What is the difference between the two circuits?

The circuit on the left is CORRECT, if and only if, ...
you use the proper size resistor to allow approx 10ma - 20ma current per LED, or as needed.
Typically, each LED needs it own current limiting resistor because each LED will have slightly DIFFERENT Voltage Drop.
To illuminate all LED's at the same brightness, the LED's will need the same current, this circuit does that = OK.

The circuit one the right is a very POOR design because ...
The LED's will NOT share the current equally because of the above mentioned DIFFERENT "Volt - Amp" characteristics of each LED.
Now, due to the varying current flow in each LED, some LED's may be way too bright and some LED's may be way too dim.
The circuit on the right is forcing EQUAL voltage across each LED, but we know equal voltage does not mean equal brightness.

Each LED must be allowed to have a have a slightly different VOLTAGE.
The circuit on the left - Allows for varying voltages, equal current and equal brightness - GOOD Design
The circuit on the right - Forces equal voltages but varying current and varying brightness - POOR design

 

Did up a short 4 1/2 minute video demonstrating what happens when LEDs are not matched for forward voltage. In the video I use two red super bright LED's at 1.95 Vf, one green super bright LED at 2.95 Vf and six blue super bright LED's at 2.82 Vf. In the calculation for the current I use the average value of the LEDs so that I can not over power any single LED. Where you see two red LED's lit, they're sharing 25.5 mA of current. Each is seeing about 12 to 13 mA. IF I were to up the current so that each saw 20 mA, when I unplug one the other would blow out because it would be seeing 40 mA of current. Enjoy the video.

 

In the left circuit you will get 0.5mA per LED, in the right circuit you will get 0.5mA to split between the 3 LEDs. ON PAPER, the 3 LEDs on the right would all be the same brightness, and 1/3 the brightness of the LEDs on the left circuit. IN THE REAL WORLD, it is very unlikely that all 3 LEDs would have the same forward voltage (due to manufacturing tolerances) so it is likely that they would not all be the same brightness. Assuming the resistors are all the same size, and that they are sized appropriately for a single LED, then the right circuit is not more at risk of failing, because even if one LED took all of the current it would not be too much for a single LED. However if the resistance on the right circuit were to be 1/3 that of the resistors in the left circuit (would pass too much current for a single LED), THEN the right circuit would be at risk of component failure, because of the likelihood that the LED with the lowest forward voltage would take too much current.

In the real world, you would use the circuit on the left, though with much smaller than 10k resistors if they were typical LEDs.

 

So, Tony…which one of those blue LEDs was going to run away with all of the current?

 

So, Tony…which one of those blue LEDs was going to run away with all of the current?

If any single LED were to run away with all the current it would not have been harmed because the current was limited to about 15 mA. As the video demonstrated, LED's with Vf with the potential variance of 30 mV, a standard deviation of 10 mA, they were all pretty well matched. If you noticed, the red LED ran away with all the current when it was plugged in parallel with the blue. The blue went out. IF I had designed the circuit with the intention of 15 mA per LED and 6 LED's, in other words a max of 90 mA (so that each would share about 15 mA), if one of them failed the rest would see an increase. Eventually a second would fail, leaving 90 mA to be divided amongst 4 LED's constituting about 22 mA. These numbers are not detrimental to LED's, yet it illustrates how if one LED hogs the major share of the current then fails it leaves all that current for the rest. Suppose three LED's fail. The remaining LED's would see approximately 30 mA, and that's higher than they should be. One would fail, quickly followed by a second and then the last LED would go out in rapid succession.

When I built the circuit it was limited to the max possible current of 45 mA (in the event of a short circuit). When you subtract the Vf you get lower current. Two red LED's at (lets call it) 2 Vf; 4.5 - 2 = 2.5. Divide that with a 100Ω resistor and you get 25 mA. That's a lot for a single LED but not so much that it will blow in short order. Put two red LED's in parallel and they share the 25 mA. Assume that they are identical electrically. Each would see only 12.5 mA of current. Again, if I designed for two LED's sharing current I would have been able to supply them with 50 mA. Under the assumption they are absolutely identical, they each would see 25 mA. At that amperage one (or both) would not last 10,000 hours. Assume for the sake of argument that one fails after 10 hours. At 50 mA - how long do you think the remaining red LED would last?

 

All you have proved is, LEDs can be run in parallel if they are properly de-rated, which is something I have been saying all along.

Running different colors in parallel is a nonstarter because it simply doesn’t work…period.

Running LEDs from different batches or different styles in parallel is also a no-no.

I’ve been doing it for decades, without any failures…you just have to know how to do it. I’m a little anal so I actually bin any LEDs that need to be run in parallel, but as a general rule I try to design my projects so they don’t need to be in parallel, but every now and then it solves certain problems, and knowing how to do it really comes in handy.

 

which is something I have been saying all along.

OK, so we agree. I still say planning for max amps times the number of LED's is almost a guaranteed failure.

I've designed chase lights that have a single resistor on the down side of the LED's. Each LED is turned on sequentially, one at a time and only one at a time. So even though I've built 30 LED chase stages I've used only one resistor. Some would say that each LED needs its own resistor, but they don't. Like you say, there's times when it works and times when it won't. Even the video proves that you can't run a red and a green LED at the same time off of a single resistor. However, in dinking with it - I DID get the green to light with a blue. The Vf was close enough, but the green was quite so much dimmer than the other that it was visually obvious.

