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I bought LEDs with a built-in chip which make them blink. The type of the LEDs are Kingbright L-56BSRD-B. The complete spec sheet is here.

After reading the spec sheet I do not know if this LED needs a series resistor.

The spec states:

  • With built-in blinking IC.
  • Operation voltage from 3.5V to 14V.

But then it has a row with the forward current:

IF Forward Current, Min:8mA, Typ:22mA, Min:VF=3.5V, Typ:VF=5V

Do I need a series resistor for this LED if I stay in the specified operating voltage range?

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  • \$\begingroup\$ Didn't know this was a thing, thanks for enlightening me, so to speak! \$\endgroup\$ – Dan Aug 18 '15 at 18:15
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No you don't need a series resistor. On page 3 you see that the current is only rising a little bit depending on the voltage. On normal leds the current almost doesn't depend on the voltage

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  • \$\begingroup\$ A perfect! I absolutely missed this diagram. \$\endgroup\$ – Flovdis Aug 18 '15 at 16:51
  • \$\begingroup\$ I wonder if the chip uses die temperature to control the blinking? Normally a chip with a built-in current-limiting device would have to dissipate more heat than one without, but if the device switches on until it reaches an upper temperature threshold, and then off until it reaches a lower one, the heat dissipation could actually be useful rather than posing a problem. \$\endgroup\$ – supercat Aug 18 '15 at 17:57
  • \$\begingroup\$ @supercat I don't believe they do. Evilmadscientist did some intersting analysis of a "random" flickering LED, and from previous spec sheets I think the regular flashing ones switch the current in a similar way. \$\endgroup\$ – Chris H Aug 18 '15 at 19:07
  • \$\begingroup\$ Temperature dependence would be too slow for switching also on small scale i think it's not enough fast. I think it's a pwm or a step down converter, which generates a smaller voltage (current limiting included of course) \$\endgroup\$ – Sider Aug 18 '15 at 19:35
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No, no resistor is required, the chip takes its place.

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Normal LEDs are prone to go into thermal runaway when run from a fixed-voltage source without a series resistor. As the temperature increases, the amount of current they will pass at that voltage will also increase, until they've gotten so hot that they are at least partially destroyed. Blinking LEDs incorporate a temperature-sensing element, which will shut them off before they reach damaging temperatures. The more current is allowed through the LED when it is on, the less time will be required for the LED to get hot and shut off, but unless the drive voltage is excessive, the LED's behavior will thermally self-regulate so as to prevent damage.

Incidentally, I'd long wondered why LEDs with blinker chips were available in the 1970s, but LEDs with addressable control chips didn't appear until decades later. The reason is that the blinker chips didn't need to worry about LED drive current or thermal runaway. If one tried to have an addressable LED use temperature-based current regulation, an LED which is lit continuously would be much dimmer than one which is turned on for 100ms once per second since the current through the LED would be thermally-regulated while the current through the latter would not. Although addressable LEDs might have been usable for some purposes even with unstable brightness behavior, they would have been unsuitable for many of the purposes to which such LEDs are put today. That having been said, I would think it might be useful to have addressable LEDs include an "overdrive" option which would drive them unsustainably hard as long as they were sufficiently cool, so as to allow for sparkling or strobing effects whose brightness was higher than what the display could show continuously.

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