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I'm very young (72) but like to tinker. I have an old Christmas window decoration that doesn't work (light-up). I have a 110 voltAC to 3 volt DC adapter rated at 1 amp. Can I use this to power about 20, or so, 5mm white LEDs on this rewired window decoration, or is the amperage too high ? I'm guessing I would need a resistor or some other form of current limiting ? Would 30mA be a sensible current for each white LED ? Is there a source where I can find these answers? or do I have to take up your time ? Thanks for your help. BBs

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  • \$\begingroup\$ Are those LEDs connected in parallel or in series (or mixed)? Is there any label on that decoration with nominal voltage/current? Can you make a decent photo of the wiring and add it to your post? When you ask "can I..?" and people say "yes", it means something is technically possible, but not necessarily true in your particular case. \$\endgroup\$ – Dmitry Grigoryev Dec 7 '15 at 14:01
  • \$\begingroup\$ Typically, 20 mA is the limit for normal 5mm leds, for a normal lifespan. \$\endgroup\$ – Passerby Dec 7 '15 at 17:59
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For that size of diode, 10mA each should be fine, so a 1A PSU would be more than enough. Ideally use a series resistor per diode, as this controls the current in each LED individually. If you parallel all the LEDS, slight voltage drop differences between them could result in large current differences, but for a Christmas decoration that's probably not a job-stopping problem.

A 3v supply for white LEDs is a bit low. Is it nominally 3v, so is much higher off load, or is it a regulated 3.00v?

I recently measured some chinese white LEDs that I bought in a 1000 lot from eBay, and got 1mA at 2.65v, 10mA at 3.2v. Your mileage will probably vary. If you were to connect those to a regulated 3.00v without any series resistor, you would get about 6mA per LED, which is quite reasonable, there's even some elbow-room in there for the inevitable temperature variations.

If you have a meter, measure the characteristics of the LEDs you have, and go from there. If not, whack 220ohms in series with each LED, put them on your adapter, and see what result you get. It may be OK.

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  • \$\begingroup\$ Also be aware that if the PSU is not regulated, it may output more than 3V if not loaded to 1A. \$\endgroup\$ – Pete Kirkham Dec 7 '15 at 12:22

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