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I recently converted a beer light (plexiglass) using 20 LED's, resisters and a power block rated at 9V but it burned out the LED's. I When i tested the adapter it was putting out 14V's.

Question: I then tested several other power blocks i had on hand with varying voltage ratings and found most of them, not all, provided higher voltages that the tag indicated. What am i to believe when looking at these labels. I like playing with LED's but need to understand the voltage of these power blocks to ensure they work.

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closed as off topic by Hennes, Dave M, CharlieRB, Siim K, Daniel Beck Dec 20 '12 at 20:01

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You might want to ask that on – Hennes Dec 20 '12 at 18:32
LEDs do not burn up because of too much voltage. LEDs burn up because of too much current. – sawdust Dec 20 '12 at 20:57

Those ratings are only specified under the load shown (i.e. if you have a 20V 4A adapter, the voltage should be 20V when drawing 4A). Most unregulated power supplies, or ones that expect a significant load, will go to a very high voltage when open-circuit.

To ensure the voltage doesn't go too high, you might want to put a high-wattage, low-resistance resistor in parallel with your circuit. It's wasteful, but if you put enough of a load on the power supply, it will ensure the voltage never goes higher than what one would expect.

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