LEDs are diodes. They're not boring resistive devices. They pass
current only one direction, and they do that when their "forward
voltage" is exceeded. They cause a voltage drop equal to their
forward voltage, and pass whatever current is flowing; if it's too
high, they get fried. If you string a bunch in series, none of them
pass current until the summed forward voltage is reached. So these
things have a bunch of leds in series, with a combined forward voltage
of 75 or so. You need a resistor to limit the current. Yes, this is
less efficient than not having a resistor. On the other hand, it
works, the leds don't get fried, and it's still way more efficient
than an incandescent bulb.
Forward voltages vary with the led types. little red and orange leds
are about 2 V, blue, green and white tend around 3 V. Current
capacity is around 20 mA. So we're talking 40 or 60 mW per diode.
more power is wasted by the resistor as heat than is used by diodes.
Typically, these strings are built so the combined forward voltage is
about 70 to 85 volts. That keeps the led on about a third of the
time. If you used enough leds that the forward voltage sum was 168 V
(peak voltage on 120 AC) they'd be on a negligible amount of time, and
you'd not get any light. So you either need a chain with a much lower
forward voltage, and a resistor to control current, and accept the
watt or two of power wastage (And it's actually less than that,
because it's only a thirty odd percent duty cycle.), or you need a
more complicated driver circuit and more complicated wiring, which
would increase the cost to buy them a lot more than it would save in
power over their lifetime. (You can make them brighter, though, which
is why there are some fancy strings that work like this.)
Led flashlights get away without an external current limiting resistor
because they've got a big honking current limiting one in the battery.
They also tend to use higher power drawing diodes. But better flashlights
have driving circuits that provide both current and voltage regulation.
--
David Scheidt
dmscheidt@gmail.com
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