 

How many LEDs can be wired in parallel; just bought a 5 ft. string with 30 LEDs in parallel. Nice uniform brightness
& total drain is 58 ma. operating on 3 AAs with 6 & 24 hr. timers.
Maybe I should sub. a wall wart & see how long they last.

 

How many LEDs can be wired in parallel; just bought a 5 ft. string with 30 LEDs in parallel. Nice uniform brightness
& total drain is 58 ma. operating on 3 AAs with 6 & 24 hr. timers.
Maybe I should sub. a wall wart & see how long they last.

I’ve purchased LED strip lights that have 300 LEDs. I ran them off a 12VDC 5A power supply. Each LED has its own resistor and the LEDs are grouped in threes.

 

I have a 16 foot long set of color changing LEDs in my sauna. They run on 12 volts. Off hand I don't know the amperage, nor am I certain exactly how many LED's there are. They're grouped in sets of 3. I believe there are four lines. One ground and three lines, one each for 1 of 3 LED's, RGB. The controller varies the brightness of a string in varying combinations to produce a rainbow of colors. Set for a slow sweep through the colors is very relaxing. But I think we're getting away from the TS question.

 

Each LED has its own resistor and the LEDs are grouped in threes.

You mean each group of three LEDs has it's own resistor?
SG

 

You mean each group of three LEDs has it's own resistor?
SG

No, I mean each LED has its own resistor. Here’s a group of three I just happened to have beside me...

 

Each resistor is marked 331. That means 3 and 3 and one zero. In other words, 330 Ω. The strip is 12 volt powered. Without knowing the voltage drop across the diodes (Vf) I couldn't tell you the exact current going through the LED's. It appears as each "LED" is actually a triple LED, meaning there are three elements on a single chip. If we assume a Vf of 3 volts then we take 12 and subtract the 3, leaving us 9 volts. 9 divided by 330 equals 27.3 mA of current. That seems a reasonable assumption. 3 LED's sharing 27.3 mA gives just little over 9 mA for each element. Of course you know I'm only guessing at the values, but this demonstrates how you can use a single resistor to limit the current over 3 LED's. In this case the Vf is tightly controlled. I'm sure their standard deviation for the chip LED's is less than 10 mVf. (Ten Millivolts Forward Voltage).

If you look carefully at the picture you see that the chip LED has six legs. It appears that each element of the chip has its own set of legs. So there are three elements on the chip, all controlled via a single resistor.

In my video you see what happens when the Vf is not matched. In the case of the red and green LED's, when one is lower than the other it hogs the current. The red LED had the lower Vf.

 

So there are three elements on the chip, all controlled via a single resistor.

Yes, but maybe the LEDs are in series. Each LED would still see 9ma.
EDIT: Wait a minute, if there are 300 LEDs and the they each draw 27ma that would be a total of 8.1 amps at 12 volts. Can't run that on a 5 amp supply.
Probably means 100 modules X 3 LEDs = 2.7 amps or 100 X 9ma = .9 amps
SG

 

@sghioto Yeah, I did say I was just guessing at the numbers.

I have those RGB LED's in my sauna. I'd have to go down stairs and pull the power supply out of the wall to see what it's rated at, but if I had to guess I'd say probably 4 or 5 amps at 12 volts. But it's possible that my LED's are being PW Modulated. The red may be not on when the blue is or when the green is. The combined affect may be the perception of colors mixed in by the RGB LED's. I guess you could run the numbers backwards. 5 amps (a.k.a 5000 mA) divided by 300 LED's. That comes to 16.7 mA per LED. At that amperage they'd be plenty bright but no where near to their burn-out point. Again, I'm just guessing - without having hard numbers to work from.

 

For 30 mA at 5 V you'd need a resistor value of (and I'm assuming a forward voltage for the LED at 2V) would be a 100Ω resistor. 10KΩ is just way too high. At that resistance your LED will only see 0.3 mA (0.0003 A). IF your LED lights up - you'd probably need a match just to see if it's glowing.

[edit] BY THE WAY - - - 30 mA is a LOT of current for a typical LED. Unless you're talking about one of those LED's that are used as a light source. But then in that case 30 mA is rather small. Especially given the use of 5 volts.

Dear friend, never consider the resistor values

 

h sv,
Look at your image.
E

Dear friend ,
Thank you for your time.
Never consider the resistance. I know 10K is very high. A maximum of 160 Ohm is sufficient for 30mA current flow.

 

A maximum of 160 Ohm is sufficient for 30mA current flow.

hi,
For general purpose LED's the datasheet states approx 20mA as a maximum current.
For that type I would recommend using not more than 15mA.
E

 

Yes, but maybe the LEDs are in series. Each LED would still see 9ma.
EDIT: Wait a minute, if there are 300 LEDs and the they each draw 27ma that would be a total of 8.1 amps at 12 volts. Can't run that on a 5 amp supply.
Probably means 100 modules X 3 LEDs = 2.7 amps or 100 X 9ma = .9 amps
SG

BTW, each LED in the strip I attached draws 0.0167A.

 

BTW, each LED in the strip I attached draws 0.0167A.

What, only four decimal places of precision?! How crude!