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I bought my first LED strip and was a bit surprised that resistors are used to limit the current.

When connecting only one or two LEDs it is an obvious solution, but when ther are several hundred LEDs, why don't they use the voltage drop per diode to construct a resistor free setup. After all, LEDs may be efficient components, adding resistors just consumes energy, and worse, add heat to the strip.

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    \$\begingroup\$ Because what you suggest is not practical. If you give us the circuit you think of we can point out why. \$\endgroup\$ Commented Oct 16, 2016 at 7:23
  • \$\begingroup\$ Thanks for your replies. I don't mean putting all LED's in series, but in strings of maybe 4, 5, or six LED's, depending on the voltage drop. If voltage drop is, let's say 3 V, why don't use series of 4 LED's in series (and these short series again in parallel of course). I have the impression you don't need a current source, but just a voltage source of 12VDC as the voltage along each LED is taken down to it's drop. \$\endgroup\$
    – markjoe
    Commented Oct 16, 2016 at 10:40
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    \$\begingroup\$ If that reasoning were correct we could use 4 LEDs and a 12V voltage source. The ugly thruth is that the volatge -> current function of a LED is ill-defined (depends on manuafacturer, production run, even individula LED, temperature, etc) so applying a fixed voltage will never give good control of the current. \$\endgroup\$ Commented Oct 16, 2016 at 13:23

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Your question is not to difficult to answer.

  1. By connecting each led to its own resistor it becomes very simple to cut the strip to length without losing a whole section of led due to a wrong cut.

  2. During the production process the two carriers can be connected with led resistor combinations. That makes the production process very simple.

  3. When a led fails you only loose one led in stead of a whole chain

There are strips with leds in series with marked cutting positions but mainly the leds are put in parallel over the supply line.

A different aspect is that when putting leds in series you are in need of a current source and not a voltage source. That also make is for the non professional user more difficult to handle ledstrips with series connection.

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If you put LEDs in series, then indeed you can have a reduced number of resistors. As you say, their voltage drop provides the exact type of drop that a resistor does. But if you have a hundred LEDs in series, then all of a sudden you need 100x the voltage drop to drive them. If they've each got a 2V drop (to make the maths easy), you'd need a 200V supply!

However, now all of the LEDs are controlled by one voltage - you can't turn them individually on or off. If you could, and turned half of them off, then the other half would be being fed by double their rated voltage - poof!

So the LEDs in a strip aren't in series, they're in parallel. That's why you only need a 5V (or whatever) supply - but a higher amperage one. Now the wiring is low-voltage safe, and you just need a beefy power supply at 5V (or whatever) to supply them. You can turn them all off, reducing the current to basically zero, or turn them all on, increasing the current to 100x 20mA (or whatever). However, each LED will be powered by 5V - so each LED needs its own, individual voltage-dropping resistor.

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    \$\begingroup\$ They also make 12V strips with LEDs in series groups of 3. \$\endgroup\$ Commented Oct 6, 2020 at 10:06
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You can design your own LED strip without resistors by buying individual LEDs online and using a constant current led driver.

LED strip length depends upon the customer requirements. Imagine if a customer wants 100 meters LED strip how many LEDs we have to provide and we have to increase voltage for each rise in meter by keeping current constant. Resistors are placed in strip because small changes in forward voltage can result in very large forward current changes. Those resistors are forward current limiting resistors.

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  1. LEDs output voltage does not match with input voltage, constant voltage systems are cheaper than constant current systems so we are using constant voltage system.

  2. LEDs are non linear, resistors are linear devices, small rise in voltage level causes significant rise in forward current.

  3. Rising temperature forward current goes up in summer forward current would be high and in winter forward current is low. Also when the LED strip heats up the rise in temparature causess a rise in forward current of LEDs. It is called thermal runaway and it causes failure of strip.

  4. Current limiting resistor helps mitigate the effect of voltage increases by virtue of its linear curve. Additionally, resistors behave oppositely from LEDs in relation to its temperature - as temperature increases, resistance also increases.

  5. LED devices are inherently current-controlled devices, and do not respond well to fluctuations in voltage.

If you are building an LED system using constant voltage power sources, you must absolutely be prepared to use current limiting resistors to ensure stable and safe operation of LED devices.

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It is possible to make a series/parallel arrangement of LEDs without resistors. Here is an example:

enter image description here

But this comes with several compromises:

  1. LED forward voltage must be matched for good current sharing between parallel strings, which implies:
  • Either LEDs must be sorted at the factory to make sure their forward voltage is matched for good current sharing between parallel strings, which adds cost, or the fabrication process must be tight enough to ensure Vf match, which also adds cost.

  • Since Vf depends on temperature, all the LEDs must be at the same temperature which means a metal core PCB on an aluminium heat sink, not a flexible strip. If the adhesive on a flexible strip fails and it unsticks, or if the user doesn't stick it on a proper heat sink, the LEDs on that spot will get hotter, and without current setting resistors, these LEDs would hog current, get even hotter, and end up burning.

  1. The "strip" can't be cut to desired length by the user since it's a metal core pcb. It is much harder to use.

  2. Drive voltage is not standard 12-24V, and it requires constant current drive, not constant voltage drive

  3. It isn't compatible with existing 12-24V constant voltage PWM dimmers

The other side of the compromise is better efficiency since there are no resistors.

Thus you'll find this setup in high power lights where the efficiency gain is worth the hassle. Efficiency doesn't just save money on electricity, it allows the manufacturer to save money on heat sinking. It is also used in pre-packaged lights where the driver and the LEDs are sold together.

But it is not practical for flexible LED strips. You get useful features like ease of use, flexibility, ease of driving, at the cost of lower efficiency.

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schematic

simulate this circuit – Schematic created using CircuitLab

So here let's consider if all the LEDs have a 2V drop then a total of 6V drop across all the LEDs. 1V will drop across R1 and due to this 1V/10Ω = 100mA current will flow.

If we don't use a resistor then 7V will cause a large current to flow and the LEDs will burn.

1. For limiting current to a certain level, a resistor is used, it is a low-value resistor so that power loss is minimized.

2. If voltage spikes come due to any reason it helps ptevent the LEDs from burning.

But if an LED driver is used such resistors are not needed.

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