LED Wiring & Resistors - 1 'Power' Resistor?

[Necro]

I'm wondering - would it be possible to do one resistor for the entire line of LEDs on a CP since the LEDS are all running at the same voltage and the hot wire is the common on these?

Key points in my question:
- Using an Ultimarc PAC-Drive
- Planning on using 3.0-3.5v LEDs (Groovy Game Gear Blasters)
- Most extensive board will require 12 LEDs (240mA) which should be able to run directly from the board.

What I would like to do is make a resistor 'plug' that I can put directly into the power out 'ports' of the Pac-Drive, then have connect to my LED DB cable power wire (it's a swappable control panel setup).  Not all LEDs will always be lit up

This SHOULD work right?  Given I use a resistor that can handle this level of current? (and modifies the voltage appropriately)


A quick diagram would be like this (expand out to 12 LEDs):

5V----Resistor----------LED ------- ground wire 1 on PACDrive
                         |-----LED ------- ground wire 2 on PACDrive
                         \-----LED ------- ground wire 3 on PACDrive

(It's only going to be yellow and red LEDs, so same voltage)

[Necro]

I think I actually found the answer...no.  It would be fine if all the LEDS were always on, but because they aren't I can't do it.

I guess the other big question is having the resistors on the 'ground' wires for each LED ok? The LEDS will have the same voltage requirements across all control panels,  so it should be straight forward but there's conflicting guidance on where the resistor has to go (before or after the LED).

(I'm trying to simplify under the CP as much as I can - LEDS on the  PAC-Drive side are easier/cleaner)

[SavannahLion]

The LEDS will have the same voltage requirements across all control panels,  so it should be straight forward but there's conflicting guidance on where the resistor has to go (before or after the LED).

Unless you're doing something hokey, it doesn't matter if the resistor is before or after the LED. For example, there are bicolor LEDs that have no real anode/cathode so there's really no "right" place to attach a resistor, you just have to attach one. A tricolor LED, it would be cheaper to put it on the cathode side unless you want to fine tune the brightness (I guess) of the alternate colors.

[Yvan256]

I would also advise about testing one LED with your target resistor value before wiring them all up.

Just because a LED is rated at 20mA doesn't mean you have to drive it to its maximum. New LEDs are far brighters than the old ones from a decade or two ago.

[MonMotha]

In general, you should have one resistor per "string" of LEDs in series.  The reason for this is that the current/voltage characteristics of the LEDs does have process variance, and the relationship is exponential, so small process differences can make a big difference at application.  The idea behind the resistor is to swamp this exponential dependency with linear behavior of the resistor, but if you put the LEDs in parallel, they each see the same voltage, and the exponential behavior still applies to a great extent (I can explain this in more detail, but the math starts to get a bit complicated - just trust me).

The extreme case is that, despite your best efforts, the LEDs are so mismatched that one ends up taking the entire set current which would be several times the rated current (since you intended it to be split between all your LEDs).  Poof.

Will it work? Probably.  Then again, you might pop a few LEDs, too.  You'll also have to adjust the resistor value as compared to just a single LED.  Since you'll have more current (rated LED current x number of parallel LEDs), you'll have to set the resistor to drop the right voltage for that multiplied current.  You'll also want to round it up some (and accept a slightly dimmer LED) to avoid popping LEDs due to mismatch.  Mismatch will be especially evident if the LEDs aren't the same color or type.

If you're trying to control each LED as per your diagram, this won't work at all.  Don't bother trying.

If you want to light several LEDs from the same control line.  It's better to wire them in series.  You then add up all their forward voltages (at desired current) and set the resistor based on that (the math is as though it were a single LED with all the voltages added up).  This greatly lessens the problem of mismatch.  As long as you're close, you won't pop anything, but you may get slightly uneven brightness.

As for where you put the resistor, it doesn't matter.  As long as it's in series with the LED, it can go anywhere.