# Does an LED connected to a C.C. Power Supply require a series resistor?

For the purpose of creating/repairing a lighting fixture where one or many LEDs are the only component in a circuit driven by a constant current power supply, is there any benefit to using a series resistor with the LED(s) if the forward voltage after the LED(s) is 0V?

Let's say 5 LEDs, given:

• Power Supply Vout: 14-20V
• Power Supply CC: 350mA
• LED Voltage: 3.2 - 3.4V
• LED rated current: 280 - 350mA

Since the current is being regulated by the power supply itself, and the voltage will match the necessary voltage for the circuit, can the LED(s) be the only components in the circuit and the series resistor omitted?

Thanks!

• Yes. A true CC source can safely drive a series string of LEDs. May 25 at 7:08
• potential benefits: limit inrush/failure current to LEDs, increase stability (dampen oscillations on noisy current sensors), act as a fuse (metal film only), allowing cheap/simple ad-hoc external current monitoring, and tweaking brightness or dissipation. May 25 at 7:22
• The question in the title and the question in your text require the opposite Yes/No answer to each other. May 25 at 7:25
• The LED doesn't need a resistor, but there may be rare cases where the CC supply does. In your example, 5 LEDS are OK but 1 or 2 leave most of the voltage drop in the CC supply. If it was linear (unusual!) that could cause it to overheat, and an extra resistor to increase the string's voltage might be needed. May 25 at 11:09

When an LED / string of LEDs is powered from a constant voltage source then the current can vary a lot. A voltage that is a little too high can result in a much too large current which can damage the LED(s). So in that case, a current limiting resistor is needed.

When an LED / string of LEDs is powered from a constant current source then the current is set by that source so whatever the LEDs do / how they behave, the current remains constant. The current is set by the source (LED driver) so a current limiting resistor is NOT needed.

Since the current is being regulated by the power supply itself, and the voltage will match the necessary voltage for the circuit, can the LED(s) be the only components in the circuit and the series resistor omitted?

Correct.

And you need 1W LEDs that run on 350mA without overheating.