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LEDs are marked as a particular voltage. If I have 4x1.2v LEDs I need a 4.8V battery. But if the battery is, say, 9V I will need a resistor in series to prevent damage.

Resistors are not marked as a particular voltage, they are marked in Ohms. The voltage depends on the other resistors in series (including the internal resistance of the battery). But LEDs are not marked as having a particular resistance.

If I have a number of LEDs of given voltage, and set of batteries (normal commercial AA/AAA etc), what size resistor do I need to protect the LEDs?

spraff
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    If you have 4x1.2V LEDs, you can't (reliably or safely) use them in series with a 4.8V battery. – JRE Dec 08 '20 at 10:25
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    @JRE Why not? What's the 1.2v rating for if not for that determination? – spraff Dec 08 '20 at 10:43
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    Because there's nothing to limit the current. If the battery voltage is just a tiny little bit above the forward voltage of the LED, then the LED behaves like a short circuit. LED forward voltage is not "set in stone," and battery voltage varies as well. Just a tiny difference in either is the difference between lighting your LED and exploding your LED. – JRE Dec 08 '20 at 10:48

2 Answers2

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LEDs need to be current driven. Any voltage rating is nominal.

If you plot their voltage against current, you'll find a very flat line, whereby a large change in current gives rise to a tiny change in voltage, unlike a resistor where the two are proportional.

If you had very good precision on your power supply (possible with spending a lot of money) and good precision on your LED specifications, you would still have a problem as the temperature of the LED affects the voltage drop. As the temperature changed slightly, you would find the LED drawing a current varying over a huge range.

The solution is to fix the current the LED takes, and let it establish whatever forward voltage drop it likes. For high power LEDs, we invariably use a switch-mode current source for efficiency. For small LEDs, a series resistor from a higher voltage power supply is adequate.

Your nominally 1.2 V LEDs would run just fine from a 9 V battery, dropping the roughly 4 V excess across a resistor.

Neil_UK
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You need to know the maximum current that the LEDs are intended to run at. Call this Imax amps. Let the battery voltage be Vbat, and the combined voltage of all the LEDs be Vled.

The resistor you need is given by

R = (Vbat - Vled) / Imax

For a safety margin, round the resistor up to the next commonly available value.

Simon B
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