kinda newbie electronics question

pimpsmurf · Post by **pimpsmurf** » Thu Nov 10, 2005 10:15 pm

Ok, I bought 10, 12v 5mm Wide-angle red LEDs from radioshack (4 in the store, and 6 on order.)

I'm going to use 4 of those on a custom alluminum light fixture that mounts when the anti-roll bar holder goes in the rear. The other 6 are going to light some stuff on the inside of the body, and maybe 2 or 3 on the ground. (I know, I know, I'm a rice king)

Anyway, I want to use some low-voltage white LEDs for some headlights which will mount to another custom rig on the front. I figure 4 lights up front would be killer.

Now my issue is, I can't find a voltage regulator, and I have two requirements to use them. They can't mount to the body (no problem), and they must use my battery pack.

If I put them in parallel, they will all have the same voltage, but if I put them in series, will the voltage on each of the 4 be 1/4 the total of the power source?

A 9.6V battery pack would mean 2.4v to each of the 2.5v LEDs, which would rock serious ass. Am I off my rocker? "Teach yourself electricity and electronics" has me confused on voltage spliting...

Anyone have a clue? I don't want to burn $20 in LEDs if I can avoid it...

*Cross post bitpimps/halfeight*

-JNY

Cheesehead · Post by **Cheesehead** » Thu Nov 10, 2005 10:44 pm

10 2.5 volt LEDs? Odd. LEDs are usually 2.1.

It looks to me as if you're only considering voltage, and not amperage. Most LEDs cannot put out more than 20 mA, yours likely cannot take more than 50 mA. If you wired them in series, they would likely have a fine amount of voltage. However, they would promptly catch on fire, because LEDs have almost no impedeance.

Now, I'm no good at this myself, but here's my attempt:
Because you're using a 9.6 volt battery with 2.5 volt LEDs, you can't use more than 3 in series. (four would require 10 volts, and would'nt work as well.) Put 'em in a 3s3p configuration, which means 3 paralell sets of 3 LED's wired in series, for a total of 9.
I'm assuming that each LED draws 50 mA, each string would take 150 mA, and so would anything in series with the strings, such as a resistor. After the 7.5 volt draw of the LEDs, you would have 2.1 "leftover" volts.
This allows us to use Ohm's Law to determine the size of resistor you need.
Ohm's law: Voltage/Amperage = Impeadence.
So:
2.1/.150 = 14 ohms. You could use 3 5-ohm resistors in series, and it would probbably work nicely.

Now, will someone come along and point out the errors in my calculations?

betty.k · Post by **betty.k** » Thu Nov 10, 2005 10:44 pm

first up, you won't wanna run led's straight from a hefty power source without resistors. and you'll need separate resistors for the different coloured led's too.

as for the maths you've pretty much got it.

let's just say you're using 4x 2.4v, 20ma led's.

in series (wired +-+-+-+-, like a continuous curcuit) they will require 9.6v and draw 20ma.

in parallel (all the + wired together, all the - wired together) they will require 2.4v and will draw 80ma.

similar deal goes with resistors. in the case of 2x 10 ohm, 1 watt resistors;

in series they will handle up to 1 watt and have a resistance of 20 ohm.

in parallel they will handle up to 2 watts and have a resistance of 5 ohm.

and batteries too! if we have 2 x 1.2v, 1000 mah batts;

in series they will supply 2.4v and have a capacity of 1000 mah's.

in parallel, well you get the idea now

pimpsmurf · Post by **pimpsmurf** » Fri Nov 11, 2005 12:19 pm

Damn people. you are like Al Qaeda with the simultaneous attacks! haha

Great info! This all makes sence now. Sometimes you just need someone to rephrase something for it to sink in... Much obliged sirs!

*EDIT*
Now that I've had time to read this over, I'm confused again. The red 12v leds have an integrated resistor and are working great. The LEDs I've decided to use are these:
http://www.radioshack.com/product/index ... =techSpecs
They are 5v (6v max) 30ma max blue leds. I'm planning to use a 2s2p setup with 4 of them on the front. So I need to use a 320 ohm 1/4 watt resistor on each of the parallel circuits? If I got this right, I'm tatooing ohm's law on my forearm...

