Soarer Central: LED Resistor Selection

Soarer Central How-to and Tutorials LED Resistor Selection

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Russell Rohde
Tinkerer
Queensland
Soarer 1JZ 2.5L TT (JZZ30)

Posts: 63
Reg: 07-2006

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Votes: 1 (Vote!)

Saturday, March 08, 2008 - 12:30 pm, by: Russell Rohde (Ax0n)

LED Resistor Selection
Tutorial

By
Russell Rohde

Okay, So your going to mod your A/C, Radio, Shifter Selector etc with LED's. First thing you need to do before jumping into things is to select which LED you want to use (color and brilliance). Then select which resistor is needed to drop the 12V supply voltage in your car so the LED wont burn out.

LED's are like a One way valve for electricity. If installed backwards, they will not light up. It's wont damage them - you can just remove and turn them around. when Installed Forward, they will allow voltage and current through and the result will be Light - YAY! They are a much better idea than regular Globes as you have a greater choice of colors, you can select different brighneses (brilliance) adn they will last for years longer than a regular filament style globe.

Here is an example from the Jaycar website.

Red Rectangle:

If mA: 30
If mA (peak): 100

I = Current
f = Forward
If = Forward current ("no sh1t" I hear you say)
mA = milli Amps (rating of current)

This is the current range that your LED is required to be within - If current too low, LED wont illuminate properly, if too high, LED will burn out. Besto try and aim somewhere around half way. Closer to tolerence the LED is run, the shorter the life span. For this particular LED, I'd be aiming somewhere around 55 - 60mA

Min: 3.2
Vf (V) Typ: 3.5
Max: 4.0

This is the voltage required to 'fire' up the LED
Minimum of 3.2 Volts to start illumination, 4.0 volts max before burnout.
Vf = Forward voltage
Typ = abbreviation for "Typical"

Long story short, this particular LED should be used at current of 55-60mA with a voltage very close to 3.5V.

Blue Rectangle:

MCD Typ: 1000

This is the light output. For an LED in your car, I would not reccoment going above 1000mcd. Some LED's can go as far as 10000mcd or higher. choosing an LED this bright is plainly dump. It will WAYYYYYyyyyy too bright at night. Even 1000mcd is toward the upper threshold before it's getting 'too bright' at night.

Black Rectangle:

Viewing Angle: 20

This is (obviously) the angle of which the light comes from the led...

This viewing angle can cause 'Bright spots' if not installed correctly behind LCD displays etc.
A nifty trick to oversome this is to get a fine file and file the tip of the LED flat. make sure to leave the top rough - this will help disperse the light more evenly. you can even use a dab of hot glue on the top of the led after filing it flat (note though, hot glue sets semi-transparent and will therefor take away from the brilliance of the LED output)

One last thing to look at is the
Pd W: 130nW

Pd = Power dissapation
W = Watts (power rating)

130nW is 130 nano watts or 0.00000013 Watts

So...

Now you have your LED in mind, you can use some thing like this to select your resistor.

I would highly reccomend (kudos to Damian Ware for pointing this out) that when calculating your resistor for use in a car - Calculate using supply voltage of 14.5V as this is the actual output voltage due to the output voltage from the alternator (again, cheers Damo) - If you are not sure of the supply voltage (i.e. - in a dash where the lights re dimmable) get a multimeter and check it - where the LED's are being wired - not at the battery. You can buy a Very cheap multimeter from Jaycar for around $10 - wont be super accurate but will do the job for this.

http://led.linear1.org/1led.wiz

To help select the resistor you need, or, if you want to go more fancy, you can use something like this
http://led.linear1.org/led.wiz

The only difference is that the first one is for individual LED and resistor while the second is for multiple LED with single resistors.

Resistors have 2 ratings.
Ohms = value of resistance
Watts = Power handling capability

What you need to remember if using the second one to select multiple LED's is that the total of the Forward Voltages of all LED's can not exceed the supply voltage. If your new to this and stumbling through with it, Just use One LED per resistor. An 8 pack of resistors is about 50cents. It's not worth the screwing around. Also, what can happen (cos I've done it) is the LED's in series will be slightly duller or brighter than the individual LED's and can give crap final results if your using both methods in one device (i.e. - the AC contol module)

Also, the resistor calculators tell you out need a resistor rated at 'so may' ohms and Minimum of 1Watt (how much power it can handle) - Realistically, if you go the 1LED 1Resistor method, you will Never need to go above Quarter Watt resistor rating. (the LED I chose uses 0.00000013 Watts and a Quarter Watt resistor will handle 0.25Watts - well within tolerence and the resistor is physically smaller and easier to fit into tight places)

Here is how to determin the Positive and Negative (anode and cathode) side of an LED

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Easy enough to figure out from the pic above - Long Leg is the Posotive and the Short leg is negative. If you trim your LED legs and all of a sudden forget which was long and short, firstly, think to yourself "well, aren't i a dipsh1t" then look at the Blue Square above. The negative side of the LED will have the Smooth edge at the base.

****NOTE: It does not matter which leg of the LED your resistor is attached to. Anode or Cathode -It does not matter

****NOTE: Try www.jaycar.com.au for your LED specs. You can be fairly precise in your search. Try something like "5mm red LED" or "3mm white LED 1000MCD" - and there you go, all of Jaycars RED LED's are there to find. Get a part number and go to your local Jaycar dealer and pick them up.

I've also thrown in a resistor color chart for you in case you buy several different resistors and get them mixed up.

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This is easy and straight forward - no explination required.

for those who want to know how the calculators work - It's way easy.

Supply voltage minus LED Forward voltage divided by LED Forward Current (in WHOLE amps) equals MINIMUM resistance required

i.e.

12V minus 3.5V equals 8.5V

then

8.5V devided by 0.055A (55mA) equals 154.54 Ohms.

therefore, purchase the next size resistor up (I think is 180 ohm) and your set.

Hope this helps make the mods to you Soarer easier for those not overly electrically minded.