Electronic – Using resistors in a simple Led circuit

ledresistors

I'm using an 18650 (4.2v unknown mah) to power 14 leds (2v 20ma) in parallel with a 100ohm 1w resistor on each led. Everything has been running fine for 12 hours now with no problem.

My understanding is the resistors should drop voltage from 4.2 to 2v but my volt meter shows no change. Am I understanding resistors right? Any help in explanation would be greatly appreciated.enter image description here

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. How to measure battery and LED voltage.

When taking the measurement across an LED measure at the points shown on the left most voltmeter.

From the comments:

LEDs are flickering yellow 2.1 V - 2.6 V around 20 mA and operates as low as 1.8 V and as high as 3 V. ... VUPN527.

WOAH!

enter image description here

Flickering LEDs are not LEDs. They are an integrated package containing an LED and a blinker circuit. There is no datasheet available on the Flicker Flame Orange LED 5mm web page. The only specifications are:

Just supply 2.1 to 2.6 Volts DC and you are ready to go. They will NOT operate on any less than 1.8 Volts, and are quite dim at that voltage. You can run them at 3 Volts DC too, but you may shorten the life if you do that. Anything above 3 Volts is almost sure to kill the LED.

These use around or less than 20 mA each.

A couple of points to note:

  • The 20 mA current is not specified as peak or average. I suspect average.
  • The current will vary from close to zero while off to, I suspect, a lot more than 20 mA when on, depending on the voltage applied and the current limiting resistor.
  • The blinker circuit will cause some additional voltage drop so the lamp's on forward voltage will be higher than a standard orange LED.
  • Therefore the voltage drop you measure across your series resistor will vary between 0 V (LED off and no current through the resistor) and whatever the voltage drop is when the LED is on.
  • You (presumably digital) meter will give the average voltage. So, if the LED is on for 50% of the time and off for 50% of the time we would get the LED voltage switching between 4 V (off) and 2 V (on) at 50% duty cycle. The average is 3 V and that's what you saw on your voltmeter.
Related Topic