# Resistor for LED-series in parallel



## Corday (Jun 14, 2011)

Hi =)

[FONT=&quot]I have been led to believe that i can use LED-series in parallel and thereby decrease the number of resistors needed. I will explain.[/FONT]

[FONT=&quot]*Volt source*: 12v DC[/FONT]

[FONT=&quot]Say i have 3-LEDs in series with the following specs: 3,5v 30mA[/FONT]
[FONT=&quot]The three LEDs will consume 30mA total when soldered in series.[/FONT]
[FONT=&quot]This will require one (1/4w) resistor of 50 Ohm. Resistor will dissipate 0,045w.[/FONT]
To calculate the resistor for this circuit i use the formulae: (12-(3,5*3))/(30/1000)






[FONT=&quot]Now I add one more series of 3 LEDs of the same source but solder it in parallel it to the first series.[/FONT]
[FONT=&quot]The 6 LEDs will now consume 60mA total.[/FONT]
[FONT=&quot]This would require one (1/4w) resistor of 25 Ohm. Resistor will dissipate 0,09w.[/FONT]
To calculate the parallel resistor value i use the formulae: 1/((1/((12-(3,5*3))/(30/1000)))*2)





[FONT=&quot]I will continue adding 10 more series of LEDs in parallel - adding up to a total of 12 series of 3 LEDs in parallel connection.[/FONT]
[FONT=&quot]These 36 LEDs will now consume 360mA total.[/FONT]
[FONT=&quot]This would require three 1/4w resistors of 4,17 Ohm Total (1,39 Ohm each). Total dissipation 0,54w.[/FONT]
To calculate the parallel resistor value i use the formulae: 1/((1/((12-(3,5*3))/(30/1000)))*12)






[FONT=&quot]Is this all wrong? Does each of the 3-LED-series need a separate resistor?
[/FONT]


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## CKOD (Jun 14, 2011)

Yep, you need a resistor for each string of 3. The voltage of each string can vary, and they wont share current equal. Repeat the first schematic 12 times, (go with 49.9 ohm resistors, thats a standard value, and 50 ohm resistors can be annoyingly pricy ) If youre feeling lazy and dont want to do 12 resistors, you could probably get away with the 2nd schematic 6 times and be fairly close. But with 12 strands in parallel, you'd definitely have some strings brighter then others by a noticeable amount, and since you calculated for 30mA, you would have some drawing more current then they should, and overheating quicker.


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## bbawkon (Jun 14, 2011)

And then.. There's the correct way to do it.. Well, one of the correct ways to do it anyway..






ETA: I thought about this for a minute after I threw that schematic together.. Problem with using a linear regulator like the LM317 for this circuit is that you probably don't have enough head-room with your supply voltage.. Still, Something along these lines is much better than a resistor (or 6 or 12).

The LM317 circuit above will require about 2.5V higher input voltage than the combined Vf of the string of LEDs that its driving.


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## Corday (Jun 16, 2011)

Thank you CKOD and bbawkon!

I see both your points, and thats what caused me to question the theory to begin with 

However, bbawkon although your way would be a stable, safe and correct way of doing it - the cost and workload would increase to huge amounts.

To make me even more confused however, i just found an OEM-worklight that supports the original-post theory... I have taken photos of it, and drew a schematics aswell.

The LEDs are high-intensive white with Vf around 3,5 (i tested with multimeter and it said 3,05v - but worklight-battery was half empty).





Photo #1 - Front with LEDs
Photo #2 - Back with circuit 
Photo #3 - Battery pack

Hmm what do you think about this? Its weird if they would manufacture and produce a product if the theory wasnt eligible?


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## bbawkon (Jun 16, 2011)

The fact that the two resistors are of different values suggests that they are actually testing the Vf of the actual LEDs installed on a unit and then populating a proper resistor for that Vf.


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## Lynx_Arc (Jun 16, 2011)

bbawkon said:


> The fact that the two resistors are of different values suggests that they are actually testing the Vf of the actual LEDs installed on a unit and then populating a proper resistor for that Vf.


 
If you look at the picture the resistors have the same color code on them so it is just a tolerance issue that the values are not exact.


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## Corday (Jun 17, 2011)

Been doing alot of googling, testing and calculating...
However, i just found an "e-book" written by Colin Mitchell <--- Link to the book



Colin Mitchel said:


> LEDs "generate" or "possess" or "create" a voltage across them called the CHARACTERISTIC VOLTAGEDROP (when they are correctly placed in a circuit). This voltage is generated by the type of crystal and is different for each colour as well as the "quality" of the LED (such as high-bright, ultra high-bright etc). This characteristic cannot be altered BUT it does change a very small amount from one LED to another in the same batch. And it does increase slightly as the current increases.
> For instance, it will be different by as much as 0.2v for red LEDs and 0.4v for white LEDs from the same batch and will increase by as much as 0.5v when the current is increased from a minimum to maximum.
> 
> If you get 2 LEDs with identical CHARACTERISTIC VOLTAGE-DROP, and place them in parallel, they will each take the same current. This means 30mA through the current-limiting resistor will be divided into 15mA for each LED.
> ...



