Author Topic: LED Resistor  (Read 8007 times)

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Offline dark_hawk

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LED Resistor
« on: July 02, 2013, 04:38:54 pm »
Hello,

This is a very newbie question so please be nice. (It's in the beginners section).

I'm trying to connect 18 LEDs to a 6V power source. LEDs Voltage drop is 3.5V @ 20ma. Question is: Which is better and why, Connect a resistor (130ohm 1/8watt) to each LED or Can I connect a resistor (13ohm 1watt) to 9 of them?

Thanks.
 

duskglow

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Re: LED Resistor
« Reply #1 on: July 02, 2013, 05:17:47 pm »
EDIT:  I think I answered the wrong question here.  In this case, I think it would be better to have one resistor per LED.  The reason being that since E = IR, as the current through the resistor changes, so will the voltage drop, so you might end up with a toasty LED if you have a resistor rated for all 18 and only one is on.  With one resistor per LED, the voltage drop and current are both known, so you can be reasonably comfortable with the value of the resistor.

And someone pleeeeze correct me if I'm wrong, I want to know. :)
« Last Edit: July 02, 2013, 05:29:00 pm by duskglow »
 

Offline casper.bang

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Re: LED Resistor
« Reply #2 on: July 02, 2013, 05:30:19 pm »
Hello,

This is a very newbie question so please be nice. (It's in the beginners section).

I'm trying to connect 18 LEDs to a 6V power source. LEDs Voltage drop is 3.5V @ 20ma. Question is: Which is better and why, Connect a resistor (130ohm 1/8watt) to each LED or Can I connect a resistor (13ohm 1watt) to 9 of them?

Thanks.

18 small-signal resistors are going to be cheaper than 2 power resistors, but then again, you could just put small-signal resistors in parallel. Parallel coupling always opens up the chance of creeping single-point-failures, otoh it will also allow you to hit an optimal target resistance more precisely. Regardless, either solution is going to dissipate about 1W of energy into heat, which may or may not warrant a look at more efficient alternatives like a cheap step-down switching regulator.
 

CompElitePC

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Re: LED Resistor
« Reply #3 on: July 02, 2013, 05:54:39 pm »
Hello,

This is a very newbie question so please be nice. (It's in the beginners section).

I'm trying to connect 18 LEDs to a 6V power source. LEDs Voltage drop is 3.5V @ 20ma. Question is: Which is better and why, Connect a resistor (130ohm 1/8watt) to each LED or Can I connect a resistor (13ohm 1watt) to 9 of them?

Thanks.


Hello,

In my experience, it is customary to typically use one(1) resistor per LED. Mainly because it is cheaper like mentioned before.
« Last Edit: July 02, 2013, 05:56:13 pm by CompElitePC »
 

Offline dark_hawk

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Re: LED Resistor
« Reply #4 on: July 02, 2013, 06:04:16 pm »
All of the LEDs will be lit together, so there is no fear of a toasty LED taking the full voltage.
If the expense is not an Issue, Will I able to get away with 2 resistors instead of 18?
The space is really small and putting 18 resistors in there will be a pain.
 

CompElitePC

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Re: LED Resistor
« Reply #5 on: July 02, 2013, 06:27:09 pm »
All of the LEDs will be lit together, so there is no fear of a toasty LED taking the full voltage.
If the expense is not an Issue, Will I able to get away with 2 resistors instead of 18?
The space is really small and putting 18 resistors in there will be a pain.

they both pretty much weigh the same.   (9)  1/8th watt resistors is 1 and 1/8 watts   

in other words I do not see a major problem with it, so long as your resistances are correct for your load(s)
 

Offline sleemanj

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Re: LED Resistor
« Reply #6 on: July 02, 2013, 07:14:52 pm »
Since you can't run more than one in series off that 6v supply (unboosted), then ideally go with a resistor each, as you correctly conclude if you used one resistor for all the LEDs in parallel, every LED that goes out, increases the current to the remaining ones, causing a cascade failure, eventually - not likely instantly.

If you can use say 12v, then six series strings of three with 1 resistor per string would be a good compromise.

