wiring up LED's

[ozboater]

hey gents... i m looking to wire up 6 led's for my slash... 4 for the front and 2 reds for the rear... going for a desert truck looks... i have downloaded the rpm instructions and it looks pretty clear but i m wondering if some electronic experts here can help me... sorry about the size of the downloads...

i d like to add 2 more sets of wires (for the rears) to the bottom set of instructions... would it be OK to just run 2 more sets of wires (4 total strands - to power the red RED led's) where the 4 sets of wires exit AFTER the 'connector' ??

hopefully that makes sense... i don t see any reason why it wouldn t work... maybe i ll need to put in different resistors or lower 'mcd' LED's ??

i ll be running the lights off the rx... it s a spektrum sr3000... using a dx2.0 tx...

i m piecing together the stuff i need this week and i d like to get this thing dialled ASAP... hopefully some people with alot more experience with wiring LEDs can help me out

cheers

[Diegoboy]

I didn't read through your post, I'm in a hurry right now, but from what I skimmed through, I think you may find this useful.

[ozboater]

thanks diego, i ll check it out

[ozboater]

link is good diego but it doesn t seem to give me the option to link up LED's of differing specs in a series...

[ozboater]

c'mon lads, someone must have half a clue (i have 1/16 of an idea of what i m doing here)

[BigBen2k]

It's somewhat easier than it looks.

First, you have to know that the voltage drop across an LED is fixed, and that each LED has a current limit (check the specs).

If you're doing LEDs in series, then you simply add up each LED's voltages, and subtract the total from your battery/supply's max voltage: the result is the voltage that your resistor must drop. (there will always be a resistor at the end of a chain of LEDs)

The current is determined by the LED with the lowest current maximum (typical LEDs max current is 20 mA). The same current will flow through your resistor.

Once you know the voltage and current of the resistor, simply use V=R * I to calculate the resistance value (V= voltage, I= current, R= resistance).

Example:
3 LED in series.
Spec of LED: drops 1.4 volt, max 20 mA
Battery: 7.4 volts max

voltage of resistor = 7.4 - (3 * 1.4) = 3.2
current through resistor = 20 mA (0.020 Amps)

Resistor value = 3.2 / .020 = 160 Ohms.

*always* pick a resistor that has a slightly higher value, to make sure that you don't exceed the current limit.

[ozboater]

bigben2k... thanks, that help me out for sure...

i m a little confused a to the voltage of the LED's...

the WHITE one have the specs: 3.2 Vf @ 20mA w 3200mcd


the RED ones : 1.85 Vf @ 10mA w 19mcd

so the voltage of the LEDS is 3.2 and 1.85 respectively ???

no problems then hooking up 4 WHITE LEDS and 2 RED LEDS even tho they are of different brightnesses ??

sorry for all the questions mate... can you tell that i m a newbie at this stuff :)...

in the above supplied RPM instructions, they have a resistor for every LED... do i need that or is just one resistor ok ??

cheers

[Diegoboy]

MCD = millicandelas
The unit of measure commonly used to describe LED intensity is the millicandela (mcd), 1000 millicandela equals 1 candela. Candelas measures how much light is produced as measured at the light source.


Your white ones are far brighter than your red ones.

Try getting some red ones that are more compatable to your schematic and brightness.
If they're 5mm try these
If they're 3mm try these

[ozboater]

i guess that they spec the lower mcd for the RED LEDs since they are for the rear tail light and are therefore meant to be dimmer ??? sounds reasonable...

the truck will be using 3mm LEDs all round.. thats what the canisters hold..

is it not recommended to wire up LEDs that differ greatly in the 'mcd' rating... i mean the whites are 3200 mcd and the reds are 19 mcd... is this too much of a difference ??

cheers again lads, i appreciate the help... i ll get this bastid sooner or later (with some help of course)

[Diegoboy]

You can wire them up no problem. I, myself, would like brighter red ones.

Here's your schematic.

[ozboater]

sweet diego... cheers

so there is no worries running this off of my spektrum rx ??? what voltage typically does a rx provide ??

i can make good sense of the diag... but does anything differ since i m only using 2 red LEDs ???

you guys have been a great help...

thank again

oz

[Diegoboy]

This is being powered from the RX? Hold on, that changes things...

[Diegoboy]

Ok... I fixed it

[ozboater]

dude, thats awesome... now i can get it going... i ve been procrastinating with this damn project since i didn t really know how to wire it up...

so that schematic is golden assuming 4 X 3200 mcd WHITE LEDs and 2 X 19 mcd RED LEDs ??? if i read above correctly then it really doesn t matter about the mcd rating... just that a different mcd rating will change the resistors i use... sound correct ??

do ya reckon it ll effect my run time significantly ?? i plan on putting in a simple switch so i can turn them off for daytime running... ought to be a sweet project, i ll post photos when shes done !!

[Diegoboy]

The MCD has no direct affect on the resistor, it's the fwd voltage and the current, but the mcd does affect the current, therefore affecting the resistor indirectly.
LED's draw such little current, you will never notice any runtime difference.

