Simple LED in series Q

Discussion in 'General Electronics Chat' started by jappy, Nov 28, 2008.

  1. jappy

    Thread Starter Member

    Nov 28, 2008
    Hey All,
    Please refer to the attached image:

    My Question is, does the circuit at the bottom of the image equal the circuit at the top of the image? I have 4 LEDs in series with a 220ohm resistor powered with 12vDC to run the 4 LEDs at 20mA. Does the circuit at the bottom, provided the same specs, run the LEDs at 20mA?

    Thanks for any insight.
  2. leftyretro

    Active Member

    Nov 25, 2008
    No not the same. Each string of four LEDs must have their own 220 ohm resistor installed.
  3. jappy

    Thread Starter Member

    Nov 28, 2008
    Thanks Leftyretro. Does it matter which side the resistor is on?
  4. SgtWookie


    Jul 17, 2007
    Just so you know:
    The 220 Ohm resistor is limiting the current through a single string of 4 LEDs to 20mA.
    If you tried placing four strings of 4 LEDs in parallel with a single 220 Ohm resistor, each string would get around 5mA.
    You might be tempted to decrease the resistor to 55 Ohms which would then give 20mA per string, but this is very risky!
    LEDs don't all have the same forward voltage (Vf) even in the same batch. It can vary by as much as 10%. This means that one of your strings may draw more current than the others, due to a lower than average Vf on one or more of the LEDs. As the string draws more current than the others, it heats up more, which causes the Vf to get even lower, drawing even more current; this is commonly called "thermal runaway".

    Eventually, the string drawing the most current has one or more LEDs fail, resulting in an open circuit. This forces the other three strings to share the 80mA that used to be shared by four strings. Now the remaining strings are all running with an average of 26.7mA current, which is over their current limit. The rest of the strings "pop" in rapid succession.

    If I have a choice, I usually put the resistors on the anode end (more positive side) of a string just to be consistent. Current is in series however, and it really doesn't matter at all where it is in the string, just as long as it is there. In a 4 LED string, it could be at either end, or between any of the diodes. To current, it's all the same.
  5. jappy

    Thread Starter Member

    Nov 28, 2008
    Thank you SgtWookie, I am going to separate the strings into individual 4 LEDs per series with their own 220 r.