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Offline IanB

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Re: LED Resistor
« Reply #7 on: July 02, 2013, 07:24:18 pm »
I'm trying to connect 18 LEDs to a 6V power source. LEDs Voltage drop is 3.5V @ 20ma. Question is: Which is better and why, Connect a resistor (130ohm 1/8watt) to each LED or Can I connect a resistor (13ohm 1watt) to 9 of them?

It doesn't look like anyone has given you the full answer to your question. You should normally use one resistor per LED to ensure proper current distribution between the devices (each LED should get 20 mA).

If you use a 13 ohm resistor you will have 200 mA available, but how will you ensure that is split evenly between the LEDs so that each one only gets 20 mA? Instead of, for example, one LED getting 40 mA and the others getting 18 mA? Using one resistor per LED ensures this cannot happen. The individual resistors act as current dividers.

If space is at a premium, have you considered using surface mount resistors? They would take up much less room than through hole resistors.
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Offline lapm

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Re: LED Resistor
« Reply #8 on: July 02, 2013, 07:24:47 pm »
I give one more reason to use resistor per led....

There is manufacturing variations in leds. At same voltage some take just little more current then others. Putting several of them in parallel would result usually one led burning itself out by hoarding massive current, then next... I have seen this happen myself.

Resistor per led, avoids this by adjusting single led's current with single resistor. This way one led cant rob all the current from its neighbors.
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Online mariush

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Re: LED Resistor
« Reply #9 on: July 02, 2013, 07:34:16 pm »
Well, 6v input , 3.5v on led ... that means you drop 2.5v on resistor.   V = I x R   = > I = 2.5 / 130 = 19mA.

I guess the value is right for 20mA but you should pay attention to the actual rating of the leds. You don't want to run the leds close to the maximum power rating.

20mA might be super bright leds which may shorten their life and if the leds are close together this brightness may be annoying or cause ugly effects. 

If you connect multiple leds in parallel and somehow one fails, the other leds get more current so their color may be distorted, for example a red led might turn to slight orange tint.

You might want to investigate using led drivers instead of resistors. They'll be slightly more expensive than simple resistors but they'll work better with varying voltage, since they do constant current on a wide range of input voltages.

For example, this BCR405U can do 50mA constant current by default, up to 65mA with an external resistor, with input voltages up to 40v :

http://uk.farnell.com/infineon/bcr405u/led-driver-low-power-leds-6sc74/dp/1791064
datasheet: http://www.infineon.com/dgdl/bcr405u.pdf?folderId=db3a30431400ef68011407a9cfc70181&fileId=db3a304333b8a7ca0133f9d1e2ca4349

You can put 4-6 leds on one of these, 10-15mA per led should still get the leds quite bright.

There's also other led drivers in similar footprints, and even smaller ones that are like smd resistors, such as these:

http://uk.farnell.com/on-semiconductor/nsi50010yt1g/ic-led-driver-50v-0-01a-sod123/dp/1794979
http://uk.farnell.com/on-semiconductor/nsi45020at1g/ic-led-driver-45v-0-02a-sod123/dp/1794973
http://uk.farnell.com/on-semiconductor/nsi45030at1g/ic-led-driver-45v-0-03a-sod123/dp/1794976RL
 

Offline casper.bang

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Re: LED Resistor
« Reply #10 on: July 02, 2013, 07:50:07 pm »
Just thought it should be mentioned, that you can also avoid the discrete resistors entirely if space is really a concern, by using the appropriate (length and thickness) constantan wire. It's very cheap and will last you many projects, just don't try to dissipate more than 1W/30cm as it starts to feel warm to the touch.
 

Offline dark_hawk

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Re: LED Resistor
« Reply #11 on: July 02, 2013, 09:24:22 pm »
Got to love this community.
I asked a simple question and in the process learned of the existence of an LED driver and constantan wire.

Will go with one resistor per led for this project and will get some LED drivers, smd leds/resistors and contantan wire next time I'm making an order.

Thank you all for your answers.
 

