led resistor at Vss or Vdd

[fiveten]

I have seen many diagrams using leds with limiting resistors. Everyone has his or her own design. Some with a resistor then led cathode to 0v. Instead of having 5 resistors for 5 leds why can't you have a common resistor between cathode and 0v?

Thanks, fiveten

 

[Styx]

because you cannot be sure of the current sharing. With 5resistor you can limit it to 10mA (for each LED) but for 1res it would be set for 50mA, who is to say 1LED wont take the whole 50mA and kill itself then overstress the others

 

[k7elp60]

Stxy is correct. One way around the problem is to put the LED's in series if you have enough voltage. The current will be the same in the LED's and only one current limiting resistor is needed. The voltage varies with the current, but the forwrd voltage for red, yellow and green LED's @ 10 Ma the forward voltage is typical of 2V, for orange it is 1.9, for blue it is 3.8 and for white it is 3.2. You can mix colors in series. Just make sure the forward voltage of all the LED's is series is a few tents of a volt less than the applied voltage and then use a current limiting resistor to drop the difference.

 

[Oznog]

It happens with any component which is essentially constant voltage, especially those with a temp coefficient so the voltage goes down as it warms up since this means the warmest one takes all the current, thus warming itself further to guarantee it takes it all. The same problem shows up if you try to make a 5 amp rectifier by putting two 3 amp rectifier diodes in parallel. In some battery technologies, this can mess with a plan to charge them in parallel too.