ROBERT via EV wrote:
The other advance that made LED lighting possible was low cost PWM LED drivers on a chip or with few components. Even today an LED cannot take a continuous high current.
I agree with Cor. LED prices and efficiency have improved markedly, which is what has made them practical for lighting.
But there is no reason a high-tech PWM driver is required. A simple dropping resistor works. I don't know of any LED that can't operate at a continuous current.
The resistor wastes a little power, of course. Good design practice is to use it when 80-90% of the supply voltage is still going to the LEDs. For example, an LED car tail light will have four LEDs in series, each having a 2.8v drop for a total of 11.2v. The series resistor only needs to drop the remaining voltage. Since a car's electrical system delivers about 13.5v to the tail light socket, the resistor drops 13.5-11.2 = 2.3v. That's 83% overall efficiency. A consumer-grade PWM circuit isn't going to be any more than 90% efficient; hardly worth the bother.
If you're running the LEDs on 120vac, the voltage drop needed is much larger. But as Cor said, you can use a series capacitor instead of a resistor. Capacitors drop the voltage by shifting the power factor; not by making heat. So the efficiency is in the high 90% range. Again, no PWM needed.
PWMs are good a) when dimming, or a large input/output voltage differential is required, b) to impress gadget-happy consumers, and c) c) for marketing and to enhance profit.
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