Robert Bruninga via EV wrote:
The 60W equivalent I just took apart had a full 14 LEDs in series but
still used a PWM controller.
It took 80 volts DC to get the LED's to their proper brilliance. So just
a small resistor will do.
Right. And a rectifier. :-)
80/120v means a simple series resistor would make it 67% effient. Not
bad. 40v at 60ma is 2.4 watts of waste heat.
What was interesting was that 3 came on at around 15 volts, then more and
more came on as the voltage was raised. And they shifted back and forth
at various levels until they were all on.
My guess is that there is more to the circuit than may appear. The
flashing could be a side effect of the PWM circuit's "brownout"
characteristics.
It's also possible that there are other parts inside the LED package
itself, too. Some are designed for applications like 5v indicators or
12v car lights, and have their own internal regulator or limiter.
All these LED lights are made in China. They will use whatever part
falls to hand this week, regardless of its specs or source.
PS, this divides out to be about 5.7 volts per LED. Much higher than the
2.8v mentioned?
Was that actually measured at the LED? Or just 80v at the PWM input,
divided by 14 LEDs? It is likely that the PWM itself has a considerable
voltage drop, so this won't properly estimate the LED voltage.
It's also possible that each LED actually has more than one chip inside.
The 2.8v I used in my example was for the red LEDs inside a cheap car
tail light. That's actually rather high for red -- typical red LEDs are
usually 1.6-1.8v. But they were driving these red LEDs at excessive
currents; about 100ma, when the part's data sheet says 60ma absolute
maximum. (This is unfortunately a common practice).
White LED voltages are generally higher; in the 3.x volt range.
--
Problems that go away by themselves will be back with friends.
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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