Quick LED question, details inside?
I have a circuit board which currently uses standard 3mm LEDs. These operate at 1.8-2.4v.
I wish to replace these with Ultra-Bright 5mm LEDs. These operate at 3.2-3.8v.
Will this circuit light the ultra-bright LEDs? (I'm not bothered about them not being bright, they just need to be visible up close in day light)
I hazard a guess that the circuit board runs these LEDs at 1.8v to save power. But I don't have a voltmeter to test this.
See if you can trace the LEDs to power and ground. The path will likely go through a resistor. If the LEDs turn on and off, one end will likely terminate at the output of a 74xx logic IC.
To get the best brightness out of the LED you'll have to swap resistors. To do this, you'll need to know the actual supply voltage, but if it is connected to a 74xx chip, that voltage will be 5V. The correct resistor will then be (5 - 3.5) / 20 mA (typical numbers) or 75 ohms.
You should then check and make sure that the resistor will be able to handle the power dissipation. For example, 75 ohms x (20 mA)^2 = 30 mW, which is less than the typical 250mW rating of a through-hole resistor.
If you don't know the voltage, you can assume the LEDs are running no more than 10 mA (chances are it's closer to 5) and reverse this formula to find out the voltage. For example, if there's a red LED there, (V - 1.2) / (observed resistor) = 10 mA.. you can then solve for V and plug it into the formula above.
(By the way, LEDs can be run at only one voltage; they are current-controlled devices.)
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