LED Dimmer Circuit: Questions & Answers

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Discussion Overview

The discussion centers around an LED dimmer circuit that utilizes a Darlington pair of transistors to control the brightness of LEDs. Participants explore the functionality of the circuit, the role of the transistors, and considerations for driving multiple LEDs.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the necessity of the Darlington pair, suggesting that a single transistor might suffice for the task.
  • Another participant explains that with the switch open, the transistors are in cutoff, preventing current flow through the LEDs, and describes how the capacitor affects the base voltage of Q1, leading to a gradual increase in LED brightness.
  • Concerns are raised about the need for transistors to smooth the brightness, with a question about whether the collector voltage versus collector current graph illustrates this effect.
  • A participant inquires about the implications of adding more LEDs, specifically whether any component values would need to be changed to accommodate the increased collector current required by Q2.
  • Another participant asks for details about the transistor used for Q2, including its package, power dissipation, and whether heatsinking might be necessary as the number of LEDs increases. They also suggest that the base bias for Q1 may need to be increased to support more LEDs and inquire about the Beta specifications for both transistors.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and functionality of the Darlington pair in the circuit. There is no consensus on whether a single transistor could achieve the desired effect, and the discussion remains unresolved regarding the implications of adding more LEDs and the corresponding component adjustments.

Contextual Notes

Participants discuss the impact of component values on circuit performance, particularly regarding the base biasing of transistors and the power dissipation associated with driving multiple LEDs. Specific calculations and assumptions about the transistors' characteristics are not fully explored.

ranger
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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.
 

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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.
 
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?
 
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)?
 

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