Question about resistors and LED's

[Ray H]

Surely, it is interpretation time again.

 

It matters not whether we are talking about LEDs or (for example) motors.

 

I believe from what has been said before that it is correct to say that individually they can each take, suffer, accept, use, call it what you will, different current values.

 

However if they are all connected in series then from what is also said above it is correct to say that they will all see (or whatever the correct term is) the same current which may not be their ideal current and, in some instances, presumably this could no doubt cause a malfunction.

 

I wouldn't even hazard a guess as to which of a string of series connected components sets the current that flows through them all (but I wouldn't mind knowing out of interest).

 

For a change it isn't the electronics that is doing my head in, it is all the disputes.

[Roger.s]

I wouldn't even hazard a guess as to which of a string of series connected components sets the current that flows through them all (but I wouldn't mind knowing out of interest).

 

All of them!

 

In simplified terms: add together the Vf of all the series LEDs and take this from the supply voltage.This gives the voltage across the series resistance and Ohm's law will tell you the current.

 

Now for the less simple bits. The Vf varies according to the current - see the diagram in post 35 or the data sheet for you particular LED if you have it. This means that you do not know the exact Vf to work out the current. With more than 1 LED in the circuit, power, colour and manufacturing tolerances will all have an effect making it difficult to calculate the current.

 

For example with 1 LED with a Vf of about 2V and a 12v supply the Vf might vary by 0.3v giving 9.7v - 10v across the resistor this is approx 3% and the current is undetermined by the same percentage.

With 5 LEDS the total Vf could vary by 5 x 0.3v giving between 12 - (5 x2) and 12 - (5x2.3) volts across the resistor. that is the voltage can be between 0.5v and 2v and the current vary by 4 : 1.

 

The practical solution is to arrange that the sum of the Vfs is not to high a portion of the supply voltage ant to aim for a current on the low side and then increase it if the LEDs are note bright enough.

 

Regards Roger 

[DCB]

Has anyone reading this thread actually tried connecting a string of diodes in series?  Or more usefully a string of random LEDs of various colours and sizes like the ones which rattle around in my toolbox

Did the red one burn brightly, emit smoke and die while the others barely glowed?   Or do they all consume the same current and glow at similar brightness?

I will have a play later if no one responds, maybe put it on YouTube.

 

On a different tack ...Surely the reason to avoid series connection is that when one LED or its connection fails the whole string fails with all the problems of working out which one is at fault?

[Roger.s]

Has anyone reading this thread actually tried connecting a string of diodes in series?  Or more usefully a string of random LEDs of various colours and sizes like the ones which rattle around in my toolbox

Did the red one burn brightly, emit smoke and die while the others barely glowed?   Or do they all consume the same current and glow at similar brightness?

I will have a play later if no one responds, maybe put it on YouTube.

 

Yes I did.  IIRC I had about 15 red LEDs on a 24v supply tastefully draped over a Christmas tree. They were all the same type and they all light up with equal brightness. I forget what value resistor I used but be assured, I did use one.

As has been said before the current passes through the LED the same current then passes through the next LED and so on until the end. If the LEDs are well matched ie same make, type number and manufacturing date they should all be about the same brightness. If the LEDs are not all exactly the same then some may well be brighter than others.

 

On a different tack ...Surely the reason to avoid series connection is that when one LED or its connection fails the whole string fails with all the problems of working out which one is at fault?

Another consideration is the power dissipation. Suppose you want to light several LEDs at say 10mA each from a 12V supply. For 1 LED with Vf = 2v a 1k resistor gives 10 mA

for 5 LEDs if 5 x Vf = 10V then a resistor of 200 Ohms gives 10mA (but see my post no 52) However both dissipate 120mW so the 5 in series use 120mw while the 5 separate LEDs use 600mw.

These power levels may not seem much but become significant when the are dozens or even hundreds of LEDs in use, or for example in train lighting circuits when there may be a limited current available and excess heat can damage plastic.

 

Regards Roger

[Crosland]

Has anyone reading this thread actually tried connecting a string of diodes in series?  Or more usefully a string of random LEDs of various colours and sizes like the ones which rattle around in my toolbox

No need. I know what will happen.

 

Did the red one burn brightly, emit smoke and die while the others barely glowed?

You do need to do a simple calculation first (as has already been explained) to set the current to be below the maximum allowed for any individual LED.

 

Or do they all consume the same current...

The same current flows through all of them. The word "consume" is wrong, the current doesn't disappear. What is consumed is power.

 

...and glow at similar brightness?

If they are identical types, and well matched, then there will be no discernible difference in colour or brightness. If they are different colours, well, I'm sure you can work that one out :-)

 

If they really are a truly random selection, from a variety of LED types, sizes and ages, then there will almost certainly be differences in brightness between LEDs of the same colour.

 

On a different tack ...Surely the reason to avoid series connection is that when one LED or its connection fails the whole string fails with all the problems of working out which one is at fault?

As has already been posted.

 

Andrew

[DCB]

L

 

 

Posts showing my LEDs working quite happily from a low voltage Transformer with no resistor 16.6ma at 2.04 volts measured from a single LED one of 14 wired in parallel.

[Tricky Dicky]

 

 

Posts showing my LEDs working quite happily from a low voltage Transformer with no resistor 16.6ma at 2.04 volts measured from a single LED one of 14 wired in parallel.

Oh no! Not again?

 

Richard

[SHMD]

 

Posts showing my LEDs working quite happily from a low voltage Transformer with no resistor 16.6ma at 2.04 volts measured from a single LED one of 14 wired in parallel.

 

 

It's a good job you live in a country with a "well regulated" mains supply, because all your LEDs would have popped long ago in some of the countries I've visited!

 

 

Kev.

[Crosland]

Posts showing my LEDs working quite happily from a low voltage Transformer with no resistor 16.6ma at 2.04 volts measured from a single LED one of 14 wired in parallel.

 

As pointed out already, no one disputes you can do it.

 

Relying on the internal resistance of the power supply, to limit the current, is never good design practice in any situation.

 

You do what you like, but please do not promote it as the right, or even acceptable, way to do things, without also pointing out the caveats that could catch a novice unawares.

 

Andrew