Steve_T Posted April 3, 2007 Report Share Posted April 3, 2007 HiI have a 12vDC transformer and when i put my meter accross the poles its showing 15v? anyone have any idea why?Anyway back to the question in hand. I have calculated that i will need a 470ohm resistor to connect to my led too but it appears to be slowly getting duller and duller.The led spec is as follows:Color: BlueSize: 3mmMCD: 5000-7000Viewing angle: 20Forward Voltage: 2.8-3.2Forward current: 20-30 mADoes anyone know why this is happening?ThanksSteve Quote Link to comment Share on other sites More sharing options...
audioguru Posted April 3, 2007 Report Share Posted April 3, 2007 Hi Steve,Your "transformer" is an AC adapter with a DC output. It is rated at 12V with a full load. A single LED is hardly any load so its voltage is higher because it doesn't have a voltage regulator.The LED is about 3V so the 470 ohm current-limiting resistor has 12V across it. Ohm's Law calculates a current of only 25.5mA which should be fine.Maybe the 470 ohm resistor is a lower value.Maybe the LED is no good.Maybe you looked at the LED too long and you are getting blinded. Quote Link to comment Share on other sites More sharing options...
korrawit Posted May 13, 2007 Report Share Posted May 13, 2007 more idea! Steve_TYou can reversed think for ac to dc transformer to the sum of ac transformer with diode bridge and some capacitor if ac transformer told that output 12 Vac ,it's an average voltage. Next, 12Vac is conversed via diode bridge and pass it to capacitor. Finally,you will get Vpeak of voltagee.g. I bought some adaptor which insides of it contains a Supplied circuit above. Side of it told that 12Vdc but i measured output voltage with multimeter ,i got 18 Vdc Have Fun!! Quote Link to comment Share on other sites More sharing options...
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