Opening note. LED, led, led's, l.e.d..all refer to the same device.
LED's are basically a bit like light globes, but much tougher.
If you pass electricity through them, they glow.
Too much, they die. Not enough, they dont shine at all.
A few points. An LED cant quite match an incandescent light globe for sheer light output yet.
A halogen downlight, will kick a LED'S bum for lighting up your boot.
However, leds provide a nice "focused source". They also run off sweet stuff all power, have
a nice focused dispersion, and a very pure wavelength of colour.
They are also shockproof, and treated nicely last for ten thousands of hours.
LED stands for "Light Emitting Diode". If you know what a diode is, you know about polarity.
Basically, it means a LED will only work one way. They have a positive and a negative side.
Ordinary LEDs have both a short leg and a long leg, from the factory. (not luxeons, more on that in a second)
The longer leg of an LED is the positive one.
You should mark those legs somehow, if you trim them down (which you almost always will).
You can see a longer and a shorter leg in the image below.
You should note that while the led will only work in one direction, it can still be blown up if you try to connect it, either way,
to a voltage beyond its design.
Wiring up multiple LEDS is a fairly simple process. Your car's voltage ranges from 12v to 14.8v.
An l.e.d, however, may only need 2v to run. If you try to run it off 12v, you'll blow it up.
We use resistors to control the amount of current going to an L.E.D.
This is necessary because LEDs, like any device that converts energy from one form to another, are NOT perfectly efficient.
Crank them at high current, they get hot. The hotter they get, the more current they pass, and hence they get even hotter.
LED's have to have resistance, a resistor, somewhere to stop this happening.
They can be used for some fantastic effects. Firing them into the edge of perspex, for example, yields a result like this
Resistors. What they are. Why we use them.
Correctly picking resistors is extremely important. Its also the hard part of planning out an L.E.D installation.
Heres a basic rundown on how they operate.
A resistor...impedes current flow.
The amount of resistance, is measured in Ohms.
The maximum amount of current they can dissapate, is measured in watts.
When they dissipate current, they get hot. They can get very hot.
If your resistor is connected to say, 3 leds with a maximum draw of 5watts, but can only dissapate 1 watt, its going to explode.
In this situation, you'd use a bigger resistor with the same ohmage (resistance), and higher wattage (dissipation capability).
Bigger resistor = more surface area to dissapate heat.
A high ohmage resistor will restrict the current too much. Your l.e.d's wont light up.
Too low an ohmage, will pass too much current, and fry your l.e.d's pretty much instantly.
How to calculate resistor values
There are three ways to wire up leds.
1. Single (one led only).
2. Series, (multiple leds, but a limited number)
3. Parallel-series (multiple leds, unlimited number, built in redundancy).
I'll walk you through how to calulate the resistor value for a single led first.
You need to read this to understand the basic concepts of how ohms, volts, and amps are applied in the world of led wiring.
If you dont know what these things are, dont stress!
You dont need to...you can simply plug in values into the equations below to get a good answer.
For a single led
Image by me.
To calculate the value's of the resistor you need, you must have these.
***Supply voltage. Units, volts. Use 13.5v for in car use.
***Led voltage. Units, volts. What it needs to operate.
***Max led Current. Units, Amps.The maximum current a led can handle.
- Note: most led current's are specified in millamperes. 1 amp = 1000 milliamperes. Divide milliamperes by 1000 to get a value in amps.
The equation is as follows:
(supply voltage - led voltage) / max_led_current
Example:
*Supply voltage, 13.5 volts.
*Led voltage, aka Forward Operating voltag, Aka VF. 1.5 volts.
*Maximum led current,
(13 - 1.4) / 0.020 = 10.6 / 0.02 = 605ohms
You just worked out that a 605ohms resistor will protect a 20mA (0.02amp), 1.5volt LED running on a 13.5volt circuit.
Sounds much harder than it is, doesnt it?
But wait, lets not forget about wattage. The maximum amount of power the resistor can handle.
The formula for working out how many watts a resistor must be able to handled is as follows.
W = I x V.
*W is "power in watts (What we want to find)
*I is "current in amps"..which we already worked out above. 0.02 amps.
