LED lights info

[dgfischer]

I recently came across a bunch of cheap LED bolt lights. I am interested in wiring them up ( a couple of blue LEDs mounted to the inside of the tail light for a different look).
The LEDs have what look to be a resistor built in to the ground wires(each one).
My question is, Can I daisy chain the ground wires together and use 1 resistor (this would get rid of alot of excess wiring) or do each of the LEDs need a seperate resistor?
Daniel

[IverJ]

Howdy Daniel,

I just got registered so will reply for everyone to read. I sent you an eMail on this yesterday but will try to be clearer on this. Unfortunately, while this is really simple there is no simple way to explain it without drawings

The answer is Yes...but (there is always a but)

There is no such term as daisy chain in electronics. Components such as LEDs can be wired in series or in parallel or in a combination of these.

Before I get going on this I need to point out that LEDs must be hookedinto the circuit with the proper polarity. In other words the LED anode must be connected to the positive side of the circuit and the cathode must be connected to the negative side of the circuit.

Trying to describe electrical circuits without drawings is very difficult but here goes. (Before I get going let me specify that I will use R to indicate resistance, V to indicate voltage, and C to indicate current. I mention this because these are not the normal letters for these units but to someone not familiar with electronics it keeps thiings simple)

SERIES CIRCUIT: We will use an example of hooking up 3 LEDs in series. With this type of circuit the negative lead from the first LED connects to the negative terminal of the battery and the positive lead connects to the negative lead of the second LED. The positive lead of the second LED connects to the negative lead of the third LED. The positive lead of the third LED connects to the positive terminal of the battery.

So is a resistor required? It depends on the particular type of LED being used. Lets assume you are using an LED with a forward voltage rating of 4 volts dc maximum and a current flow at this voltage of .020 amps. In this case an individual LED drops 4 volts whichl eaves 8 volts that need to be accounted for. You could add in a resistor by using ohms law to calculate its value. R = V / C or 8/.020 = 400 ohms. So for a single LED in this example a 400 ohm resistor is what is needed (color coded yellow, black, brown). What happens if we hook two LEDs in series?

We have two LEDs each dropping 4 volts for a total of 8 volts. Current remains the same in a series circuit so we have 4 volts left over to account for. In this case the resistance would equal 4/.020 = 200 ohms (color coded red, black, brown).

Now lets get down to our example of three LEDs in series. Each one drops 4 volts for a total of 12 volts so no resistor is needed.

Could you add 4 such LEDs in series. yes but they would be dimmer than desired and by the time you added 5 you would probably be below the threshold at which the LEDs would light at all.

Remember the above applies only to the specific LED we are using in our example.

PARALLEL CIRCUIT: No conside hooking up the LEDs in parallel. In this case the postive leads of all of the LEDs would be wired together. Then the negative leads would all be wired together. In this case there is no limit to the number of LEDs you wire in parallel as long as your battery can supply enough current. Unlike the series circuit where the current was the same everywhere in the circuit, in a parallel circuit the total current flow in the circuit will be the sum of all the individual branches (LEDs). For example, if we wire three LEDs in parallel, the current is .020 + .020 + .020 = .060. The voltage across the parallel set of LEDs will be 12 volts so once again we must drop this to 4 volts or risk burning up the LEDs. Since the LEDs are dropping 4 volts we have 8 volts left to account for. Again we use ohms law to calculate the needed resistance. Remember that R = V / C or 8/.060 = 133.33... ohms. There is probably no 133 ohm resistor available swo lets use a 140 ohm resistor (color coded brown, yellow, black). This resistor will be wired IN SERIES with the parallel wired resistors. Remember we hooked all ot the LED positive leads together and all of the LED negative leads together. The resistor will get wired to either the poitive side or the negative side. It does not matter but lets put it on the negative side. Onelead of the resistor will be hooked up to the negative batttery terminal and the other lead will be hooked up the the negative leads from the LEDs. The positive leads from the LEDs will be hooked up to the battery positive terminal.

If you want to measure the LEDs you have a simple way to do so would be to hook one of them up to your bikes battery with its resistor. Connect a volt meter accross the resistor leads with the meters black lead connected to the resistor lead that hooks to ground (battery negative terminal) and the meters red lead connected to the resistor lead hooked to the LED. Write down the voltage measured. Then place the meter black lead where the meter red lead was and hook the meter red lead to the LED lead which is connected to the battery positive terminal. Write down the voltage measured. When you add up these two voltages they should = or be close to 12 volts.

Now look at the resistor. It should have three color bands. Starting at the side closest to one end of the resistor write down the colors. The first two are simply single digits. The third is the number of zeros placed after the first two. (you can also measure the resistance directly with most multi meters. Just make sure yuo read the correct scale. The resistance will not be an order of power greater than in my example.)

Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet = 7
Grey = 8
White = 9

(there may also be a 4th band or no 4th band. It will be silver or gold and indicates tolerence. no band = 20%, silver = 10%, gold = 5%. Don't worry about this.)

Resistors also come in different sizes depending on there ability to dissipate heat (power) and are rated in the number of watts they can handle. Even the smallest commonly available resistors will handle 5 watts. In the examples above the series circuit needs a resistor capable of handling .16 watts and the Parallel circuit requires a resistor that can handle .48 watts so don't worry about physical size of the resistor. It will handle the power.

So now you know the voltage across the Resistor and the resistance of the resistor. To find the current thru the LED simple divide the voltage across the resistor by its resistance. Since a series circuit has the same current flowing everywhere in the circuit you now know the LEDs voltage rating and current rating. USe Ohms law as demonstrated above for either a parallel or series circuit to do what you want with the LEDs.

Hope this helps. Feel free to contact me if you have any questions.

Regards,
Nelson

[IverJ]

Howdy Daniel,

I had a bit of brain fade in my post above. Resistors are commonly available as small as 1/2 watt and even 1/4 watt. For yuor application make sure you get at least a 1/2 watt resistor for up to 3 LEDs in parallel. For 1, 2 or 3 LEDs in series a 1/4 watt resistor is ok.

For 4 or more LEDs in parallel go with a 1 watt resistor.

Sorry

Nelson

[crazypj22]

Using your example of 4volts, does this mean that if you used 12 LED's you could wire3 sets series/parrallel and not require any resistors?
I had some thoughts about stop/tail lights but wasnt sure.
(I got so many projects on the go whats another one? ;D)
Maybe its a dumb question but I need to check I understand what you said.
PJ

[fnlboy]

You can wire many sets of 3 using no resister. Remember that in a motorcycle, the voltage changes from about 12 to about 14 volts. You can find the voltage rating for the LEDs you want to use, (white LEDs are often 3.3 volt) . Using a LM317 regulator from Radio Shack and a simple circuit like this one
www.muzique.com/schem/lm317.htm
using R1 as 470 ohm 1/4 watt resister and a 10K trimmer pot and a heat sink kit all from the "Shack" you can built a 1.5 amp regulator that would power up to 75 20ma LEDs with 3 LEDs in series and the voltage set to 9.9 volts. Other color LEDs drop different voltages so check what you need.