Schorschi
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Many LEDs But One Resistor In Parallel?

Tue Aug 27, 2013 8:43 pm

Ok, I know this topic has come up before... Can someone reiterate the technique? Many LEDs needing many resistors, one to one. The rule is, if I understand or recall correctly... the LEDs in parallel each have a resister to protect overload of each single LED, either cathode or anode side of LED, right? But if only one LED is active, for multiple LEDs in parallel, all said LEDs could leverage one resister because the total current draw can be handled by the single resister given a single LED is active. There is the twist, what if a few or all LEDs in parallel are active at once, a single resister in common is not sufficient? In theory each LED in parallel gets the same current source level, say cathode to anode direction, right? So the single resister anode side could be overloaded especially if a given LED in parallel fails? Do I recall all this right? A resister is throttling effect, controlling the maximum volume flow of electrons... so why is only one resister insufficient if total source voltage/current does not change, LED forward resistance is taken into effect, then resistance is established in common to limit total current draw. The issue really is LEDs vary right? So in parallel, you can't trust consistent current draw, consistent forward resistance, across multiple active LEDs, even for LEDs (cheap ones anyway) that are supposed to be consistent in manufactured specification, right?

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redhawk
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Re: Many LEDs But One Resistor In Parallel?

Tue Aug 27, 2013 9:06 pm

How many LEDs do you intend to wire up and will they all be active at the same time??

It is possible to connect LEDs in parallel with only 1 load resistor but the mathematics gets a little more complicated since you need to take into account the total forward current of all the diodes.
You also need to make sure the forward voltage specification is identical for all diodes if you mix different types like red + purple, red + white then only the red LEDs would light up because they have the lowest forward voltage of all the colours.

Richard S.

klricks
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Re: Many LEDs But One Resistor In Parallel?

Wed Aug 28, 2013 4:11 am

You can use a single resistor with parallel LED's.... Just remember that the number of lit LED's must be constant.
If you add more LED's then the R value must be decreased. Simple example: A circuit has 1 resistor and 1 LED and draws 20mA. Now if you add a second LED in parallel with the first, you will still have 20mA though the resistor..... but the LEDs' will split the 20mA and take 10mA each ...... and be half as bright .... or maybe not even light at all.

If you want both LED's at full brightness then you have to cut the resistor value in half so that 40mA will flow through the circuit or 20mA each LED. Also note that the power or wattage thought the resistor will increase. So you have to choose a resistor for it's R value as well as it's wattage value. If you have many LED's on 1 resistor the the resistor may need to be quite large (wattage) and expensive compared to circuits with 1 resistor / LED.

Here is one of the many on-line LED/Resistor calculators:
http://www.hebeiltd.com.cn/?p=zz.led.re ... calculator
Unless specified otherwise my response is based on the latest and fully updated RPiOS Buster w/ Desktop OS.

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pluggy
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Re: Many LEDs But One Resistor In Parallel?

Wed Aug 28, 2013 8:37 am

The problem comes when the LEDS don't have exactly the same forward voltage (very common even with seemingly identical LEDS) and one or more of the LEDS take the lions share of of the current. It never fails to amaze me how often this comes up, considering the resistors are the cheapest part of the equation, it is incredibly short sighted to save pennies to compromise the project. Get a life and wire one per LED.
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

shuckle
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Re: Many LEDs But One Resistor In Parallel?

Wed Aug 28, 2013 9:14 am

It is a bit difficult to understand why
pin1 -> LED 1 -> resistor 1 -> Ground
pin2 -> LED 2 -> resistor 2 -> Ground

Has to be used instead of:

Code: Select all

pin1 -> LED 1 -> -+
                           |
                           +resistor  -> Ground
                           |
pin2 -> LED 2 -> -+
What is wrong with that common resitor setup? Even if pin1 and pin2 are on and off variable time. I have never seen that explained properly.

This is not a cost issue, but if you have 10 leds, it takes time, space and effort to use 10 resistors if one is enough?

mikerr
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Re: Many LEDs But One Resistor In Parallel?

Wed Aug 28, 2013 9:25 am

I've used both these layouts:

Image

Image


The only downside it that all LEDs ON at the same time may be dimmer, or at different brightnesses (LEDs may not be identical).

In practice I didn't see any downsides, the second one is just less wiring (never skip the resistor entirely though !)

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pluggy
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Re: Many LEDs But One Resistor In Parallel?

