Led christmas lights

What's your line voltage? Country?

The no driver solution: Go direct to 120VAC if you don't mind the low brightness.

D from BC Amateur smps designer. British Columbia, Canada Posted to usenet sci.electronics.design

OK I'll byte,

What colour LED's , as each LED has a voltage requirement between 1.2volt to

4Volt (dependant on LED choice and colour).

So for a std RED LED, the typical voltage across each LED is say

1.2volts.... so for 100 RED LED's you will need 120V DC not AC (which is a dangerous voltage to work with for a hobbyist).

Strings of 10 RED LED's will be 12V so this might be the preferred option.

google found tons of links with people causing fires by connecting LEDs to the AC line,

but here is a link I have used in the past:

don

Fire??? At

Thanks for reply...

Europe 230V

I have two question about this link:

1. why capacitor is used? "the capacitor acts as a constant current source", but series resistor isn't enough?
2. power dissipated by this circuit isn't high?

Novices and those with textbook-level questions are encouraged to post to the beginners' group. From Google, that is

I'll be brief since you're asking scary questions..(Will you be safe?)

The capacitive reactance sets circuit current. The resistor is especially required for surge limiting upon power up.

Don't think DC with this circuit.

I'd agree with the other guy that this is too basic for SED.,

D from BC Amateur smps designer. British Columbia, Canada Posted to usenet sci.electronics.design

Bur doesn't "amateur" mean "not up to textbook level"? These days, even remotely on-topic is a breath of fresh air - no question is too basic for S.E.D. if it is well posed.

-- Joe

A word of warning on the capacitor: It should have an AC voltage rating. I have had capacitors fail as ballasts with peak voltage of the AC less than the DC rating of the capacitor. The capacitor usually went POP or BANG when it failed.

I have had 250 volts AC blow two oil filled capacitors with 600 volt DC ratings and no AC ratings, as well as one foil / waxed paper capacitor with an 800 volt DC rating and no AC rating. One of the oil filled ones left a large oil stain on the ceiling, a misting of oil on nearby objects, and oily capacitor guts on the floor nearby in the process. I also once had some sort of poly cap with a 600 volt DC rating and no AC rating fail loudly from 240 or 250 volts AC.

All of these occurred during a 2 week period of my more reckless youth, towards the end of which had any BANG being followed by my mother or one of my brothers asking "capacitor?".

Whenever I saw a capacitor with both DC and AC voltage ratings (always after those adventurous two weeks), the AC voltage rating was always a lot less than 70.7% of the DC one, usually 1/3 to 1/2. In addition, some dielectrics heat up more than others do with AC.

- Don Klipstein ( snipped-for-privacy@misty.com)

The OP is fuzzy about capacitor operation.

I have 'amateur' in my usenet signature cause my stuff still blows up.

The short answer to the OP maybe to simulate the circuit and to study the waveforms.

D from BC Amateur smps designer. British Columbia, Canada Posted to usenet sci.electronics.design

On Sat, 17 Oct 2009 17:46:48 +1100, Joe G (Home) top-posted: [top-post repaired]

Don't try to run LEDs from a constant voltage. They need a current-limiting resistor, or be driven by a current source or sink.

IOW, you could put 9 in series, with enough resistance to drop 1.2V at

10 mA - I'm too lazy to do the arithmetic, but you get the point.

And don't top-post.

Hope This Helps! Rich

Only if you want to end up with 100 dead LEDs, and possibly a fire, as don noted.

It's almost impossible to get an LED to work properly with a constant- voltage drive, because of the exponential V-I curve.

And they won't share reverse voltage like rectifiers would try to; even rectifier diodes in series are usually paralleled by resistors or capacitors to divide the reverse voltage equally.

Hope This Helps! Rich

Why dead??? Say each LED is 2 volts.. then a 100 LED string is 200volts. How is the peak from a 120VAC line going to kill the LEDs. Heck..it maybe too dim or not light up.

Use fuse for fire prevention.

Are you implying LED voltage drive is not possible at any voltage?

Oh...yeah.. the LED PIV... mmmm :(

Then stick real diode in the string? :P

D from BC Amateur smps designer. British Columbia, Canada Posted to usenet sci.electronics.design

The problem is the temperature dependance of the forward voltage drop. The outside temperature as well as the internally generated heat will alter the voltage drop and hence greatly affect the current.

While there may be flash lights in which the LED is directly connected to the batteries, but the internal resistance of the battery will function as a current regulator, so this is not a "constant voltage" source.

Paul

Ever see an LED on a Vsource glowing dim then go 'poof'! from thermal runaway?

D from BC Amateur smps designer. British Columbia, Canada Posted to usenet sci.electronics.design

Not if you want to maintain sane current levels.

THe V-I curve is not only exponential, but it has a tempco that will put them into thermal runaway. The Chinese throwaway flashlights get away with it because of the internal resistance of the battery.

Cheers! Rich

No, I've always had sense enough to limit the current.

Cheers! Rich

Have always been a bit concerned about the safety of tree lights and run them through an isolation transformer, irrespective of type. Also, the incandescent ones run for years if you reduce the rated voltage by around 10 or 20%...

Regards,

Chris