On 10/4/2010 9:28 PM, David Scheidt wrote:
> 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.
Nor are they just boring diodes. The dissipate energy in the form of
light and therefore have resistance. But you're correct they are not
just simple resistors.
> 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.
Can't say for sure if the strings I have resistors. Could be in the
plug or the LEDS themselves. But there are 35 bulbs, if you assume 3.3
volts that's 115 volts total, about where you want them to light. The
optimum limiting resistance at that voltage is zero.
> 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.)
Typically they don't brighten, but instead the on cycle is lengthen.
> 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.
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