LED Xmas lights

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[This followup was posted to sci.electronics.repair and a copy was sent to the cited author.]
snipped-for-privacy@SPAMBLOCKfilmteknik.com says...

The brightness is mainly due to a greater on-percentage. It probably depends on your sensitivity and ambient light. One easy test to see if the rectifier is working properly is to wave a bulb sideways in front of you. You should be able to see it turning on and off.
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Andrew Rossmann wrote:

will see very short off period (when waving the lamp). With half wave, you see the 50% on and 50% off. I did notice a strange thing in the strings which I bought (LED Lights). The one color 70 LED string (2 series strings of 35) has the limit resistors in sockets 2,3,33,34 for the 1st half and 37,38,68,69 for the second string of 35. You can actually feel the heat on these sockets. I have them on a full wave rectifier. The multicolor sting of 35 apparently has distributed resistors in every socket as there seems to be no socket getting warm. Also, on their web site they mention that some color LEDs (white and blue) are run at higher currents than red. So, my guess is that there are resistors in each socket.
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In my experience, halfwave in an LED string has duty cycle a lot less than 50%, since much of the time the instantaneous line voltage is too low to make any current flow through the LED string at all. With fullwave, expect the duty cycle to be not that high despite being doubled (or a bit less than doubled due to voltage drop of the bridge rectifier).
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On Mon, 5 Dec 2005 01:17:16 +0000 (UTC), snipped-for-privacy@manx.misty.com (Don Klipstein) wrote:

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.
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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.
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On Mon, 5 Dec 2005 17:18:47 +0000 (UTC), snipped-for-privacy@manx.misty.com (Don Klipstein) wrote:

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).
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> Yes, it does have to reach the LED's forward breakdown voltage

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.
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wrote:

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.
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Andrew Rossmann wrote:

Yeah I waved two bulbs back and forth, one on a string running on AC and the other on the rectifier and it was easy to see that there were twice as many flashes on the rectifier.
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On Thu, 01 Dec 2005 17:18:44 GMT, Steve Kraus

I was actually going to try that, but don't have a 120V rectifier now (I do have a 50V one).

I'd expect it to be brighter, with nearly twice the power.
Does adding the capacitor make any difference (that is, if you have a large capacitor that'll work on 180V or so)?

You should see the drop across 2 diodes, 1.4V. Also, are you sure you switched the meter from AC to DC?

Neither an analog VOM nor a DMM (other than a very expensive one) will give accurate RMS readings except on a good sine wave. They respond to average voltage (or current, if that's what you're measuring) and attempt to display this as RMS (correct for sine waves).
I made some measurements of current for Xmas lights, including:
string of 25 C9 lights: 1.5A
string of 25 C7 lights: 1.3A
string of 50 miniature (incandescent) lights: 200mA
string of 70 LED lights: 50mA
Supply voltage is almost exactly 120V here.
BTW, with the LED lights, the blue ones are noticeably brighter than the other colors (I have some red, some blue, some green, and some yellow).
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Mark Lloyd wrote:

Haven't tried that yet.

Soitenly. Putting it on DC gives an even more ridiculous number.
If enough LED's would be connected in series could they dispense with the resistors entirely?
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Steve Kraus wrote:

need a series diode to rectify the AC to half wave DC. The capacitor filters the bumps and provides smooth DC. I used a very small electolytic capacitor; 2.5uf (that's what I had in my junk bin) provided smooth DC. I didn't pull out the scope, however, waving the LED showed NO blinking. This was for a double string of 70 single color LEDs. The difference in LED brightness between full wave rectified DC and pure DC (with a capacitor) was very small.

statement, I'll try to reply, so I hope this makes sense. First, every meter reads differently; some read peaks, some take an average and some are true RMS. When taking these type of measurements ( AC and pulsing DC), you should use a true RMS meter, however, any meter can be used to compare one reading to another. The readings just may not make mathematical sense. Also, the voltage drop on LEDs may not be, and is usually not, the typical .7 volts. Differernt color LEDs may also have different voltage drops.

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wrote:

A capacitor works with a full-wave rectifier too. you could use a smaller capacitor (It won't have to maintain 120V for as long).

scopes will show peak. A new one may also show the RMS equivalent (may work right for sine waves only).

Aren't those expensive?

.7V (700mV) is the common forward breakdown voltage for silicon diodes (300mV for germanium). That's the voltage required for the diode to conduct when forward biased.
LEDs emit light when the forward voltage exceeds the forward breakdown voltage (some people might think "breakdown" is something destructive, but it isn't when current is limited). LEDs are made with different semiconductor materials, and so can have different breakdown voltages. I have measured about 1.5V on a red LED. I have not measured any blue ones yet, but have seen advertisements claiming around 5V.
Also, I measured 50mA current to my string of 70 LED lights (I measures this on an all-red one). 50mA is a common maximum forward current for LEDs and exceeding it would damage the LED. It could be the string has 2 35-LED series, running on opposite polarity and drawing the maximum current (blue ones may be different). Once I use a scope, I might find out more about this.

I haven't measured a blue LED yet. I've seen catalogs that claim around 5V.

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On Fri, 02 Dec 2005 05:10:00 GMT, Steve Kraus

Would a capacitance of 220uF be OK for this?

Oscilloscope?
Supposedly you can't, since a LED in breakdown (necessary for it to light, voltage needs to be at least 1.5V or higher for blue LEDs) has a very low resistance, and would draw too much current without a resistor. I have seen LEDs used without resistors on small batteries, with high internal resistance.
Resistors do dissipate some power (reducing energy efficiency of the lighting system). Maybe someone could figure out a way around this someday. Maybe a duty-cycle controller like that used in incandescent light dimmers.
BTW, I did once (accidentally) connect a LED to 12V with no resistor. There was a POP and half the LED package disappeared (moved too fast to be seen, like a flea jumping). The remaining half (attached to the leads) did look burned around the chip.
Another thing, I learned about LEDs in college. The semiconductor material used determines the color. There were no blue LEDs at that time (blue is at the high end of the visible spectrum). There are blue ones now. However, I still don't understand WHITE LEDs (there is no single frequency for white, like for the other colors). White could be made from red, green, and blue but I see no evidence of them in a white LED.
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Mark Lloyd wrote:

Supposedly it's a blue chip exciting a phosphor.
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So then, white LEDs should have the same forward voltage as blue LEDs? I do intend to test that sometime.
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That is correct - give or take production tolerances.
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clifto wrote:

Then they could actually make it any color they want.
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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.
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But why would twice as many flashes or continously lit look only a little bit brighter than the original half wave arrangement? I know in film projection a projector shutter that is open twice as long will seem more or less twice as bright even though peak instantaneous brightness has not changed.
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