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.
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