RGB LED Newb Question

[markronz]

I recently got my two RGB LEDs from Paradise arcade.   Now I'm starting to play around with how to hook them up.
http://www.paradisearcadeshop.com/paradise-arcade-exclusive-products/556-rgb-5mm-led.html 
I'm relatively new to this sort of thing.   I've only wired up a normal one color LED before, the kind that comes in buttons normally.   Those are pretty straight forward.  They are 5v and have two wires needed for them.  One is ground and one is the +5V.   But anyway, this is my first time with a RGB led.   Is there just one ground wire, the black wire, and then the rest (the RGB wires) are each hooked up to 5v?  Or is it the opposite?

Then on Paradise website it says:
"Please note these do not have resistors installed so they will need a current control circuit (or resistor) to run properly."
I'm not entirely sure how this applies to my situation.  I am using this switching power supply from GGG to power the lights:
http://groovygamegear.com/webstore/index.php?main_page=product_info&cPath=93&products_id=364
Do I need resistors?  Anyone know if these RGB LEDs are 5v?   Can I just wire them into the +5V from my power supply?

Any help with all this would be most appreciated!

[PL1]

The four wires are 5v and 3 grounds for red, green, and blue.

Bryan doesn't spell out the specs for the LEDs very clearly , but he does mention "Resistor suggested: 1/8W VAL 120E".

Playing around with an online LED calculator, a 120 Ohm resistor works for a 5V supply, 2.8V LED voltage, and drawing less than 20mA. (fairly common specs)

This setup will dissipate 40 mW across the resistor -- use one resistor per channel. (red, green, blue)

Most LEDs operate at around ~2.8-3.3V -- if you don't include a current limiting resistor, the LEDs will burn up like a fuse.

The single color LEDs have the resistor hidden in the LED holder.



Protip: Buy a bag of resistors and match the resistance values for each button as closely as possible -- cheap 20% tolerance resistors are NOT your friend.


Scott

[markronz]

Thanks for the help so far!  OK, so I assume the resistor goes between the 5v line and the LED?  So the black wire is the 5v wire then right?  And the RGB wires are naturally the ground for each color?

I've never used a resistor before due to the sort of bulbs you have from your picture.   However, I did buy this big package of electronics stuff a while back.  I always intended to read this Make: Electronics book, but I haven't gotten around to it yet.   At any rate, I have a whole mess of resistors. See the attached picture.   They appear to be labeled:  100ohms, 220 ohms, 1K, 10K, 20K, 100K, 330K, 2.2KOhm, 4.7K, 1M and then the larger piles with the brown tape on the ends say, I think, 220 and 470.

You said 120Ohm before.   Does that mean none of these are the correct size I'd need?

And as sort of a follow up question, can I put heat shrink around the resistor once it's in place, so that it's not just hanging out there in the open?

[PL1]

OK, so I assume the resistor goes between the 5v line and the LED?

Disclaimer: I haven't used RGB LEDs and controllers so my advice is based on other threads on BYOAC, electronic theory and extensive experience with single LEDs.

Not certain which side the resistor for your setup would go on since the current draw of three channels would require the resistor to dissipate a combined 120mW -- a bit too close to the 1/8W (125mW) rating for my taste, but I always stay well within specs on these things.

The problem with having 3 resistors on the ground side is matching them up so all 3 channels balance evenly.

So the black wire is the 5v wire then right?  And the RGB wires are naturally the ground for each color?

That sounds correct.

I've never used a resistor before due to the sort of bulbs you have from your picture.   However, I did buy this big package of electronics stuff a while back.  I always intended to read this Make: Electronics book, but I haven't gotten around to it yet.   At any rate, I have a whole mess of resistors. See the attached picture.   They appear to be labeled:  100ohms, 220 ohms, 1K, 10K, 20K, 100K, 330K, 2.2KOhm, 4.7K, 1M and then the larger piles with the brown tape on the ends say, I think, 220 and 470.

You said 120Ohm before.   Does that mean none of these are the correct size I'd need?

None of those are 120 Ohm resistors.

