Parallel Circuit question?

When connecting LEDs in a circuit in parallel, why do you need a resistor for each LED or LED series within the parallel. Couldn't you use 1 resistor connected to the power supply before the current even got to any of the LEDs in parallel?

westfw10 years ago
If one of the parallel LEDs has a lower Vf than the others, it will end up "stealing" the majority of the current (possibly to its destruction, if R was calculated to supply ALL the leds.) You can see this pretty easily using an old red (Vf ~= 1.8V) and green (Vf ~= 2.2V) leds in parallel, fed from a single current limiting resistor... Only the red LED will light...
jarv3410 years ago
In an ideal world this wouldn't be a problem, most resistors you have lying around are probably rated at 1/4W meaning that the voltage across the resistor multiplied by the current it is sinking must be less than .25 (usually you try to be conservative so pick something less than that) hope that answers your question
Goodhart10 years ago
As Nacho points out, one or more LED's burning out could result in disaster, create a current-sensing circuit that would detect the rise in current and shut it down, working like an electronically controlled breaker. Sounds too complicated for me.....I would just add a resistor to each LED myself.
NachoMahma10 years ago
. You can, but if one of your LEDs opens, then there will be less current flow through the single resistor, which will decrease the voltage drop, which will supply more volts to the remaining LEDs, possibly causing them to burn out.