Leds can be Wired In Series: In series wiring the forward voltage drops are additive, but the current requirement stays the same If you had 6 red leds in series, then 12 volts at 20 milliamps is required. In parallel wiring, the current requirements are additive, but the voltage drop remains the same; for example, if each red led has a 2 volt forward voltage drop and a maximum 20 milliamps, and you put 2 in parallel, then you will need 2 volts at 40 milliamps to drive the leds to maximum brightness If you had 6 red leds in parallel, then 2 volts at milliamps is required.
The object is to find which method or combination of methods gets your power requirements in your led wiring scheme, to closly match the power source. Having voltage much higher than needed results in the need for very large, expensive, hard to find, and inefficient current limiting resistors that waste energy by getting real hot. By using a power supply that is just slightly higher voltage than needed,or by putting leds in series will result in a more efficient design, and the current limiting resistors needed will be much smaller and very inexpensive and easy to find.
If you need to hook up many leds The correct way is to place several leds in series, and then several series strings in parallel with each other, thereby needing neither the high voltage needed with series wiring, or the high current needed with all parallel wiring. Automotive voltages fluctuate from under 12volts dc to as high as We suggest using current limiting resistors based on your highest battery reading with alternator running full blast. The leds wont be as bright when the car isnt running, but you wont blow them up.
Alternately, you can use an IC as a Buck Regulator and put your set point at 12vdc and calculate resistors based on 12 volts.
Parallel LEDs – the problem
Although you can use a potentiometer or rheostat or use a transistor as a variable resistor to brighten and dim leds, both methods have serious drawbacks. In all the above cases the parts can get real hot, the circuit is very inefficient, and with batteries it can lead to seriously decreased battery life, and in some cases, more power is being wasted limiting the current than is going to the leds themselves.
Some people also refer to pwm circuits as a duty cycle controllers.
The advantage of PWM over Pure DC is, it is much more efficient, resulting in lower power consumption, longer battery life, less heat in the pwm circuit versus the other circuits mentioned, less led heating, smaller heatsinks can be used on parts requiring heatsinking. At 2 volts, the LED is taking 20 mA. If the LED was manufactured slightly differently it might require 2. Imagine what happens when two LEDs are in parallel - if they "suffer" from normal manufacturing variations, an LED that only needs 1.
Wiring LEDs Correctly: Series & Parallel Circuits Explained
The device that needs 2. This assumes a "common" current limiting resistor is used to provide about 2 x 20 mA to the pair.
Now multiply this problem out to 8 LEDs and the one that naturally has the lowest terminal voltage will turn into smoke taking the best part of over mA. Then the next one dies then the next etc Not only will you have inconsistent brightness which you may not care about you will have a cascade failure mode. You've got 8 LEDs in your circuit with one resistor.
As long as each LED takes roughly the same current we are fine. Now lets say one of them gets a bit warm, and it now pulls more current. Now we have 7 LEDs receiving current meant for 8. Rinse, lather and repeat until the final LED gets all mA and comes to a fiery end. Yes you can do that, and to balance the brightnesses you can put a resistor in series with each individual LED varying the value of the resistor to compensate for the difference in brightness.
Also, if you put a single current-limiting resistor in series with the complete LED group, you will need to take into account the fact that you are using multiple LEDs and calculate the total required current allowing for their differences in forward voltage drop and the values of their individual resistors if you have put those in , and you cannot use the same value of resistor that you would use if you had just one LED.
As "Andy aka" has mentioned above, you need to watch out for such situations. But if you use the correct current-limiting resistors for each individual LED then there shouldn't be a problem although you will need to determine the correct value for the resistor by measuring the forward voltage drop across each LED, which is not a very elegant approach to circuit design.
Yet another possible solution is this. More expensive than resistors, but it pretty much guarantees equal brightness for each LED with much more relaxed voltage requirements. Just be careful of power dissipation and temperature de-rating.
For example, let's say you have 2-volt red LEDs, and you are running them in a car, with a supply voltage between 12V and 15V more or less. You can them connect as many of these modules in parallel as you want.