When selecting resistors, there are several specs to consider:
- Value (Ohms) -- Determines how much resistance it provides.
- Wattage (Watts) -- Determines how much energy it can dissipate as heat, larger values like 1/4W are OK to substitute since they will run cooler.
- Tolerance (%) -- Determines how close to the stated ohms value the resistor can be i.e. a 20% tolerance 100 ohm resistor can be anywhere from 80 to 120 ohms, but a 5% is between 95 and 105 ohms.

And as sort of a follow up question, can I put heat shrink around the resistor once it's in place, so that it's not just hanging out there in the open?

That's what I did for the 1/8W 10KOhm pullup resistor on VCC (3rd pin from left) on the KADESTICK.




Scott

[wxforecaster]

The typical forward voltage for 5mm LEDs is 2.1V for red and 3.3V for green/blue, at 20mA.

Thus, a 100ohm resistor rated for 1/8W or greater will be perfect for the blue/green leads...and something around 150 ohms is good for the red lead. You don't have to be exact, but try to get close.

Of course these forward voltages depend on the semi-conductor used to create the red, green and blue...and can vary as much as 1.5V depending on the composition which is rarely known.

The best approach is probably to start with a 150 ohm resistor applied to each of the three colored leads. If you find one of the colors is too dull...work back toward 100 ohms. I probably wouldn't go much below that.

[markronz]

Thanks for the help everyone.  I talked to Susan from Paradise about it and confirmed that all four wires need to have a resistor on them.   They are going to sell me some resistors that should be sufficient for the joysticks.  So I think I'm all set on this!

[Nephasth]

I talked to Susan from Paradise about it and confirmed that all four wires need to have a resistor on them.

This doesn't make sense. You only need a resistor on each of the 3 grounds of an RGB.

[markronz]

Got me man.   They just said a resistor goes on "all wires".  Maybe I misinterpreted that.  Would it harm anything if the 5v line also had a resistor on it?  Or is it just the fact that it shouldn't be needed?

[Nephasth]

Would it harm anything if the 5v line also had a resistor on it?  Or is it just the fact that it shouldn't be needed?

Harm anything? No. But it may add enough resistance to the circuit (depending on resistor value) that it would prevent the LED from lighting.

[wxforecaster]

Folks, Google is your friend. The resistor goes between the supply voltage (+5V) and the cathode. There are 3 cathodes on an RGB LED, one for each color. Please read my post again. A resistor on the exiting anode lead connecting back to ground is not what you want. It's not going to harm anything, but your LEDs won't operate correctly.

[Nephasth]

Folks, Google is your friend. The resistor goes between the supply voltage (+5V) and the cathode. There are 3 cathodes on an RGB LED, one for each color. Please read my post again. A resistor on the exiting anode lead connecting back to ground is not what you want. It's not going to harm anything, but your LEDs won't operate correctly.

Did you know that there are two types of RGB LEDs? Common cathode and common anode. So it really depends on what type you're working with, but the most common type in this hobby is common anode since most LED controllers we use control the ground side of the circuit. In either case, positive voltage gets connected to the anode (the "entering" lead), not the cathode. You've got it backwards.

With a single color LED, it doesn't matter which side the resistor is on (but it does matter which side you apply positive voltage to). But since an RGB LED is three LEDs in one, the best practice is to install an appropriate resistor on each cathode of the RGB LED.

Edit: This is from your friend, Google:

[wxforecaster]

You are correct. I would not have expected anyone to buy a common cathode RGB LED from any of the arcade sources, but you're right, someone could also just pick up a load of them on Ebay on the cheap in which case you definitely need to be aware of which type you are dealing with. Great followup post.

[AndyWarne]

Whether or not you need resistors is dependent on what you are driving the LEDs with. If its a constant current source such as our PacLED64 board you don't need resistors at all.

If its a non-current limited source, as has already been noted the values of resistors might need to be different for each channel. One way to get an exact white balance is to connect pots to each channel and adjust so that the current draw of the highest channel is within spec (usually 20mA) and the white is actually white, then measure the pot values and fit resistors.