LED's in Series Vs LED's in Parallel

1. May 8, 2013

CraigH

This should be a simple question yet I am struggling to see the benefit of using one over the other.
I was building an LED and LDR based sensor head for a line following robot, and I needed to choose a method of connecting 3 LED's inside of it. I figured that the 3 options are:

1) All in series with a resistor

2) All in parallel, with the parallel bit in series with 1 resistor

3) All in parallel, each with their own resistor

4 important points:
.The LED's are rated at 3V, 20mA, 60mW. This is what I am aiming for.
.The LED's should all be as bright as possible, and more importantly they all need to be the same brightness (so that the sensor head is accurate).
.The 5V is set, this cannot be changed, however the resistors can be any value at all.
.I'm guessing the LED's are 150 Ohms as 3V/20mA = 150 Ohms, but the resistances of each may slightly vary due to manufacturing inaccuracies.

The voltage and current through the resistors can be controlled equally well in each case as the value for the resistor in each case can be changed.

I was told that option 3 is the best option, and option 1 is completely useless, but I don't see why. Is it something to do with the fact that a diode does not have a linear I-V characteristics?

So the relationship between current and voltage is linear like with a resistor. I have always assumed that this doesn't matter for an LED, as the main reason there is a voltage across this is because energy has been converted to light, (like a resistor does as heat). This is unlike a diode whose "resistance" changes as you change the voltage across is.

Thanks!

Last edited: May 8, 2013
2. May 8, 2013

phinds

You are saying that each LED requires 3 volts, yes? If you put 3 of them in series, how many volts would you need?

3. May 8, 2013

davenn

Do you see the problem with this scheme ??

look at your supply voltage 5V, Vs you voltage drop for each LED 3V and the resistor

thats why parallel LEDs are better if there's going to be a lot of them

Dave

4. May 8, 2013

davenn

hey phinds SNAP LOL

you typed less ;)

Dave

5. May 8, 2013

Introyble

Im trying to understand this:

Why would you ever put lights you wished to be of equal "brightness" in series?

6. May 8, 2013

Integral

Staff Emeritus
A instrument I build has LEDs in series. But this is to guarantee that either both are working or neither.

7. May 8, 2013

davenn

Ohhh its no problem 3 LEDs in series and with a current limiting resistor work well
all the same brightness.
But as the OP will discover he will need more voltage ;)

Dave

8. May 8, 2013

CraigH

Ah yes, it would be impossible to have 3 Volts across each in series, as this would add up to more than 5 Volts. I see now, thanks for answering.

Although I'm still not sure why each having their own resistor is better than just having one in series with the all the parallel resistors? What is wrong with having one resistor that splits the voltage between itself and the parallel part as 2V and 3V?

If they are all of equal resistance (or at least very close) then the voltage across all of them will be the same, and as they are in series the current through all of them will be the same, so they will all be equal brightness, no?

9. May 8, 2013

Staff: Mentor

With newer "matched" LEDs, you can put them in series. You do however, need enough voltage headroom to make it work, as pointed out by other posters.

The disadvantage is that matched LEDs tend to be a bit more expensive. The advantage is that you can run them from a higher voltage, and lose less power in the series current-setting resistor for each series string.

Series LED configurations are common in LED lighting applications, where you may have several strings of series connected LEDs (each with its own current setting resistor or low-side current source transistor) placed in parallel.

10. May 8, 2013

Introyble

Yeah, I have to much WCAD (who cares attention deficit) to work the simple math stated by the OP. I'm going to guess equal "brightness" is on the less than probable result of the hypothesis

11. May 8, 2013

Introyble

Guess what I'm really trying to say is in the situation where the "brightness" is so paramount to the problem itself that I would never consider series with a 5V supply. We can assume your supply is VDC? Yes, I know you can also go AC with LED. Just curious...... what happens to the equation as your supply begins to deplete it's ampacity???????

12. May 8, 2013

Staff: Mentor

Actually, the best way to do this for good battery life would be to do a boost DC-DC circuit to drive the 3 LEDs in series, and use current-mode control (with a small low-side sensing resistor) to keep them operating at 20mA...