-JNY

PS. Bush just had a speach where he grew some balls... congradulations. I guess they won't kick him out of skull and bones now (no girls allowed)

Cheesehead · Post by **Cheesehead** » Fri Nov 11, 2005 5:10 pm

12 v LEDs should just be wired in paralell, connected directly to the pack. On 9.6 volts, you're likely to have 'em be a little dim.
The 5v 30mA LEDs you'll want to run in paralell, as well. (1S4P). Running 2 in series would leave each with less than 5 volts, and that's not a good thing.
Assuming a 5 volt drop, there would be 4.6 "leftover" volts.
4.6 volts/120 mA (for 4 30mA LEDs) = 38.3 ohms. A 40 ohm 1/8 watt resistor wired in series with the lot of them should work hunky-dory.

pimpsmurf · Post by **pimpsmurf** » Fri Nov 11, 2005 6:17 pm

A man is allowed to change his mind too. I decided blue headlights would look too much like a cop car... and KRS-1 says "that's not hot". So I decided to blow the few extra dollars and get 2 white LEDs. They are 1100mcd, 3.6v (4v max), 20ma, 100 degree viewing angle . woot. Should be hella visible. So I'm thinking it is going to be pretty nice (better be for $5+ ea!)

So, for 3.6v, I would want to put them in series because there is enough voltage to accomodate it, right? So 2.4v left over means I need a 120 ohm resistor in the circuit. If I put them in parallel, there would be a 150 ohm resistor. correct? 1/4 watt because .144w > .125 (1/8th watt), correct?

Maybe this time I got it right. Cheesehead, you the man. mad props.

-JNY

Cheesehead · Post by **Cheesehead** » Fri Nov 11, 2005 7:06 pm

You might want to just go with wiring them in series. It's not efficient, but LEDs, as it turns out, do not like being wired in paralell. (Check it out at Wikipedia. The same problem does not apply to standard bulbs.) Plus, the efficiency should be higher.

A 120 ohm resistor is required, but RS is prone to giving some rather overstated specs on their LEDs (or so I've read), so you may as well go with the 150. If it's not bright enough, a replacement resistor is'nt likely to cost you much.

betty.k · Post by **betty.k** » Sat Nov 12, 2005 2:00 am

white, blue, ultra violet, etc, led's usually require 5v to operate properly.
red, yellow, green, orange, infra red, etc, are happy with 2.4v.

pimpsmurf · Post by **pimpsmurf** » Sat Nov 12, 2005 7:52 am

typically betty.k, but are the ones (like the 12v 5mm reds I got) only able to do 12v because of the integrated resistor?

-JNY

Cheesehead · Post by **Cheesehead** » Sat Nov 12, 2005 10:33 am

pimpsmurf wrote:typically betty.k, but are the ones (like the 12v 5mm reds I got) only able to do 12v because of the integrated resistor?

-JNY

Yes.

Also, 5v directly (sans resistor) will fry any LED I've seen so far.

betty.k · Post by **betty.k** » Sat Nov 12, 2005 3:23 pm

well it depends. if you're using those little button cells you can get away with no resistor as they can only supply limited current. i have white led lights on my bike and they're powered by 2x 3v button cells (6v in total).

pimpsmurf · Post by **pimpsmurf** » Sat Nov 12, 2005 5:12 pm

Unfortunately, these are going on my United RC Xtreme 42s pack, so I don't think that would be a good idea.

Something awsome I've discovered... the mounts on the arms for anti-roll bars (which I don't use when the track is loose like it has been), are 5mm wide! =)

The 5mm LEDs fit in PERFECTLY which puts them 1/2 an inch from the ground. A tiny dot of hot glue makes for sweel ground effects

I have 2 on the rear wing mount, 4 on the lower arms, and 4 left over to shed a little light on the inside of the body (white backed lexan should look nice when it's done.

I've got to hit up the LHS for a small piece of alluminum to make the headlamp holder. I've got the design all worked out!

It's going to look fantastic at night. I can't wait to get my digi video cam out of the shop. They "don't know when" it will be fixed. buncha' putzez... Even have a camera man on standby and they can't get off thier ass and do some work.

-JNY

Cheesehead · Post by **Cheesehead** » Sat Nov 12, 2005 6:27 pm

betty.k wrote:well it depends. if you're using those little button cells you can get away with no resistor as they can only supply limited current. i have white led lights on my bike and they're powered by 2x 3v button cells (6v in total).

"Button", or zinc-air batteries, tend to have a very high internal resistance. 3 in series is probbably 15 or more ohms.