For example here. Why waste electricity and produce heat? If I'd supply voltage enough for three LEDs in series, what is the problem? Why are there two resistors?
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3\$\begingroup\$ Read all these posts to understand why \$\endgroup\$– Andy akaCommented Jan 5, 2023 at 13:04
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3 Answers
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The resistors are there to limit the current flowing though the LED.
The amount of current flowing through the LEDs will be given from the diode forward characteristic, without a limiting resistor this amount of current will be excessive (generates too much heat and destroy the LED).
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\$\begingroup\$ Why it has to be limited? LED will consume as much current as it needs. \$\endgroup\$– NameCommented Jan 5, 2023 at 13:48
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2\$\begingroup\$ @Name No, LEDs will conduct as much current as it is given and then go up in a puff of smoke if you exceed the current spec. They are very different from light bulbs. \$\endgroup\$– LundinCommented Jan 5, 2023 at 13:52
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\$\begingroup\$ Well I did put LED to lithium battery to test it - it definitely did not explode. \$\endgroup\$– NameCommented Jan 5, 2023 at 14:01
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1\$\begingroup\$ @Name - it worked that time, and probably the next ten times thereafter, but it will fail (and burn out the LED) eventually. However, if you want to save energy, then use a 9V supply without a resisior... it will work, yes, but you will also be polluting the planet with burnt out LEDs that constantly need replacing. \$\endgroup\$ Commented Jan 5, 2023 at 14:02
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3\$\begingroup\$ @Name What's a "lithium battery"? In case of Li/ion coin cell batteries they have ESR (equivalent series resistance) similar to a resistor, so you can usually drive LEDs from them directly without a series resistor. If I were to put a LED across my e-bike Li/ion 36V battery, then the LED would instantly explode. \$\endgroup\$– LundinCommented Jan 5, 2023 at 14:10
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This is a simple LED strip (no driver chips etc.) so there would be 3x white LEDs in series. A series resistor is required to control the LED current. Three white LEDs add up to about 9V, so 3V will be dropped across the resistance so they can operate from a 12V supply.
In this case the designers have used two resistors per 3-LED group, probably to increase the power dissipation compared to a single resistor and avoid a larger resistor package which would not allow the strip to bend as easily. The two resistors may be in series or in parallel.
answered Jan 5, 2023 at 13:37
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\$\begingroup\$ But why would I use 12V on 9V LEDs? \$\endgroup\$– NameCommented Jan 5, 2023 at 13:47
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1\$\begingroup\$ @Name Because proper engineering involves designing with margins. If you were to supply this with 9.1V then any power fluctuation or temperature variations would cause the LEDs to intermittently go dark. \$\endgroup\$– LundinCommented Jan 5, 2023 at 13:56
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\$\begingroup\$ What Lundin said (you need the resistors) and also because 12V is a standard power supply voltage. But even if you had 9V, a competent designer would use 2 LEDs in series and a resistor. \$\endgroup\$ Commented Jan 5, 2023 at 13:58
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\$\begingroup\$ But if I had ideal 9V supply (or 8.5V) then I would not need resistors? \$\endgroup\$– NameCommented Jan 5, 2023 at 14:02
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\$\begingroup\$ In practical terms to make a good design you need the resistors. Alternatives include putting LEDs in series and using a switching power supply, but that costs more and is more complex (and doesn't work with a modular strip unless you have a lot of switching supplies). \$\endgroup\$ Commented Jan 5, 2023 at 14:19
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If you used a 9 V supply, as you suggest, and put it directly across the three LEDs (without a series resistor), then they will greedily take as much current as they can, up to the limit imposed by the power supply, and then, either burn themselves out, or the power supply, or both. They have no concept of "just taking as much as they need".
Hence a higher voltage power supply is used along with the addition of a resistor, with the resistor limiting the current by Ohm's law, using the voltage drop across the resistor - where the voltage drop is, as Spehro states, difference between the power supply voltage and the voltage across the three LEDs in series:
$$12 - 9 = 3 V$$
So, if you use a 1 kΩ resistor then the current is limited to 3 mA - which is probably too low. Therefore, a 100 Ω resistor would be better, giving 30 mA.
Check the specification of the LED strip to see what the current should be, and spec out the resistor accordingly.
You can indeed power an LED directly from a power source (which has a voltage comparable to the voltage drop of the LED), without a resistor, often you will see people checking small LEDs with a hearing-aid or watch battery.
You can try it yourself... it will work that time, and probably the next ten times thereafter, but it will fail (and burn out the LED) eventually.
Nevertheless, if you want to save energy, then, by all means, go ahead and use a 9 V supply without a resistor... it will work, yes, but you will also be polluting the planet with burnt out LEDs that constantly need replacing.
answered Jan 5, 2023 at 13:55