13. May 8, 2013

CraigH

Yeah the supply is from a 5V DC battery (a pretty big rechargeable one). This battery is also powering a motor drive circuit (in parallel with the LED circuit).
As the source depletes the voltage across it will decrease, and therefore the voltage across everything in the circuit will deplete, and hence the current through everything will deplete, so they will become dimmer.

I've never heard of a boost DC-DC circuit, but my robot is running about of space to mount new circuits anyway so it probably couldn't be done. But out of curiosity, how do they work?

14. May 8, 2013

Staff: Mentor

http://en.wikipedia.org/wiki/Boost_converter

They are one of the topologies of DC-DC converters. Normally you use voltage feedback from the output voltage supply to control the duty cycle of the switching action. But in LED applications, you put a small-value resistor at the bottom of the string of LEDs, and use that sense voltage to regulate the current out of the boost DC-DC to the desired value (20mA in your case).

15. May 8, 2013

davenn

google DC - DC Boost Converters.
And for your interest also look at DC - DC Buck Converters, for dropping and regulating voltage
They are much more efficient than linear regulators like the LM78xx series.

Dave

16. May 8, 2013

davenn

also for the obvious reason .... to get 3 times the brightness out ;)

I have a commercial product I produce that has 3 sets of 3 LEDs in series
all hi-intensity 5mm LEDs, 3 sets of red, of green and of blue

Dave

17. May 8, 2013

CraigH

I will do thanks, I should have done that before asking really haha.

Just one last question though:

I was told option 3 is better than option 2, do you know why this is? To me it just seems like a waste of resistors.

18. May 8, 2013

davenn

well yes more resistors ... but the cost is irrelevent, they are so cheap.
but if you are just going to use 1 resistor supplying ALL the parallel branches, it will be a higher wattage to handle the extra current. and if it fails then your whole display dies where as if you have individual resistors in each branch and one failed then you would only that one branch

Dave

19. May 8, 2013

Introyble

Let's do this for fun please????

Debate this topic from the true stand point of modern engineering. No, I don't mean academic theory vs real world experience.

I mean, the cheaper the better!!!!

Now pretend you own the market share and like you are trying to gain market share.

Accounting balance sheets lol

20. May 8, 2013

Introyble

Dave, why not the1970's christmas light argument on the series option? Which is more efficient? Do the math, I don't want to. Which loses the most due to heat? Which ever one is mathmatically using less wattage is the option I pick. Would you consider that a load resistor?

21. May 8, 2013

davenn

several of us have already said why series option is bad( not as good)

Dave

22. May 9, 2013

Staff: Mentor

But you said you use that configuration in your product...no?

And I mentioned how many LED lighting applications use series connected matched LEDs, with DC-DC converters regulating the current of each series-connected string.

23. May 9, 2013

sophiecentaur

Why not? If they're in series, they would have the same current through each - which would make them equally bright - if they are of the same type.

24. May 9, 2013

davenn

No, not a christmas tree light configuration where there would be many many LED's in series

My units are a calibrated sound monitors for clubs and pubs etc to monitor noise level and keep it under the level required by local council ordinances.

My sets of hi intensity LED's 3 red, 3 green, 3 blue are on a remote display case separated from the main base unit by 15 - 30 metres of cable ( depending on the installation circumstances). They are also being driven by 24VDC that overcomes cable voltage drop over that distance

Anyway this is getting way off the original point .... the OP not realising that he wasnt going to power 3 x 3V LEDs from a 5V supply ;)

Dave

25. May 10, 2013

sophiecentaur

Re-reading this thread I can't see there's any argument about how it must be done. If you only have 5V available then you can only connect 3V Diodes in parallel. Using separate series resistors for each will probably ensure that their brightnesses are nearer the same value and you won't 'waste' any more power.
There are practical reasons not to bother with a DC-DC converter (cost for a start).
If you are worried about battery life and what happens when it starts to die, the volts will dip, of course but most rechargeables, these days, maintain a good voltage until they are nearly dead so I don't think it's worth worrying about that too much. Just buy a bigger battery which will last long enough.