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I've just bought an illuminated sign that contains 25 orange LEDs and runs off two AA batteries. I've opened it up and the LEDs appear to be wired in parallel, but I can't see a resistor. (I assume a resistor is unnecessary here due to the number of LEDs running off just 3v?)

I'm going to replace the batteries with a mains adapter, but I would also like to increase the LED brightness if possible, as they are quite dim at the moment. How could I accomplish this?

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    \$\begingroup\$ Crappy product assuming high resistance of the battery. The LEDs will not last long if you use good batteries. Do you have any means to measure the current drawn from the battery, like a multimeter? \$\endgroup\$
    – winny
    Commented Jul 27, 2016 at 19:40
  • \$\begingroup\$ @winny yes I've just tried it and is reading 11ma. So would you recommend adding a resistor if I was to use a mains adapter? \$\endgroup\$
    – Andrew Stephens
    Commented Jul 27, 2016 at 19:49
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    \$\begingroup\$ 11 mA per LED or total for 25 of them? If they are writes in parallel, you will make the uneven distribution if current a bit worse if you go for a say 5 V supply and drop 2 V over a resistor versus 3 V and no resistor. If you have a variable power supply and a few resistors, you can play around with it. Only problem is if one of them have significantly lower Vf, then that will go very bright for a short while before burning out. \$\endgroup\$
    – winny
    Commented Jul 27, 2016 at 20:00
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    \$\begingroup\$ Ah! No wounder it's dim. 20 mA per led is resonable max, but with all wired in parallel, it's impossible to tell other than brightness how well they share. And increase slowly because if one gets too much current, it will get so hot it dims itself, which is very counterproductive. \$\endgroup\$
    – winny
    Commented Jul 27, 2016 at 20:24
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    \$\begingroup\$ Oh! Warm white and 3.0 V! No wonder it's dim! If they are 5 mm standard though-hole LEDs, aim for 10 mA. I suppose someone has made even weaker chips in even smaller packages but you can go higher than 0.5 mA. Yes! Please do aim for a far more resonable series-parallel setup with say three in series and a resistor for 12 V and 20 mA per group and wire eight of them in parallel. \$\endgroup\$
    – winny
    Commented Jul 27, 2016 at 21:25

1 Answer 1

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Unless there is a hidden resistor which may be in line inside the battery compartment or by the first led, the string depends on three things.

  1. Matched diodes with close if curves.

  2. Less important, the number of diodes in parallel causing a voltage drop.

  3. MAINLY that the batteries should be lower than the led VF for a decent current.

Ways to fix this:

  1. Add appropriate resistor for n Leds in parallel. Since we don't know the VF or IF, would could assume 3.4V at 20mA. I'd hesitate to size it at 500 mA due to potential runaway issues (one led draws more, blows out, allowing all to draw more, then next one blows, rinse repeat). So aim for 100 mA total, meaning 4 mA each if they are all equal ((Vs - Vf) / 0.1A). Risky due to unknowns.

  2. Rewire all the leds for their own resistor at 10~18 mA. Part heavy.

  3. Rewire for multiple parallel strings of series leds, basically like typical led strings. Depends on how high your power supply goes. If 12V, then it's three in series plus an appropriate resistor.

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  • \$\begingroup\$ Will finish layer \$\endgroup\$
    – Passerby
    Commented Jul 27, 2016 at 19:51
  • \$\begingroup\$ Don't tell me - your battery went flat! \$\endgroup\$
    – Transistor
    Commented Jul 27, 2016 at 20:40
  • \$\begingroup\$ @transistor thermal runaway cause me to reboot :) \$\endgroup\$
    – Passerby
    Commented Jul 27, 2016 at 21:17
  • \$\begingroup\$ I don't think rewiring will be feasible/practical so I'll go with option 1 and accept the risk of using one resistor. For the purposes of resistor calculation would I just treat all 25 LEDs as one? If so, going with a cautious 4mA per LED gives me one "LED" of 3.4v @ 100mA. Using a 6v supply, the calculator tells me 26ohms. Have I gone about this correctly, and is a 6v supply suitable or would you recommend something else? \$\endgroup\$
    – Andrew Stephens
    Commented Jul 28, 2016 at 8:57
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    \$\begingroup\$ @AndrewStephens yes. 6 Voltage Supply - 3.4 Forward Voltage / 0.1 Current in Amps wanted. A 6 volt supply is as suitable as any other. I suggest you make sure you have a switching supply and not a bulky transformer one, as those have some issues with voltage vs current pulled. Frankly, just use a cheap USB power supply or old cell phone charger. \$\endgroup\$
    – Passerby
    Commented Jul 28, 2016 at 21:17

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