Use an old Nokia charger to power 1W LED

Here is one useful trick I invented when looking around the transformers/power supplies I had.

NOKIA ACP-7 Europe Standard Charger (Original) has a specification of 230 V, 50 Hz, 4.8 VA. Output 3.7 V, 355 mA. A normal 1W white LED is driven with 350 mA maximum continuous current and typically 3-3.8V voltage drop over the LED. Those are pretty close specifications.

Next task was to verify my theory that this should work well.I took the charger and a multimeter. I found out that the charger gave around 8V output when not loaded, the promised 3.7V when loaded with the 355 mA current and around 700-800 mA current when short circuited. This would indicate that the transformer has quite high output impedance. That’s quite good for this application, meaning that some changes on LED voltage drop do not change the current too much.

First I connected a 1W LED directly to the charger output. I got nice bright light output. I measured the current, and found out that the voltage drop over LED was around 3.5V and current was almost 400 mA. The current is slightly over the specifications for the LED and charger, which is not good.

The fix to the problem was to add one 1N4001 diode in series with the LED. This increased the voltage drop (LED + diode) to be always slightly higher than 3.7V. This will mean that the charger will always put out less than 350 mA to the load. This is on the safe side on the specifications of both LED and charger.

The end result is that an old Nokia charger combined with one diode and 1W LED (with suitable heat sinkl) will make a very simple and well working mains powered LED lamp.


Nokia ACP-7 charger



29 Comments

  1. Faiz ahmed says:

    Thanx for u r explanation …it’s nice . i was facing a problem due to output of 9 v …

    Reply
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  21. waste says:

    Thanks for the tutorial, I came across a couple of old nokia chargers and opened them to toy around.

    Actually the charger is quite simple. A transformer The only added electronic apart from the transformer and 4 1n4001 is a 1.4Ohm resistance between the transformer and the bridge rectifier.

    So I was wandering why did they put a resistance there. Has this to do anything with high drop from 8V (rectified) to 3,7 when loaded?

    thanks for the inspiration

    Reply
    • tomi says:

      The idea is to do with the current limiting and voltage drop on the system.
      When the power source output current limited to some sensible current range at needed charge voltage, the charge circuitry inside the mobile phone becomes simpler to design.

      That 3.7V is typical battery voltage that is in lithium battery packs used in cell phones.
      And that around 300 mA current is pretty suitable charge current level…
      You needed to ass some electronics to start and top charging depending on battery voltage levels etc..

      Reply
  22. Tomi Engdahl says:

    Another somewhat related project:

    How to convert charger into night led lamp
    http://www.youtube.com/watch?v=o-qscjkYP5M

    Reply
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  24. Nokia 8210 says:

    That’s a great little idea. I’m going to try something similar. I can’t see any pictures on this post that show your invention? It would be nice to see what you made.. I’ve just followed you on Twitter so I can catch any new posts here. I’m number 34. Cheers.

    Reply
  25. LED Kerze says:

    Hi admin
    I read your blog, its really awesome, I love it.
    I have additionally this related website.
    An LED flameless candle looks like a real candle but does not burn. Instead, it becomes luminous due to a fitted bulb shaped like a flame, while the exterior is similar to a candle.
    LED Kerze
    Best Regards
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    Reply
  26. tomi says:

    LED candless are safety candles and good idea in many applications.

    I have also read that LED candles can have on hidden safety risk when combined with stupid people.
    There are incidents where some people have used matches to light up the LED candle light a normal candle.
    According the article I have read the burning LED candle look pretty much like normal candle for some time
    (clear plastic on top of LED burns prety nicely), then it starts to burn more heavily and finally explodes throwing burning part around.
    The thing that explodes in them is the lithium battery in them heating up too much.

    Reply

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