[BigBen2k]

"In series" won't work here, because the current to the two types of LEDs isn't the same (you'd end up with a white LED at less-than-half brightness).

Here's what you do: use my advice as above for the white LEDs, and run a separate chain for the red ones. Each chain will have its own resistor.

The supply voltage from the receiver should be the same as the battery you've got hooked up to it (If you're running a BEC/ESC, assume 6.0 volts).

[BigBen2k]

Oh I see... 4 whites at 3.2 volts, and 2 red at 1.85. That complicates things a bit.

You can't run more than one white LED, because you won't have enough voltage for them:
2 * white LED require 6.4 volts, and your supply only has 6.0 .

The two red LEDs in series will be fine, with the right resistor.

Ok, time for a new design.

4 white LEDs in parallel, will drop 3.2 volts, and run a total current of 80 mA, so the resistor will be:
V = 6.0 minus 3.2 = 2.8
I = 0.080
V = R * I
R = 35 Ohms or higher.

There you go (this assumes that all 4 white LEDs are *exactly* the same, and were manufactured from the same batch. Otherwise some of the LEDs might appear slightly brighter than others. To get around this, treat each white LED separately, with its own resistor, like Diego suggests).

[ozboater]

ok.. now i m really confused... RPM, in the first original attachment i used specced 4 X 3.2v LEDs to run off the rx... these are for the front set of 4 canister white LEDs..

is that even possible ??

ben, you think diegos schematic won t work ?? reckon you could sketch me out one that will work ?? if possible i d like to power all 6 LEDs from the rx... if thats even possible...

thanks again for the help from you blokes... i ve definately learnt something here... i m not sure what it is, but i ve learnt something...

[BigBen2k]

Yes, Diego's diagram will work. Mine's just a bit different.

Give me a bit of time, and I'll draft up something.

[Diegoboy]

My design is simple but effective (like old Vacuum tubes)

Ben's desgn will probably be more efficient with less components (like integrated circuits or IC's)

[BigBen2k]

I borrowed Diego's pretty diagram

For the two red LEDs, I calculated 230 Ohms:
6.0 - (2 * 1.85) = 2.3 volts
V = R * I
where I = 0.010 A (10 mA)
so R = 230

Now there's no such thing as a 37 Ohm resistor, but the next up, standard value, is 39 Ohms. You'll also find a 240 Ohm resistor to be standard.

To answer your other question: no these LEDs won't have any significant impact on your run time: they only draw 30 mA, which is insignificant compared to whatever electric motor you use. Just remember to turn them off when you're done though; they will drain your battery pack, in a few days. A 3'000 mAh pack will provide 3'000 mA (or 3A) for one hour, or 30 mA for 100 hours.

Diego: no, I'm not going to go into switching regulators here :) but I'm working on one for myself.

[ozboater]

great... looks like the project is moving forward... i owe you lads a cold one

thing i ll pick up the stuff to build it up today...

you fellas recommend 22 AWG stranded for wire ???

thanks a million you guys..

cheers

oz

edit - bigben2k, i m assuming your schematic takes into account the largish 'mcd' rating of the white LEDs i figure... i see you listed the specs of the red LEDs (2 x 1.85), but not for the whites... sorry to second guess ya but is this diag. all go for the white LEDs with a rating of: 3.2 Vf @ 20mA w 3200mcd...

[BigBen2k]

22 AWG ought to be just fine.

Measure twice, cut once

The mcd rating has nothing to do with how much power it draws. However... this is a significantly brighter LED (160x) for something that draws 4x the power. Maybe you could double check the mA that the white LED draws?

I have a couple of white Luxeon LEDs, and they are nothing like a standard red LED: they're mounted on a tiny board, and draw a lot more power than 20 mA. Is this what your white LEDs look like? It would draw at least 200 mA, and that's a whole different situation.

Maybe a picture?

[ozboater]

unfortunately i don t have anything yet... the white LEDs were recommended by RPM for their light canisters and i only have the part numbers... Linrose manufacturer, part # bcmd204uwc

this is them:
http://www.frys.com/product/1681761#detailed

i checked the linrose webpage and located the same LED part number...
http://www.linrose.com/

i clicked LEDs and scrolled down, they are listed just over 1/4 of the way down the page... available as single and in a five pack...

cheers again bigben2k, this project would be a pipedream without the help of kind folks like yaself and diegoboy

[BigBen2k]

Oh, ok, then everything's all right, cheers!

The Luxeon LEDs I have are similar to these:
http://www.luxeonstar.com/

They draw at least 300mA

(I'll be using them on my plane, as landing lights).

Diego: this is the kind of LED that could really use a switching regulator

[ozboater]

awesome, thanks mate... really appreciate ya help... diego too, great forum and some good folks around here... stoked

gonna cost me about $4 for each damn white led... little steep, but i hope it turns out good... i ll post some photos of the finished product !!

cheers

oz

ps - australia is gonna rip it up in vancouver in 2010