I’ve made LED strings that run off 28v, but is it possible to make a string of 75 3.2v LEDs run from 240vdc? I don’t see why if the current and voltage ratings are not exceeded, but I don’t want to smoke a whole pile of LEDs just to find out!!
Yes, I know it’s high voltage, I have plenty of experience and know how to keep safe.
Thanks!
No, you're still going to need a current limiting resistor (unless the power supply itself regulates the current), and those parameters leave no voltage to be dropped across that resistor.peejay:
is it possible to make a string of 75 3.2v LEDs run from 240vdc?
On the other hand, where is this 240VDC coming from? If you mean you’re going to rectify 240VAC mains voltage, keep in mind that’s going to produce a much higher DC voltage - close to 340V at peak. You might actually want more LEDs in the string, so that the resistor doesn’t have to dissipate as much power.
Yeah, I should put in a resistor.
If I rectify 240vac it will still be 240vdc on average if I don’t use a filter capacitor, so the dissipation in the resistor will be the same for ac or dc.
But on second thoughts, the LEDs would be off for most of the time on unfiltered DC so I guess I’ll have to go for 340v after all.
Good point!
You may want to avoid a filter capacitor anyway, and take advantage of the fact that the LEDs won’t be on continuously to drive them with a higher peak current (but keeping the average under 20 mA or whatever they’re rated for).
Avoiding having a capacitor that’s charged up to mains voltage is a pretty good safety feature, too. Keep in mind that the LEDs will never discharge the capacitor on their own, current flow will effectively stop once the voltage across each LED gets far enough below their operating voltage.
how about an LED rectifier =P
. . .bad idea, probably
threepointone:
how about an LED rectifier =P. . .bad idea, probably
It’s usually is a bad idea. The reverse breakdown voltage of LEDs tends to be pretty low.
-Bill
A directly line-connected LED string is a bad idea. The current limiting resistor will get hot, line regulation will be poor (small changes in AC voltage will cause big differences in LED current), and building/testing such a thing yourself will be highly dangerous to do.
If you don’t own an isolation transformer, 100x high voltage oscilloscope probes, CAT III or higher certified meter (not an autozone special), or know what I’m talking about in this paragraph, DO NOT undertake a line-connected project.
Every line-connected LED lamp I’ve personally seen (LED traffic light replacements, etc) has used a current mode SMPS, usually a flyback with a UC3844 controller.
A directly line-connected LED string is a bad idea.
LED Christmas lights are commonly available, and don’t include any fancy current-limiting devices (just a resistor). They seem to be perfectly safe.
A typical configuration has an LED voltage drop of 75 v, leaving roughly 120-75=45 v for the resistor to take care of. At 40 mA, this is less than 2 watts. Easy.
I agree with the need for safety, but there’s nothing inherently wrong with a long string of LEDs.
-Scott
spcutler:
but there’s nothing inherently wrong with a long string of LEDs.
And a dedicated rectifier.
-Bill
According to [[this](http://led.linear1.org/led-christmas-lights-from-philips/) site, even that's not universal (among Christmas lights, that is).phalanx:
And a dedicated rectifier.
-Scott](http://led.linear1.org/led-christmas-lights-from-philips/)
spcutler:
According to [[this](http://led.linear1.org/led-christmas-lights-from-philips/) site, even that's not universal (among Christmas lights, that is).phalanx:
And a dedicated rectifier.-Scott[/quote]
Never underestimate the ability of a company to compromise the performance of a product in order to save 20 cents.
-Bill](http://led.linear1.org/led-christmas-lights-from-philips/)