multiple LED array help

[grimxneko]

ok, so im a little confused on how to wire up these two different kinds of LEDs in the most optimal way. im actually trying to do a little something with my car by putting these LEDs in there (like under the seats and on the sides and stuff) -like this- so the wires connecting all this will be pretty long. anyway, im planning to use a 12v battery as the power source, and the 1st type of LEDs im using are **broken link removed**, and the 2nd type is **broken link removed**. so im just confused on how to wire all this together and determine the resistance for two different kinds of LEDs. any advice you can give would be great.

 

[MikeMl]

First kind: Blue, Forward Voltage (V) : 3.0-3.2, Forward Current (mA):<= 30
Second kind: Purple, DC FORWARD VOLTAGE:3.7V TYPICAL, DC FORWARD CURRENT:20mA

If you mix the two kinds, you will be limited to running 20mA per string. If you run them on separate strings, you can run 30mA through the blue ones, and 20mA through the Purple ones. I would recommend the latter.

Three blues in series: 3x3.1V = 9.3V. When the car is running, the supply voltage is 14.2V, so put a resistor in series with each string of three. Resistor is Er/Ir = (14.2-9.3)/0.03 = 163Ω, round to standard value of 162Ω. Power dissipation in resistor is E^2/R = 0.15W so a 1/4W resistor is fine.

Three purples in series: 3x3.7V = 11.1V. When the car is running, the supply voltage is 14.2V, so put a resistor in series with each string of three. Resistor is Er/Ir = (14.2-11.1)/0.02 = 155Ω, round to standard value of 156Ω. Power dissipation in resistor is E^2/R = 0.06W so a 1/4W resistor is fine.

 

[grimxneko]

oh. well thanks very much for your response, but... im not sure if that's so... accurate. just the supply voltage is supposed to 12V exactly, not 14.2. you see im not actually wiring this up to the car its self, im using a 12V battery. so i guess that also threw off the other calculations as well. thanks anyway though (though it did help me see more clearly the process of all this).
but ok, i just simply mapped out a quick schematic or what im thinking would be a correct circuit/ layout -here- (and please correct me if i laid it out wrong), but im just (still) having trouble figuring out the resistor values. so if anyone could help me, it's be greatly appreciated. (...im sorry, as you can tell im pretty new at all this -but still interested in learning the basics) ^_^;

 

[MikeMl]

Is the 12V battery the car's battery? Is the car running when the LEDs are Lit? If so, the battery voltage with the alternator charging is 14.2V +-0.2V. Trust me...

The resting voltage of a fully charged 12V lead battery is 12.65V.

 

[mneary]

On the other hand, if your "12V Battery" is eight NiCd or NiMH cells it will be only about 10V fully charged, and 9.6V most of the time.

 

[grimxneko]

nah, it's just a regular 12V battery form Energizer.

 

[mneary]

A regular 12 battery from Energizer is probably 8 "1.5V" Alkaline cells. (Most people call Alkaline "regular" batteries now.)

If you aren't using a lot of current, it will start out at about (1.6 x 8 = 12.8) volts. When you have used about 5% of the power, it will be about (1.4 x 8 = 11.2) volts.

From there, it will steadily fall to (1.1 x 8 - 8.8) volts, where they say you've received about 90% of what you paid for.

But if your LEDs stop glowing around 9.6 volts you are going to leave about 35% in the battery.

Reference: https://www.electro-tech-online.com/custompdfs/2010/06/alkaline_appman.pdf

 

[grimxneko]

oh, ok. that's quite interesting. thank you.
but... that still doesn't quite answer my question. i just simply want to know what resistant values i should use in this schematic/ circuit design? what resistors should i use?
im really sorry, i dont mean to be nagging or annoying. i just cant figure it out and would really appreciate help.
**broken link removed**

 

[MikeMl]

I gave you a method of how to calculate the resistor values and their dissipation. The method will work for any starting voltage, and any number of leds in series. How about if you take a hack at it, post your answers, and we will check your work.

 

[grimxneko]

oh, alright, ok, soooo... i gave it a shot; am i on the right track?
**broken link removed**

 

[MikeMl]

The formulas look right. I didn't check your math, but the magnitudes look about right. Do the Watts calc, too.

 

[Oznog]

Note that the voltage with the engine running is somewhat higher than with the engine stopped. 12.65v vs 14.2v is about right.

If you have a high number of LEDs in series and only a small voltage dropped on the ballast resistor, it's efficient, but small changes in source voltage can create very significant changes in LED current. The brightness is unpredictable.

In some cases it's helpful to use a voltage regulator, or a current reg on each string, or better yet a buck supply which puts out a regulated current.