which method should I use for DIY LED C7 window candles?

I recently took advantage of post-Christmas sales and purchased several strings of warm white C7 size LED lights of 25 in each string. My intention is to brighten the window candles we currently have. Initially, when I designed the candles last year, I used a single light from each string in place of the original candle bulb. I used a DC "wall wort" 12 volt power supply to power each one with a dropping resistor to reduce the current in each bulb to around the 17 mA level, the same level I measured in the 120 V string before I cut the bulbs out to use them. However, this left the bulbs too dim and the wife didn't like the dimness. Then, a few days ago, since there's enough room inside the bulb envelope, I doubled up two bulbs in series and adjusted the current going in to be around 40 mA. Using the two series bulbs in each candle envelope brightened the result considerably, but now I'm concerned that there may be too much current flowing through them. Although I didn't measure it, I believe the current in series bulbs remains the same (40mA) but the voltage divides(?). This may be too much for the bulbs.

Today, in an experiment, I decided to wire four bulbs in parallel and set the input current at 10 mA. Measurements were 2.7 VDC @ 10 mA going into the parallel combination. Four bulbs inside these small envelopes begins to get difficult, but still not impossible.

My goal is to have the longest life possible out of these candles and the maximum brightness which is the reason I've dropped the input current and/or used more than a single bulb for each candle.

I have several questions:

1) If I use either the four bulbs in parallel driven at 10 mA and/or the two bulbs in series at 40 mA input, is the resulting output brightness going to be the same for the bulb combinations?

2) Which is the best method to use for preserving the longest LED life?

Thanks in advance, Sam

Reply to
Sam Seagate
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Correct and correct. In series, both bulbs get 40 mA, and the voltage divides. And it may be too much current for the bulbs.

No. They will consume vastly different power. Power consumed by 4 in parallel at 2.7V and 10 mA equals 2.7*.01 or .027 watts.

With 2 in series, the input voltage will need to be double the

2.7V. Drawing 40 mA at 5.4V, power = 5.4V * .04 or .216 watts.

Aside from the fact that the power is so different, you don't get a linear conversion from electrical power in to lumens out.

Lower current = longer life; lower heat = longer life.

For window candles, it does not seem that lengthening LED life is worthwhile - they already have a long life if you don't abuse them.

My guess is that the easiest way for you to go is 2 LEDs in series and a resistor to limit the current to 20 mA.

Ed

Reply to
ehsjr

4 in parallel at 10mA will be about the same total liminance as 1 at 10mA, but in series it will be acbout 4 times brighter.

leds run at 1/2 rated current last esentially forever, but LEDs in ornaments are usually the cheapest available so you may see encapsulation failures etc.

light output from an LED is basically proportional to the current throuh it.

how long do you want? at 1mA they'll probably last centuries.

I'd go with 4 in series at 10mA for 4 in series-parallel at 20mA (total) if the PSU is unsuited to 4 in series.

--
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Reply to
Jasen Betts

Jasen Betts wrote in news:m7oau5$u91$ snipped-for-privacy@gonzo.reversiblemaps.ath.cx:

3 in series at 15mA is probably the best compromise between ease of construction, brightness, lifespan and voltage required.

Its extremely rare to have a LED Vf > 3.5V but many white (and blue) LEDs fall in the 2.5V - 3.5V Vf range. 12V is a common supply voltage for LED lighting. Three LEDs in series leave enough voltage headroom for a simple series resistor to be effective for current control over a reasonable temperature range, but four in series does not.

Three LEDs @ 15mA will be as bright or brighter than four @ 10mA (although the efficiency does fall off somewhat at higher currents.

I strongly recommend making up a couple of spare three LED + resistor assemblies in case you get early failure of a LED. This is especially common due to atmospheric mosture penetrating to the die due to micro- cracking of the resin around the leads caused by mechanical stress while reworking previously assembled parts.

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)  
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk  
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Reply to
Ian Malcolm

Current should not exceed 20mA per led die for an unspeced LED. Half that will give massive life.

As for light out, only you can try and see how many you need to get the output you want.

NT

Reply to
meow2222

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