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I need to adapt one of these 60 LED string-lights to be powered through a wall plug. These are micro LEDs all wired in parallel; sadly, I couldn't find specs for this kind of LED. The string's power supply holds 6 AA batteries (3 parallel-wired-pairs in series) so I know it requires a 4.5V supply, but I have no idea about the current.

Also, I have a similar LED string light with 100 LEDs (this one powered with a AC/DC power adapter) that no longer works. The adapter outputs 4.5V / 1A. So I want to ask:

-What specs should I look for in the power supply/wall wart?

-Can I use the power adapter from the broken string to power this smaller one?

-Is it a good solution to just hook the LEDs up to a wall wart with the correct specs or does this circuit need more components to work and for the LEDs not to burn out?

Thank you!

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The way these strings work is multiple leds in parallel depending on a single series resistor and/or the the battery Equivalent Series Resistance. Some simpler strings also depend on the Forward Voltage of the LEDs being higher than the battery pack voltage. A 3.3V @ 20mA led will only pull 15 or so milliamps at 3V.

In this case, there is likely a resistor or diode between the battery pack and the led string. Since these have timers, there is probably a mosfet or transistor and an IC internal to the case.

Simplest case, a 4V power supply should be used. Otherwise a 4.5V should be fine. Open the case to confirm there is a resistor or something.

As for current, measure the led string with fresh batteries using a multimeter. This will confirm how much current is being pulled.

As for the broken plug in one, what exactly broke? The string? You can solder, crimp or twist it back together, by removing the clear coat insulation. If it's the power supply, then you can't re purpose it. Check with a multimeter.

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  • \$\begingroup\$ The longer one is damaged because of poor storage decisions, but I could definitely try to salvage the undamaged LEDs. I checked the power supply with a multimeter and it works fine! I also opened up the case and there is indeed a small circuit (even a tiny microcontroller) for driving the LEDs. Does this mean that a 4V supply should be enough to power the LEDs on their own? Measurements place the current drawn around 110 mA. Is it okay to use a 1A power supply? \$\endgroup\$ – user2977634 Aug 15 '16 at 17:06
  • \$\begingroup\$ It's 110 mA at 4.5V (fresh batteries)? That seems a bit low. But yes, a 4 Volt 1 Amp supply will work. 4.5V for normal brightness. \$\endgroup\$ – Passerby Aug 15 '16 at 17:31
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If I had to guess (and I do), then a first cut on something like this is to try and work out what the string is actually requiring. (Better still, would be that you BUY ONE of them and just measure the current. But without that, and nothing more than a sales pitch on Amazon to go on you have to do some guessing. Do you have an ammeter?)

Some "warm white" LEDs, it turns out, are sold for Arduino boards. Ones I see here, LED Sequins - Warm White, use a \$100\Omega\$ series resistor and operate at \$3.3V\$ and use \$5mA\$. From this, I can guess that they actually require about \$2.7V\$ to operate (\$3.3V - 100\Omega\cdot 5mA = 2.7V\$.) That's at \$5mA\$, but I really don't know what they are actually supplying in that string. But I can now guess that they require about \$2.7V@5mA\$.

The article also mentions \$14\$ days of operation at \$25%\$ duty cycle, which works out to \$3.5\$ days. Six AA alkaline batteries, if loaded around a few hundred mA or less, will each provide about \$2\$ amp-hours of life, or about \$10,800\$ Joules. Multiply that by six to get \$64,800\$ Joules total to work with (if our assumption isn't too far awry.) Let's call it \$65,000\$ Joules, in round terms.

From this, I can now estimate that with 60 of these LEDs, the probable current is on the order of \$\frac{65000 J}{60\cdot 2.7V\cdot 3.5\, days} \approx 1.33mA\$. So the voltage is probably a little lower then \$2.7V\$ each, because a lower current implies that much. So let's refine our estimate here, plugging in \$2.6V\$ into this earlier equation to get close to \$1.4mA\$. Call it quits there.

Now we can estimate the current draw from your series-parallel stack of batteries. The current will be \$\frac{65000 J}{4.5V\cdot 3.5\, days} \approx 48mA\$. A quick double-check of the math yields \$4.5 V \cdot 48mA\cdot 3.5\, days \approx 65,300 J\$. So that seems to be about right.

Getting back to the assumption of about \$10,800 J\$ available in each AA battery, I can now see that the loading is indeed reasonably light and that my earlier guess of about \$10,800 J\$ from the datasheet (seen here) might be slightly conservative. It might work out closer to \$12,000 J\$ each. But that only suggests perhaps a slightly higher current allowed in each LED. Not enough to change the basic estimates here more than a little bit.

From all this, I'd guess that you need a power supply that can deliver \$4.5V_{DC}\$ at \$100mA\$ or better, just to be reasonably safe (I've assumed 100% efficiency up to this point and that's not likely. Odds may be closer to 50%, doubling the requirement to 100mA anyway.) More is better. Since this is about half a watt or so, I'd guess most any wall-wart would do, though you should of course read what the rating says. The only problem with those is that their power transformers are often pretty sloppy, with 30% regulation or worse. But it's probably still okay.

So I'd give it a shot, I suppose. I do think you will be just fine. Best would be to measure things, though. (I'm assuming you did work out the battery arrangements correctly.)

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  • \$\begingroup\$ Your calculations seem to be correct! I measured the current on the circuit and it pulls around 110 mA and the LEDs you linked seem to be very similar to the ones on the string. As you may have been able to tell I have a very basic understanding of electronics so is it really okay to supply a 110 mA circuit with 1A of current? \$\endgroup\$ – user2977634 Aug 15 '16 at 17:32
  • \$\begingroup\$ The LED string will take only what it requires. So if you have a power supply that can comply with up to 1A of current, it should be perfectly fine providing only 110mA. \$\endgroup\$ – jonk Aug 15 '16 at 17:35
  • \$\begingroup\$ By the way, thanks for that measurement. It says a lot about what you can do with minimal information. Yes? And as you can see, I've accumulated some rather negative thoughts for reasons I can't account for. Sadly also, for reasons they have decided not to express to me here, so that I might also learn from them. Your measurements act to support what I wrote, though. So I appreciate your taking the time. \$\endgroup\$ – jonk Aug 15 '16 at 17:36

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