0

I'm fairly new to this and I was wondering if it makes a difference if I put the current limiting resistor of a LED on the cathode or on the anode. Any "best practice" and why?

Thanks!

BadgerBadger
  • 137
  • 6

3 Answers3

9

Several reasons to put it on the high side. Though there is little practical reason to do it.

  • Its fairly common practice to put the resistor on the high side. You will make other engineers happy because they recognize what they see and understand its behavior. Most schematics are like that and its good practice to be consistent and to conform to standard.
  • If the anode gets shorted to ground, the LED simply wont come on, regardless which side you put the resistor. But its not the same both ways if the cathode gets shorted to ground. If the resistor is on the low side and the cathode gets shorted, nothing limits the current through the LED. You dont want the resistor bypassed because you could exceed the LED's current limit and destroy it. That's why putting the resistor on the side closest to the source is a good idea.
  • Testing LED's is easier if the cathode is already tied to ground; you wont necessarily have to tie the other probe to the cathode. Might make troubleshooting a tad bit easier. Just because its easier to find anode and ground than it is to find anode and cathode; limited space to put your probes may prohibit squeezing both leads in.
  • In the unlikely event that you are dealing with transmission line theory (extremely high frequency [Gigahertz] or extremely long distances [kilometers]), you run the risk of exceeding the current limit of the diode. If you put the resistor on the low end. In theory. This is because the source wont see the resistive load, thereby limiting current, until after its passed through the diode and met with the resistor. That's assuming the reflected signal translates through the diode to the source in the reverse direction at all (I havent thought about it). Depending on the duration and intensity of that transient, it could destroy the LED. But if current passes through resistor first then current will be limited prior to reaching the diode. I highly doubt you will need to consider that. You would know if its relevant by the circuits purpose. Plus, it doesn't make sense in my book to put an LED on a transmission line in the first place; a generic diode, maybe.

However, there are exceptions, depending on the circuit design. Suppose for example you used a BJT as a switch to turn on/off the LED. Depending on the design, it may be prudent to put the resistor on the high side, like normal, or perhaps on the low side. It depends on the biasing voltage available, etc. There are just other considerations, is my point. The explanation others and myself have given depend highly on the assumption of a simple circuit design.

CogitoErgoCogitoSum
  • 208
  • 1
  • 5
1

Either is fine, though if the LED is remote it should go on the high side (Vcc or whatever you are powering the LED from.)

Trevor_G
  • 46,364
  • 8
  • 68
  • 151
  • 1
    I assume by high side you mean the positive supply?? But that all depends on what the polarities are and which supply side ground represents. If you want to be safe in your wiring you should never connect an unfused supply to a wire going external. It can even be hazardous to wire a ground out through long wires since the potential to short to other supplies can occur. – Jack Creasey Apr 28 '17 at 18:35
  • @JackCreasey thanks, I guess obvious to me isn't to everyone :) – Trevor_G Apr 28 '17 at 18:42
  • @JackCreasey Thanks for the added details on the answer. So far everything is enclosed in a box, but I'll keep that in mind if I ever do anything with remote LEDs. Just to make sure, are LED strips considered external and therefore should be on a fused PSU? Because I see tons of kits sold with the generic wall wart PSU that is most likely not fused... – BadgerBadger Apr 28 '17 at 20:12
  • 1
    @Trevor : while playing around with my breadboard, I realize it's actually easier to put it on the high side since I'm using a common ground buss. It uses less tracks that way. – BadgerBadger Apr 28 '17 at 20:15
  • 1
    @BadgerBadger. Consider an example. You wire a Home control unit (runs on 12 V) and battery backup in your garage and run wiring to indicator LEDs and pushbuttons on a small PCB at your doors using low voltage cable. If you put the resistor for the LED or pullups for the switches on this small PCB ...you potentially have an unfused 12 V supply running through your walls. A short in the cables might not seem serious, but it might be enough to set fire to dust in the walls. – Jack Creasey Apr 28 '17 at 21:15
  • 1
    @BadgerBadger. If you are sure of the integrity of your ground run then you can limit any current in the wiring by putting resistance in series with sense lines. Any short is then inconsequential. – Jack Creasey Apr 28 '17 at 21:17
1

I would put the resistor in the positive supply, so:

Vcc -> R -> LEDA - > LEDK - > GND

This has no electrical benefit to the LED operation. But it has a few practical advantages...

If the LED is panel mounted etc, accidental shorting of the wires to GND or earth won't short the supply out.

When fault-finding, the LED terminals are being measured with respect to GND, letting you put the black DMM probe to GND and whip round your circuit terminals with the red probe.

TonyM
  • 21,742
  • 4
  • 39
  • 62