LED Xmas lights

[Don Klipstein]

That is correct - give or take production tolerances.

- Don Klipstein ( [e-mail])

[Don Klipstein]

Nichia manufactures yellow LEDs using blue chips and phosphor. These are a non-orangish, non-amber yellow, more like a lemon yellow, sometimes looking slightly greenish.

There are also pink and purple (lavendarish rather than violet) LEDs made with phosphors.

For colors where LED chips are available, they don't bother with the cost of the phosphor. Especially for red, orange and amber-yellow, since those chips cost less than blue chips do.

- Don Klipstein ( [e-mail])

[Don Klipstein]

I did a bit of experimentation. I posted results as well as comments on importance of peak brightness as opposed to average brightness of LEDs in:

There has been "conventional wisdom" that LEDs look brighter if you pulse them to increase peak brightness even if average brightness is unchanged. Optimum pulse frequency is barely fast enough to reliably make them appear continuously lit, although I have heard a few people say 100 Hz. The conventional wisdom there was that human vision had some "peak detection" effect.

This appears to me to have started in the early days of LED digital displays. It is true that the LEDs used in most of those looked much brighter with 50 milliamps at a 5% duty cycle than with 2.5 milliamps steady DC. However, the explanation is that those LEDs were nonlinear and more efficient at higher instantaneous current, and human vision averaged rather than peak-detected. Human vision's nonlinearity is largely after the averaging or "time integration" process.

- Don Klipstein ( [e-mail])

[Don Klipstein]

Correct. Changing F-ratio by a factor of square root of 2 halves or doubles exposure.

- Don Klipstein ( [e-mail])

[Mark Lloyd]

Yes, it does have to reach the LED's forward breakdown voltage (normally about 1.5V for non-blue LEDs) before it will conduct.

I have a rough estimate of 37% duty cycle (without a rectifier) for

35-LED strings on 120VAC. Does that sound right?

BTW, One thing I didn't do there is calculate the needed series resistance and power dissipation.

The extra voltage drop from a fullwave rectifier would be about the same as what you'd get from an extra LED in the series.

[Don Klipstein]

More like 1.9 volts except for the oldest chemistry red, not used in these strings.

I just tried my 35-LED string, and 37% looks not far off the mark to me.

- Don Klipstein ( [e-mail])

[Art Todesco]

Boy is my Trig rusty! But, after I dusted out the brain cells, I did a calculation based on a AC line peak of

177 volts, an LED turn on voltage of 1.5 volts and 35 LEDs in series, I came up with about 40% (37% was a good estimate. I think I did this right. Of course, some LEDs turn on at lower or higher voltage .... especially if the string is multicolor.

[Don Klipstein]

I'd like to revise my "confirmation" of 37%...

I did indeed try looking at my Christmas tree and rolling my eyes up and down and the duty cycle looked like somewhat more than 1/3...

Turns out, that was an illusion. The duty cycle was not over 1/3, but if anything very slightly less.

Later, I tried again, and with a red bulb, a green one and a blue one lined up, and at the right distance I tried my eye rolling trick, and saw a streak with red, yellow and blue pieces not overlapping. This means the duty cycle had to be 33% or slightly less.

Meanwhile, my line voltage is usually 121 volts, which would have a peak of 171 volts.

- Don Klipstein ( [e-mail])

[Mark Lloyd]

The measurement I remember is for one of those.

I figured the voltage across a 35-LED series, and the peak value of

120VAC. Then I looked at a sinewave (half cycle) and estimated the amount of time it would be above that (the voltage required to forward bias the LEDs).