I made this video to help some of the local hacker space people with understanding the how and why of the series resistor for LEDs. I hope it helps someone.

http://www.youtube.com/watch?v=B042Ow8hPdU

 

Hello IrelandMC,
Welcome to the forums.

Near the beginning of the video, you mention that if the Vf of the LED is equal to the supply voltage, the viewer has "lucked out".

This really isn't the case, as the Vf of LEDs will vary over temperature; it's a non-linear negative coefficient. So, if one simply connects an LED across a fixed voltage, the current can vary significantly over temperature.

Around 6:00 to 7:00 in your video, you're saying 3 diodes in parallel, but you're actually meaning connecting in series. If you had connected the LEDs in parallel, the Vf would remain about the same, but the circuit current would triple.

If you wire the LEDs in series, the total Vf will be roughly 3x the Vf of a single LED, and the current will remain the same as for a single LED.

Mentioning the impedance of the supply was quite good. However, you missed mentioning about the stability of the supply; regulated vs non-regulated/unregulated. If your supply has poor or no regulation, you need to increase the voltage drop across the resistor to keep the LED current more constant, or else use active regulation.

Most hobbyists just starting out won't have a fancy bench supply like yours. However, they can very quickly and inexpensively make a constant current supply using a "wall-wart" type plug supply that outputs from 8v-15v DC, one LM317 regulator, and a 62 Ohm resistor connected between the OUT and ADJ terminals. Then they can use an inexpensive DMM/multimeter to read the voltage drop across the LED.

In a given batch of LEDs, most of the Vf's (75% - 85% or so) will be fairly close, but the remainder can be off by as much as ±10%. This can cause difficulties if people assume that ALL LEDs in a batch will have about the same Vf, particularly if they are using low values for the current limiting resistors.

I start off by subtracting at least 500mV from the supply voltage (depending on how well the supply is regulated), then dividing that remainder by the Vf to see how many LEDs I can operate in a series string:
Rlimit >= (Vsupply - 0.5v) / Vf_LED
And then take the integer value of that result for the max number in series.