Wed Aug 28, 2013 10:08 am

shuckle wrote:It is a bit difficult to understand why
pin1 -> LED 1 -> resistor 1 -> Ground
pin2 -> LED 2 -> resistor 2 -> Ground

Has to be used instead of:

Code: Select all

pin1 -> LED 1 -> -+
                           |
                           +resistor  -> Ground
                           |
pin2 -> LED 2 -> -+
What is wrong with that common resitor setup? Even if pin1 and pin2 are on and off variable time. I have never seen that explained properly.

This is not a cost issue, but if you have 10 leds, it takes time, space and effort to use 10 resistors if one is enough?
If only one is on at once, it probably wouldn't matter, but the current going through the resistor would stay the same and that would be shared across however many LEDS were illuminated. They'd get dimmer the more that were lit. Not an issue with one resistor per LED. The space doesn't have to be an issue, I make my circuits on strip board and |I frequently solder SMD resistors to the underside of the board, cut a track with a craft knife, and solder an SMD resistor across the cut. If you're not looking very closely they are invisible, they just look like an untidy solder bridge.

It doesn't need any explanation if you understand what an LED is and how it handles current. It is not an incandescent light bulb.
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

veronicathecow
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Re: Many LEDs But One Resistor In Parallel?

Wed Aug 28, 2013 10:17 am

Hi Schorschi, if you are worried about space then search for "dil resistor array". There are lots of packages types that may give you a smaller size than discrete resistors.
Of course if your driver device automatically and safely limits the amount of current to each O/P pin then you won't need a resistor at all unless you are trying to save power.

Schorschi
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Re: Many LEDs But One Resistor In Parallel?

Thu Aug 29, 2013 4:17 am

Everyone, thanks for the discussion and input. I don't have an issue with one resistor per LED, just wanted to confirm why it is was a good idea. Nor is space an issue per se, I just happen to be maybe the worst soldering person in the world, so the fewer components to solder the better in my case. Using a resistor array is a great idea as well, I had not researched the options possible yet so this is an excellent suggestion.

At some point, once I get a few other projects completed, I plan to build a LED cube to validate a matrix driver board design, and scale up to Christmas lights for a display in my front yard at some point. So just doing basic research at this point, for a LED cube. A modular LED cube design would be preferred, something where I can build a LED cube say 8x8x8 and then build 4 or more cubes and link them together, chaining the control system together, but I am getting ahead of myself, even suggesting any of this!

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aTao
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Re: Many LEDs But One Resistor In Parallel?

Thu Aug 29, 2013 6:33 am

Schorschi wrote:I just happen to be maybe the worst soldering person in the world, so the fewer components to solder the better in my case.
Err, maybe not. Practice, and the more components you use the more practice you get. Also by far the main cause of poor soldering is poor soldering irons. If you are going to do much (any) soldering get a decent temperature controlled iron. Next comes the solder, dont buy cheap solder it dosent work. Then technique...
1 prep the wires/ solder pads: it is rare that there is a lot of oxidisation but if there is clean it off with an abrasive.
2 tin the iron : put a dab of solder on the iron then flick it off (or pinch it off with fingers if you are real hard core) never use a sponge!
3 tin the wires/ solder pads.
4 tin the iron.
5 make the joint: touch iron on, wait a gnats, feed solder in, remove solder, remove iron.
if your solder is so thick that "feed solder in" gives too much: use a thinner solder.
>)))'><'(((<

veronicathecow
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Re: Many LEDs But One Resistor In Parallel?

Thu Aug 29, 2013 7:11 am

There is a Raspberry Pi 7 page PDF available here to help with soldering.
http://www.raspberrypi.org/archives/1494
And unless you are Clark Kent or his alter ego then I would recommend a lightly damp sponge (proper soldering sponge not one taken from the sink) for cleaning rather than fingers.

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Burngate
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Re: Many LEDs But One Resistor In Parallel?

Thu Aug 29, 2013 10:12 am

veronicathecow wrote:... a lightly damp sponge (proper soldering sponge not one taken from the sink) for cleaning rather than fingers.
ASDA's best (or Tesco or ...) wash-ups are wonderful for making a gooey mess on an iron. Proper sponges made from cellulose work properly. But what I use (because I can never find the proper one - it's buried somewhere) is a damp paper towel, often supplied by Burger King. Free and disposable unlike fingers.

mindmasta
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Re: Many LEDs But One Resistor In Parallel?

Thu Aug 29, 2013 5:14 pm

pluggy wrote: Get a life and wire one per LED.


