PC2.0 and accent LED resistor double-check

[Starwinder]

So I’m planning out my first PC 2.0 build and I’m trying to figure out what resistor to use for an accent LED. The LED that I want to run off the board is a green illuminated switch LED (3.3V 20mA). The board provides 3.3.V for the accent LED pads, so according to the calculations detailed on page 12 of the manual: (3.3 – 3.3) / 0.02 = 0? Is this correct? And if so, I’m guessing I should still put a resistor on the LED to be safe, right? I see that the store carries a few 1 ohm resistors (1 ohm 5w; 1 ohm 2w), which one would work better? Does it even matter?

Hmm … that one question just turned into five.

If I’m way off the mark on this, please let me know. Appreciate the help!

[Silver Serpent]

You should use a resistor. 1 ohm is a good value. Both the 2w and the 5w are acceptable from an electrical standpoint, but space is usually at a premium inside a hilt. The 2w resistor is physically smaller, so I'd use that one.

[Starwinder]

Sounds good. I didn't know about the 2w resistor being smaller, so thanks for coming to my rescue Silver Serpent!

[hedgehog1]

For the accent LED, 20 ma will give you maximum brightness. If that is too bright, a high value resistor to bring the current down to 10 ma or even 5 ma can be used. It all depends on what looks 'right' on your saber.

Silver - the PC has a current driver on the board - do you still think the 1 ohm resistor is needed? I know if this were a NB then I would absolutely use a resistor, even in setups where it may be possible to run without one. But the PC it limits the current output to the max setting. Of course (come to think of it) you could set to max current and fry the LED (depends on the LED, really).

[Forgetful Jedi Knight]

For the accent LED, 20 ma will give you maximum brightness. If that is too bright, a high value resistor to bring the current down to 10 ma or even 5 ma can be used. It all depends on what looks 'right' on your saber.

Silver - the PC has a current driver on the board - do you still think the 1 ohm resistor is needed? I know if this were a NB then I would absolutely use a resistor, even in setups where it may be possible to run without one. But the PC it limits the current output to the max setting. Of course (come to think of it) you could set to max current and fry the LED (depends on the LED, really).

Well, if you like replacing popped LEDs, then you don't NEED a resistor. It doesn't hurt to use a small one though, think of it as "newbie insurance" for your accent LEDs. 1 ohm, 1W would be ideal, but like SS said 2W would be fine.

[Silver Serpent]

It really is just about insurance. It'll run just fine (most of the time) without a resistor. Using a 1 ohm resistor isn't going to noticeably alter the brightness. In my experience, the blue/green switch LEDs are almost painfully bright anyhow. I like slapping something a bit bigger on them to tone it down.

[Ari-Jaq Xulden]

As your still in the planning stages, you could consider having Tim bridge the smd resistor pad. Unless he has changed his method, it is done with a zero ohm link or jumper resistor. It is a 0 Ohm resistor. Then you could just wire the accents from the board.

[Starwinder]

Thanks guys for the info and confirmation of what I was suspecting/guessing - it's nice to know that I've actually learned something from all of my time spent here :p

@Ari-Jaq thanks for the suggestion, I hadn't even considered that as an option!

[hedgehog1]

Well, if you like replacing popped LEDs, then you don't NEED a resistor. It doesn't hurt to use a small one though, think of it as "newbie insurance" for your accent LEDs. 1 ohm, 1W would be ideal, but like SS said 2W would be fine.

OK - I guess I am confused. I re-re-read the first post, and I think when I saw the 5w and 2w ratings I was thinking the blade LED (which is current limited on a PC). But the OP was talking about the accessory LED still, wasn't he?

Even at a full 20 ma, a 1/4 watt resistor is what I have used for years for this type of LED. We are talking just 20 ma max here. Using a 2 watt resistor doesn't hurt anything, but uses up more space in the hilt. With the accessory output not current limited, I do agree that a resistor is very valid safety measure. In the end, I expect he will use a higher ohm value resistor anyway to bring down the accessory LED brightness to a reasonable level.

Sorry for any confusion I caused.

[Caine Drathul]

At 3.7V, you don't have to use a resistor for blues, greens and whites. The LED's forward voltage can sustain just fine at that battery's voltage.

I have never used a resistor for those colors and have never blown one.

[TrypWyr]

I think what it comes down to is "You should always use a resistor". The question then becomes "but should you really?"

Only you can answer that. Personally, I don't use resistors on accent pads for 3.3v 18mA LEDs. There are also times where I don't use them on high power LEDs either. But then, I'm prepared to replace them if/when I ever pop one. If you can accept the risk and your math is (triple-checked and) correct, go for it... but if there is any question:

"Always Use A Resistor!"

[Starwinder]

Sage advice indeed, and thanks! I plan on using a small resistor on this one mainly to step down the brightness of the switch LED. I definitely hear the argument for foregoing a resistor in order to save internal space - and I probably will heed that advice in a future build - but with the setup I have planned, it *shouldn't* (famous last words I know) be much of an issue and it's no big deal for me to do the extra soldering.

[Forgetful Jedi Knight]

Older and more experienced builders (like Caine, SS and myself) may take a shortcut or two here and there, because we know the potential risks and can live with the consequences. For the beginners, we tend to give them advice how how to do it the "safe way". ;)

[Jenny]

Okay, I understand why to put a resistor in there, and I even understand where in the circuit the resistor fits, but I'm going to stun you all with my newbish newbitutde, and ask... what do I anchor the resistor to? Or do I just cut and strip two pieces of wire, solder the resistor between them, cover the whole thing with shrink-wrap, and hit it with my hair dryer?

[hedgehog1]

Jenny:

Putting the resistor 'in-line' as you describe is a fine solution. As long as you cover it in heat shrink (which you plan to do), you are golden. Good thinking!

[Forgetful Jedi Knight]

Okay, I understand why to put a resistor in there, and I even understand where in the circuit the resistor fits, but I'm going to stun you all with my newbish newbitutde, and ask... what do I anchor the resistor to? Or do I just cut and strip two pieces of wire, solder the resistor between them, cover the whole thing with shrink-wrap, and hit it with my hair dryer?

Hedgehog is right. What you plan to do is fine. You should put the resistor on the (+) side of the LED lead (the longer of the 2 LED legs). ;)

[Jenny]

Thanks, fellows. In the next few months, I'm expecting to build three or four sabers, each with an NB board and an illuminated switch, so I needed to be sure on that.