout any variations in brightness?

3)But the thing that bothers me the most, is the need for the transistors. How exactly is it suppose to smooth the brightness of the LEDs. Is it the collector voltage vs the collector current graph that illustrates this?

http://img222.imageshack.us/img222/2923/slowledzi2.png

--thank you.

 

[SGT]

With the key open, the two transistors are at cutoff, so no current flows through the LEDs.
When you turn the switch on, the voltage at the base of Q1 will initially stay at 0V, because of the capacitor. As time goes by the capacitor will charge through the 220k resistor. The base voltage will rise slowly making the brightness of the LEDs grow smoothly.
The reason for a Darlington pair is that transistor Q2 has a considerable collector current, since it must drive several LEDs in parallel, so it will need a big base current that cannot be supplied through the 220k resistor. The reason the resistor is so large is to provide a large time constant with the capacitor.

 

[ranger]

Thanks for the input. So the more LEDs I have to drive, the more collector current will be required from Q2. If I were to add more LEDs, would I have to change (or add) any component value?

 

[berkeman]

What transistor are you using for Q2? What package is it in? How much power will it be dissipating (just multiply Vce by Ic from the LEDs)? Will you need any heatsinking as you grow the number of LEDs?

I didn't go through any numbers, but you may need to increase the base bias for Q1 to get more base drive for Q2 to support more LEDs. What are the Beta specs for Q1 and Q2 (at the currents where you are operating them)?