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Power Supplies for LED Driving by Steve Winder

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CHAPTER 3 Driving LEDs

3.1 Voltage Source

We have seen in Chapter 2 that an LED behaves like a constant voltage load with low equivalent series resistance (ESR). This behavior is like a Zener diode – in fact Zener diodes make a good test load, rather than using expensive high power LEDs!

Driving a constant voltage load from a constant voltage supply is very difficult, because it is only the difference between the supply voltage and the load voltage that is dropped across the ESR. But the ESR is very low value, so the voltage drop will also be low. A slight variation in the supply voltage, or the load voltage, will cause a very large change in current; see curve A in Figure 3.1.

If the variation in supply voltage and forward knee voltage (V

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