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[Josik Veshok'Woor]

Hi guys, sorry if this is a stupid question, but apparently I'm not very good at electronics haha.

I have (what I thought would be) a simple circuit for some LEDs in my gun. Two are blue and two are green:


They all worked great until maybe a few hours after I installed them all and the the green ones stopped working. I think it's due to the power from the 9V battery shorting tem out (strange how the blue are still working though?) and I think I'll need a resistor or two.

I'm trying to find out, what kind/how many resistors I will need?
If anyone knows please enlighten me!!

[Fi-8015]

You simply fried your green LEDs, they need a voltage of 4,2 but you supplied them 9V instead. They couldn't handle that. The blue ones are still working, because they need 8V and have a bit of tolerance.

You need to add a resistor in each branch of your circuit.

You can calculate them with this formula:

R=U/I

R: resistance
U: voltage over the resistor
I: current through the resistor


How to get "U": this is the voltage that the resistor  has to take, it's the difference between the supply voltage (9V) and the total of the forward voltages (8V for the blue LEDs and 4,2V for the green ones)
-> U_blue=1V
     U_green=4,8V

The "I" is determined by the LEDs, the blue ones draw 30mA, the green ones 10mA
-> I_blue=0,03A
     I_green=0,01A

This way you get the values for your resistors:

R_blue=1V/0,03A=33,33Ohm
R_green=4,8V/0,01A=480Ohm

If you can't find resistors with exact the calculated values take the next larger one.

I hope this was helpful.

[MMcFett]

You can also try looking here for a quick education/tutorial.
I found it rather interesting! Someone else in the forums posted it, but I can't find that post right now.
https://learn.adafruit.com/lets-put-leds-in-things/overview

[Josik Veshok'Woor]

This way you get the values for your resistors:

R_blue=1V/0,03A=33,33Ohm
R_green=4,8V/0,01A=480Ohm

If you can't find resistors with exact the calculated values take the next larger one.

I hope this was helpful.

Yes, very helpful, thank you  
I've gone out and picked up a bunch of resistors now and looking for ones in the right ball park.
I've got a 39K for the Blue and 5.6K for the green. The values are a decent amount higher so hopefully these will be ok?

So now it'll look something like this?


I've been looking at all sorts of tutorials online but it's just not sinking in 

EDIT:
Just put the circuit together with the resistors and the green LEDs are working but very very dim.
Because the only resistors I have a a way above where they need to be, can I instead achieve a lower value by using two smaller ones? Or will that not work?

[MMcFett]

Just put the circuit together with the resistors and the green LEDs are working but very very dim.
Because the only resistors I have a a way above where they need to be, can I instead achieve a lower value by using two smaller ones? Or will that not work?

I'm pretty sure resistance adds together, so you should be able to use 2 smaller ones that add up to less than the one you are using now.
Or try using just 1 smaller one and see. You could get a potentiometer and make it adjustable.
Check out this post Here!

[Fi-8015]

Those are about 1000 times larger than the ones you need. It's a surprise, that you get any response from the LEDs. The values have to match roughly the ones I've given you, lets say maybe 20% tolerance. Search for a 33 or 36 Ohm and a 560 Ohm resistor.

[Legion]

Looking through this, I might advise hooking up your LEDs in parallel rather than in series as you are. As is, in the event that the LED closest to source burns out, you actually lose two lights since power no longer reaches the second. It also makes finding out what size resistor you need easier by using this calculator.

[Darth_Vemon]

What I use can be a bit more expensive, but I use 24 VDC rated industrial LEDs.  A 9 volt battery powers them without issue, but I still need the resistor so I don't overheat the battery.  I didn't add a resistor in my thermal detonator, and it almost blew up.  Kind of ironic.

They are also a lot easier to wire and solder.  Although, they are generally larger and longer, being a complete assembly rather than just the bulb and two leads. 

I'm guessing any electrical supply company should stock them.  I get mine from work.

[Josik Veshok'Woor]

Those are about 1000 times larger than the ones you need. It's a surprise, that you get any response from the LEDs

D'oh!
That makes sense. I think I saw the number being in the thousands and assumed it'd be one labelled with "K".
But I've got that corrected and installed now and it's loads better. Thank you very much for your help vod!

If they ever do blow later on, I probably will look at getting some beastier ones but just waned to use what I had first of all.

I was beginning to doubt my sanity over this for the last couple of days, so you've all helped me a lot!
Thanks 

[Fi-8015]

You're welcome, I'm always happy to help.