Hmm... Bad news!  But still... How did they manage it with the worklight in my previous post?  Did they actually test each single LED for its individual voltage drop so they would all exactly match? Ok if it was a one-time thing but they produced thousands and thousands of those lights..

I did notice however that he writes mostly about generic LEDs and not high-intensive ones. Maybe they are better quality and dont differ from eachother as much in voltagedrop?

OR, is there a way to "play safe" and give up a bit of mcd, to gain the ability to safely use parallel arrays? I.e. if i test a few LEDs from the same batch for their average voltagedrop, and then design the circuit (the resistor etc) for that?


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## nickelflipper (Jun 17, 2011)

Here's a link from someone who works for one of the chinese flashlight manufacturers, and how they approach the parallel led strings (see post #7) http://www.electro-tech-online.com/...ideas-reviews/117927-need-help-led-setup.html

Happy Vf binning


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## Lynx_Arc (Jun 17, 2011)

Corday said:


> Been doing alot of googling, testing and calculating...
> However, i just found an "e-book" written by Colin Mitchell <--- Link to the book
> 
> 
> ...


 
High intensive doesn't mean anything it is just another word to make a 5mm LED seem more powerful than it is. There are name brand LEDs and generics and typically the name brand ones hold up better and even some can be overdriven a little without dramatically reducing their lifespan while many of the generics don't even hold up well under nominal (20ma) currents. The safest thing to do is to underdrive the LEDs a little and go ahead and use parallel arrays. If when you fire them up you notice a string that are overly bright you may want to replace them as they will probably burn out quickly while those that are dim typically won't do any harm. It is when you are driving LEDs at spec current or overdriving them that failures increase when they get mismatched and a string of them end up getting more current than the average. I have seen many lights using parallel arrays that work well enough but when the voltage drops too low it gets interesting as some go dim faster than others. 
As far as your example, if you have 20 strings of LEDs getting a total of 400ma across them (in parallel) and one string fails then instead of each string getting 20ma the remaining 19 strings divide the 400ma amongst themselves and now get about 21ma per string or an increase of only 5%. It is when you have very few sets in parallel that failure of one set can cause severe problems. Some very cheap 3AAA 8-9 LED lights have this issue as the LEDs off alkalines are driven at 30-50ma because there is no resistor to limit the current, it is only limited by the batteries internal resistance and the wiring/contacts/switch in the light. I had an 8 LED light I used nimh on it and one by one the LEDs burnt out till I had only 4 left. I replaced the LEDs and put in a resistor that dropped the average LED current to about 22ma/LED. None has burnt out but even at 22ma the LEDs seem to fade in use over time becoming dimmer.


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## MikeAusC (Jun 19, 2011)

I agree with everyone here - LEDs in parallel SHOULDN'T work reliably.

But I have a light that consists of 34 strings of 3 surface-mount LEDs close together on 6 pcbs. All strings are wired in parallel and then connected to a 5 ohm resistor to draw 0.5 amp from 12 volts. www.dealextreme.com/p/48178

It works very well and there is no sign of any variation in brightness.

I assume this will only work if all LEDs are from the same production batch for close matching of forward voltage and temperature co-efficient.


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## bshanahan14rulz (Jun 21, 2011)

Also, don't forget that 2 resistors are cheaper than 32 resistors. Why design something that works great when you can design something that works for cheaper?

You can build your array however you want, but the best way to do it is series-parallel with resistors for each series string. You can measure the Vf if you're so inclined, and adjust resistor values accordingly, but the tolerances on the resistors would likely negate any attempt at balancing. Just pick the value that works best with the average Vf of your batch of LEDs.

Think about it this way: If one of your strings has a freak diode with a low Vf, it will draw more current. Thus, it will heat up more than the others, and for that reason also will draw more current. Finally, it might come to a point where the LED simply cuts out, likely in an open circuit failure. That extra current will then be distributed to the other strings of LEDs. Depending on the number of series strings you have paralleled, this may introduce more current to the LEDs than they were designed for. Perhaps another not so freakishly low Vf diode will overheat and fail, again adding to the current going to all the other operational strings, and so on.

If what you are building is accessible and you don't mind taking it apart if it fails, do it however you want. I say, for the price of a pack of resistors at radioshack, it's worth it. (and resistors at radiojack aren't cheap)


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## KittenKat (Jun 23, 2011)

Since I saw the LM317 mentioned, I figure i should make this suggestion. ON Semiconductor makes a product they refer to as a CCR, it's a 2 pin constant current regulator. For 30mA you would want the NSI45030AT1G which from mouser is 31 cents in single Qty. If you're going from an automotive power source you need to account for a supply voltage of 10.8 (flat battery), and roughly 14v (charging battery). In this application constant current is the only way to go.


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