*V, is voltage in volts. However, this time we are using the amount of voltage at the resistor, which is the source voltage minus
the total amount of voltage drawn by the leds. In this example, using one led, its
1x(led voltage) = 1.5 volts.
So, W = (0.020amps) x (13.5-1.5) = 0.24 watts.
Or about 1/4 watts. Choose a resistor with a slightly higher value (Say, 1/2 of a watt) to avoid the possibility of failure or fire.
Too high wattage value is not a bad thing. Too low a wattage value is.
Now, heres a nice big chart showing coloured bands you can expect to find on resistor, and how to interpret them.
Sure, a resistor at the store might be nicely bagged and labeled, but what happens when you get home?
The smart people would leave at least one resistor in the bag/pack, to compare to any that might go flying around.
Because you might not need it, and because of its size, your going to have to click the link to get the chart.
http://img239.imageshack.us/img239/4113/resistor9sq.gif
As you can see, a value of 580 ohms would be
4- band. 5, 8, x10... so Green/grey/brown
5-band. 5,8, 0, x1... so Green/Grey/Black/Black
6-band, 5,8,0, x1. Dont worry about tolerance and temperature co-effecient. So Green/grey/black/black/any/any
For multiple l.e.d's
In any series of leds, each led must be the same type.
If you have two different leds (size, colour, resistance), they must be run parallel-series (see further down) each with their own resistors.
You must not run different leds in the same series..
For multiple leds, The amount of voltage each l.e.d consumes is CRITICAL.
VERY IMPORTANT NOTE: If you have a 12v source, and 5 leds which consume 3v each...you have a 15v draw. Impossible..or is it?
When wiring l.e.d's in series, the total forward voltage MUST be under your supply voltage.
If its over, like in the above example, you need to go to parallel-series wiring, which is described further down.
But lets look at series wiring first.
In series. (Image by me). Note the resistor must be placed on the positive leg.
So say we now have three leds. Lets use the same type of led as in the "single" example.
The equation for working out resistance is still quite simple. It now becomes...
(Supply voltage - (led voltage x number of leds)) / (max led current)
So, supply voltage of 13.5v. Led VF of 1.5 volts. Led amperage of 0.02 amps.
In this case, its (13.5v - (3 x 1.5)) / (0.02 amps) = 450 ohms.
This is a lower ohamge than the first example (single led), we need to let more voltage through to power multiple leds.
Now, wattage again.
W = Led amps x (number of leds x VF).
Thus, W = 0.02 x (13.5- (3x 1.5volts)) = 0.02 x 9, which = 0.18 watts. Roughly 1/5th of a watt. Go for 1/4 of a watt at least
or greater (1/2) to be safe.
But wait, i want to run more leds than my supply voltage allows in series
But what if we want to use more led's than the supply voltage lets us.
What if our total VF is greater then our supply, 13.5v?
Say we want to run 16 of our 1.5 VF leds.
16 x 1.5 = 24volts. Way beyond our 13.5 volt supply.
Hmm.
What if we ran..say...two lots of 8?
Two groups. Each only takes up 12 volts. Yes, that could work.
But how?
Parallel - series, thats how.
Image by me.
You can see here, these two rows of leds "in series" with each other, each with its own resistor.
Each row runs "in parallel" with each other.
Do not wire individual LEDs in parallel on a single resistor. If wiring multiple leds in parallel, each must have their own resistor (as above).
Wire series-wired LEDs (as above) in parallel only when each series has its own resistor (as above).
The resistor value you use, is exactly the same as series.
If your running 5 leds on one branch, and 3 on another, you'd do two seperate series equations, and
use the resulting resistor on each group. If your running multiple single leds (bottom half of the above diagram) you'd get the resistor value by using the "single led"
equations.
Advantages of parallel-series wiring
1) you can run two different strips of leds. One strip of blue, one strip of red.
2) You can run different types of led's individually, say one blue, and one brighter blue. each will need their own type of resistor, parallel-series allows this.
3) you can run more leds than a single series group would allow.
4) If one led blows out, it only takes out the group its in. In series wiring, if one led blows, the all of them wont work.
Well. Thats it. Some additional notes/reading resources to follow.
