LEDs in series

[habazoom]

Had a question regarding resistors in LED series.

For car application, I've been wiring 3 LED series as follows:
+12V-->resistor-->LED1-->LED2-->LED3-->Ground

If I were to switch the resistor placement to the negative side, would each string of 3 LEDs in series still require its own resistor? Or can I wire multiple strings off a single resistor that is connected to ground? I hope the diagram below makes sense- 2 strings coming off a resistor connected to ground. If it is possible to run multiple strings off a grounded resistor, is there a maximum number of strings that can be connected?

Ground
|
|
V
resistor-->LED1-->LED2-->LED3-->+12V
|
|-->LED1-->LED2-->LED3-->+12V


Thanks

 

[SteveDavis]

It is never a good idea to wire LEDs or strings of LEDs in parallel. The reason is that the forward voltage of an LED at a given current can vary, sometimes by almost a full volt. But by wiring them in parallel, you force the voltages to be the same, so the current between the two strings will need to be different. This will lead to differences in brightness and can lead to part failure. Once one part fails, no current will run through the string that the part failed in, forcing all of the current into the second string, which increases the likelihood that the other string will fail as well.

Actually, the difference in forward voltage is enough that I never recommend LEDs in lighting applications be driven with a resistor, but that's another story. I understand that a lot of people on this board use resistors because they're easy, cheap, and readily available, so I won't try to convince you off of them altogether.

 

[haloweenhamster]

you can run it like either of these to they are basically the same but like the other post said not recommended

```````````````/ LED1-->LED2-->LED3-->+12V
Ground-->resistor
```````````````\ LED4-->LED5-->LED6-->+12V


``````````````/ LED1-->LED2-->LED3-->Ground
+12V-->resistor
``````````````\ LED4-->LED5-->LED6-->Ground

if using resistors and power consumption isn't a worry then i would run them all in parallel each 1 with its own resistor

````````/ resistor-->LED1-->Ground
```````/ resistor -->LED2-->Ground
+12V--> resistor --> LED3-->Ground
```````\ resistor -->LED4-->Ground
````````\ resistor-->LED5-->Ground

as its for a car I don't think a constant current supply is required
it should be OK to change most lights in a car for LEDs
the only ones you might have a problem with is the indicators you need to add a dummy load otherwise they flash to fast
unless you've got a car which checks if any bulbs have blown then you will have to add a dummy load where ever you've replaced a bulb for a led

Ive fitted 3mm LEDs to cars and RC cars and running them with limiting resistors and they look the same brightness in parallel
run a standard 3/5mm led direct off a PSU and you will find there isn't much difference in a little bit of power to me it looked more stepped increments then gradual but I try to under run LEDs anyway make them last 50 years

 

[LukeA]

Kaidomain sells drivers that can run 3 high-flux LEDs at 750mA in series off of automotive power.

 

[SemiMan]

Are you talking about White/Blue/Green LEDS or RED\AMBER?

In either case, one resistor per string is what I would highly recommend. Resistors are cheap.

If you are using RED\AMBER leds, then the forward voltage per led is low enough that three in series with a resistor is generally pretty safe. Most of the LED tail lights do just this. However, you need to take into account the voltage bin of your LEDS and keep in mind the operating voltage is going to be more like 14-15volts when the car is running.....and below 12 when turned off.... and over 16 when things go wrong.

If you are using White\Blue\Green, the forward voltage of three in series can start approaching 12V. The resistor value may need to be small and you may get huge changes in brightness w.r.t. the voltage on the electrical system. In this case you are much better off with a constant current driver.

Semiman

 

[habazoom]

```````````````/ LED1-->LED2-->LED3-->+12V
Ground-->resistor
```````````````\ LED4-->LED5-->LED6-->+12V

I was planning on running white LEDs only for the interior. I've made some up as shown in the following pics. For this I did one resistor for every string of 3 LEDs in series. What I was wondering was does resistor placement on the +12V or the ground side really matter? And like haloweenhamster's diagram above, is it safe to wire multiple strings of 3 LEDs in series to one resistor. His example shows 2 strings on one resistor. What if I were to make it 4 strings of 3 LEDs on one resistor?

I'm just a DIY guy who likes to work on cars. My knowledge with car electronics is above average but limited with LEDs and power sources for it. All I'm familiar with is resistors. Any help would be appreciated. Thanks.

 

[2xTrinity]

What I was wondering was does resistor placement on the +12V or the ground side really matter?

Nope, the whole string acts like one "equivalent" resistance no matter what order it's wired in. You could even put the resistor BETWEEN LEDs is if was more convenient for some reason.

And like haloweenhamster's diagram above, is it safe to wire multiple strings of 3 LEDs in series to one resistor. His example shows 2 strings on one resistor. What if I were to make it 4 strings of 3 LEDs on one resistor?

Any time you run multiple parallel strings on a single resistor, you run the risk of one string drawing more current than others, and leading to uneven brightness. In cases where LEDs are being pushed hard/close to the limit of their specs, uneven currents can lead to early failure. In this case with breadboard, tiny resistors are pretty cheap, I'd just use a separate one. If I was wiring this instead with a constant-voltage regulator, I"d probably just wire the string in parallel rather than buy a separate regulator for each, due to cost.

 

[X_Marine]

To try and answer your question with the above mentioned variations etc. yes you can run mutible strings of 3 leds from 1 resistor, keeping in mind that each string will imply more ma draw on that resistor.

Your picture shows aprox 1/4watt if you were to run 3 or more I would suggest going to 1watt or even more just to stay on the safe side, without dong the arithmetic. And of course be sure to use inline fuses some where. Those will also require a little arithmetic to make sure you use the correct values.

Good luck, looks like a fun project.
X..

 

[habazoom]

Thanks all for your responses. I've posted these same questions in a couple other forums and couldn't get any good explanations or even a reply.

 

[CampingLED]

If you plan to illuminate all LEDs together 1 resistor in series with multiple LED serial strings in parallel should give very similar results as 1 resistor per string. The difference is that you will require a lower value resistor with a higher wattage with the single resistor. Personally I would recommend the multiple resistors.

You could play around on this site with the various options. The site will show the recommended circuit as well as resistors required. I will be good if you have the Vf value for the desired current of the LEDs that you plan to use as this will have a direct impact on the resistor value.

Also remember that there is a voltage difference in a car with engine running (13.7 to 14.2 V) versus engine not running (12.25 to 12.75 V). The main reason is that the alternator is charging the battery when the engine is running.