Offline bingo600

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Re: LED Resistor
« Reply #12 on: July 03, 2013, 03:48:39 am »
Another solution since you will light all 18 up at once , to use a LM317 to generate the voltage the LED's require from the 6v psu.
Voila ... No resistors needed.

But i'd prob go for the one resistor per led also

/Bingo
 

duskglow

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Re: LED Resistor
« Reply #13 on: July 03, 2013, 04:03:04 am »
There are SIP resistor networks that might take less space on the board.
 

Offline IanB

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Re: LED Resistor
« Reply #14 on: July 03, 2013, 04:32:14 am »
Another solution since you will light all 18 up at once , to use a LM317 to generate the voltage the LED's require from the 6v psu.
Voila ... No resistors needed.

1. LEDs do not require a regulated voltage, they require a regulated current
2. Even if you configured the LM317 as a current regulator, it would not provide for current balancing between the parallel LEDs
3. You could put several LEDs in series to ensure equal current, but then you would need a boost converter which the LM317 is not
4. An LM317 still needs resistors to set the output voltage or current

Quote
But i'd prob go for the one resistor per led also

Why would you ignore your own advice? Is it perhaps because you know your advice is bad?
« Last Edit: July 03, 2013, 04:33:49 am by IanB »
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Offline bingo600

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Re: LED Resistor
« Reply #15 on: July 03, 2013, 05:13:09 am »
Another solution since you will light all 18 up at once , to use a LM317 to generate the voltage the LED's require from the 6v psu.
Voila ... No resistors needed.

1. LEDs do not require a regulated voltage, they require a regulated current
2. Even if you configured the LM317 as a current regulator, it would not provide for current balancing between the parallel LEDs
3. You could put several LEDs in series to ensure equal current, but then you would need a boost converter which the LM317 is not
4. An LM317 still needs resistors to set the output voltage or current

Quote
But i'd prob go for the one resistor per led also

Why would you ignore your own advice? Is it perhaps because you know your advice is bad?

Whoaaa ... having a bad day ?   ;)

As i see it ,the LM317 wouldn't be worse then the "connect a resistor (13ohm 1watt) to 9 of them"
But you might be right in that it wasn't any better either.

I would chose 1 resistor per LED for getting an even light distribution on all 18.

@OP sorry about the advice.

@Ian B- Hope your day gets better

/Bingo
 

Offline casper.bang

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Re: LED Resistor
« Reply #16 on: July 03, 2013, 05:49:22 am »
1. LEDs do not require a regulated voltage, they require a regulated current
Semantics; matching the appropriate forward voltage, the current is predictable and in accordance with the datasheet.

Quote
2. Even if you configured the LM317 as a current regulator, it would not provide for current balancing between the parallel LEDs
More importantly, it would do absolutely nothing to solve the OP's problem of adapting 6V to 3.5V!

Quote
3. You could put several LEDs in series to ensure equal current, but then you would need a boost converter which the LM317 is not
A boost converter would need several resistors, caps and an inductor - now we're needing a PCB and lots of space.

Quote
4. An LM317 still needs resistors to set the output voltage or current
Sure but there *are* fixed voltage regulators that can do 3.5V like LM78D35 or AN77035SP.
« Last Edit: July 03, 2013, 05:54:15 am by casper.bang »
 

Offline IanB

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Re: LED Resistor
« Reply #17 on: July 03, 2013, 06:02:12 am »
Semantics; matching the appropriate forward voltage, the current is predictable and in accordance with the datasheet.

This statement is not correct. The forward voltage is a range that varies from batch to batch, varies between samples, and varies with operating temperature. Given an accurately regulated voltage across an LED you cannot predict the current passed with any degree of confidence. Any attempt to voltage regulate an LED without current limiting in the circuit will dispose the LED to premature failure.

The inverse statement is what is correct. Given a regulated current through the LED, you can use the datasheet to estimate the forward voltage with reasonable confidence. But datasheet numbers are typical. Individual samples may vary.
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Offline Jon Chandler

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Re: LED Resistor
« Reply #18 on: July 03, 2013, 06:04:56 am »
[

Quote
2. Even if you configured the LM317 as a current regulator, it would not provide for current balancing between the parallel LEDs
More importantly, it would do absolutely nothing to solve the OP's problem of adapting 6V to 3.5V!