Rude.

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redhawk
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Re: Many LEDs But One Resistor In Parallel?

Thu Aug 29, 2013 5:42 pm

I hardly bother with damp sponges anymore in fact I think this kind of encourages the tip to rush or wear out.
I use scrupled tissue paper this is just as effective at removing the oxidised deposits from my tip.
You do have to stroke it fairly quickly away from you though otherwise the paper may start to burn.

As for LEDs in parallel I think resistance is only critical if you intend to drive them at their near maximum rating otherwise separate resistors would be necessary.
I can't say I've had any trouble driving LEDs in parallel only when mixing different coloured because they have different voltage drops (the lowest always steals all the current).

Image

10 LEDs driven in parallel by 1 AA battery and a booster circuit - no problem. :)

Richard S.

cadcoke5
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Re: Many LEDs But One Resistor In Parallel?

Sun Sep 01, 2013 3:40 am

As another poster has described, the problem with one resistor is that minor variations in LED's can cause one to self-destruct, and then that causes the other LED's to run hot, and then another one self-destructs, and so on.

Before I had my electronics training, I wired up about 40 LED's in parallel, and directly to the battery with NO resistor at all. But, it worked! Now I understand that what happened is that I was drawing so much current, that the internal resistance from the batteries acted as the current limiting resistor. I also had unintentionally underpowered the entire array, so none of the LED's blew up.

As an aside, I have seen people directly connect a LED to a button battery. In this case, as with my early attempt, the internal resistance of the battery is what prevents the LED from blowing.

I too, wish there were an easy way to wire up a bunch of LEDs, without having to mess with a bunch of resistors. I had looked in the past, without success, but I just looked again, and found some.

Mouser carries the Avago Technologies HLMP-1620. Which is a 5volt LED with integral resistor. They are only 8mcd luminosity. I know I had searched Mouser before, but perhaps the words "LED Integral resistor" don't work for this search.

I found some brighter ones at SunLED, http://www.us.sunled.com/SearchResult.a ... ED%20Lamps. They go up to 397mcd luminosity.

Joe Dunfee

timmoore46
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Re: Many LEDs But One Resistor In Parallel?

Sun Sep 01, 2013 8:00 pm

This is really an exercise in understanding Ohms Law, power dissipation in diodes and resistors and common sense.

Direct experience is often the quickest way to gain an understanding of the core problems, including thermal runaway.

As folk often don't take good advice. Direct experience is very unforgiving but very effective as a learning experience

Try getting a good car battery that can offer over 100 Amps. Get a bunch of leds connect them to the 12 volts in various configurations and see what is left after running for 24 hours. Recommend the set up is in the back yard because some may well catch fire.

Take time out to consider what went wrong in each failure.

Then select the configuration that is mostly likely to last at least 50 years in a 365/24/7 environment.

:D

Tim

Schorschi
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Re: Many LEDs But One Resistor In Parallel?

Mon Sep 02, 2013 5:03 am

Tim... your comments and suggestions... had me laughing... for several minutes. Rest easy... I am using 1 resistor per LED, I started this discussion to drive discussion on what can be done, versus what should be done. I think in that respect, this thread has been successful. I, for one, have no plans to abuse electrical components. Short of one LED shorted out early in my bread boarding efforts with the Raspberry Pi. Or toasting one optoisolator IC, where I had a boneheaded error in my Ohms law calculation, so my resistance was away off ... I have not fried any components recently. LOL!

aTao... yes, I have a temperature controlled iron, in my past efforts in college (many years ago) we used damp newspaper to clean the soldering iron tips, but did not have controllable irons, so it made soldering less fun than it should be. My issue now... is I just don't have the steady soft touch, I think a soldering guru should have, develops with a reasonable soldering practice. I have never had great hand/finger dexterity, so soldering for me has always been a bit of challenge. But I will continue to work at it.

timmoore46
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Re: Many LEDs But One Resistor In Parallel?

Mon Sep 02, 2013 5:42 am

A good soldering iron and lots of practise will get you soldering skills up to ok. A watchmakers magnifying glass will also help in inspecting the joints anding where trouble might be.

Good luck and I'm sure you will succeed with your electronics !

:D :D :D

Tim

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AndrewS
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Re: Many LEDs But One Resistor In Parallel?

Mon Sep 02, 2013 6:35 pm

For using LEDs on a breadboard, something like this can be useful:
http://blog.ianlee.info/2013/01/quick-t ... -leds.html

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