No, Ian is exactly right.  The current must be balanced between the LEDs with individual resistors.  IF the LEDs were absolutely perfectly matched this wouldn't be true and the current would be split equally but this is never the case.

LEDs in parallel must have individual resistors.  Always.

LEDs in series only need one resistor for the group as the current is the same through each but the supply voltage must be greater than the sum of the LED forward voltages.
 

Offline IanB

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Re: LED Resistor
« Reply #19 on: July 03, 2013, 06:19:37 am »
Also, for others reading, don't fall into the trap of thinking the series resistor is a voltage dropping resistor. It is not. It is a current limiting resistor. There's a big difference.

If you do the algebra you will see that a high voltage in series with a large resistance approximates a constant current source. Since a constant current supply is what an LED needs, the high voltage plus resistor combination is a cheap and easy way to drive small LEDs.
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duskglow

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Re: LED Resistor
« Reply #20 on: July 03, 2013, 06:25:14 am »
So what makes the difference between a voltage dropping resistor and a current limiting resistor?  After all, they're the same resistor, and the current limiting resistor will also drop voltage.  Is it because ohm's law is a linear equation?
 

Offline IanB

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Re: LED Resistor
« Reply #21 on: July 03, 2013, 06:44:45 am »
So what makes the difference between a voltage dropping resistor and a current limiting resistor?  After all, they're the same resistor, and the current limiting resistor will also drop voltage.  Is it because ohm's law is a linear equation?

Typically the difference is the situation in which the resistor is used and the purpose it is assigned.

Voltage dropping resistors are typically used with other resistors in a potential divider situation. Suppose I put two resistors in series across a 6 V supply, say 1 k and 2 k. Then we know the 1 k resistor will be dropping 2 V and the 2 k will be dropping 4 V giving a total of 6 V to match the supply. The 4 V value will be stable and precisely known assuming the resistors are also stable and accurate. This voltage may now be used in other parts of the circuit.

In contrast, when we put a resistor in series with an LED or other current driven device we don't intend to regulate the voltage, we intend to regulate the current. We don't much care what the actual LED operating voltage is just as long as we don't exceed the maximum current through the LED.
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Offline Thor-Arne

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Re: LED Resistor
« Reply #22 on: July 03, 2013, 08:03:27 am »
Since no one suggested it, I'll just drop this here.  ;)
 

Offline MacAttak

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Re: LED Resistor
« Reply #23 on: July 03, 2013, 08:11:27 am »
For 18 LEDs just use some bussed resistor networks. They are not very large, and aren't expensive either. You should have no trouble finding 7 (or 10) pin bussed resistors, which will be plenty for your 18 LEDs.

A bussed resistor network is essentially a handful of resistors with one common lead tied together.

150 ohm is probably a little safer to use than 130 ohm, just in case your power source is slightly more than 6v or the forward voltage is a little less than 3.5v. 150 is also a more common value ( = cheaper). You probably wouldn't even notice the difference in brightness from the slightly lower current.

For example: http://www.digikey.com/product-detail/en/770101151P/770-101-R150P-ND/1000508

You would only need two of those since it packs 9 resistors per package.
 

Offline TerminalJack505

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Re: LED Resistor
« Reply #24 on: July 03, 2013, 08:36:48 am »
Just one more option that the OP should be aware of is the Joule Thief circuit.  These can be made fairly small.  The transformer will be the biggest component but it still wouldn't be terribly bulky. 

They're a lot of fun just to play around with.  They do require that you wind your own transformer and do some tweaking to get things optimized.  (You don't want the circuit oscillating at audible frequencies, for example.)

I've seen people run strings of 50 or more LEDs from a single 1.5V AA battery.  The circuit is also very efficient.  So if you're running your circuit from batteries and want to get the most juice out of the batteries then you should keep this circuit in mind.
 


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