Current control for lots of Led's

Hello all,

Looking for abit of guidance.

I am trying to make an array with 341 led's in it. The plan is to use 5mm through hole Led from RS.
(https://uk.rs-online.com/web/p/visible-leds/8614133/)

2.5v each 20ma current

I am limited to a voltage of 12v so will be running them in strings of 4.
I do know it would be much easier for myself to have a higher voltage but my main question is the same no matter what voltage i run at.

Now previously i have made simple constant current control devices, using an lm317, to control high power leds but they seem overkill and space constraints.

Now does anyone know of any constant current control devices/circuits that can control multiple parallel strings (doubt any could do the whole 86) or any other ideas on how i should go about driving the Led's.

Thanks in advance

Edit: So i was potentially looking at changing the led's at a future date to high power ones which will have a much larger current draw and i do not think it is efficient to just drive them with a resistor.
https://www.diodes.com/assets/Datasheets/AL3066.pdf
The AL3066 was something along the lines i was thinking off. A constant current device with four channels.

I doubt you're going to find something smaller or cheaper than 86 resistors.

Pieter

Led strips use one current limiting resistor per three or four LEDs (depending on colour).
A simple resistor works fine for that sort of current.
100ohm should get you 20mA, assuming the four LEDs in series drop ~10volt and the 12volt supply is regulated.
The resistor only dissipates 2*0.02= 0.04watt.
Brightness control can be done by PWM-ing the whole array with a single logic level mosfet, since the whole array only draws ~1.7Amp.
What are you making. Isn't a red LED strip an easier solution?
Leo..

PieterP:
I doubt you're going to find something smaller or cheaper than 86 resistors.

Pieter

Yes it would be the easiest/cheapest but unfortunately the 12v supply isn't regulated as it will be drawing from a battery. I suppose i could get a buck boost converter to get that regulated supply and then use the resistors. Definitely the back up plan.

Wawa:
What are you making. Isn't a red LED strip an easier solution?
Leo..

It is a light box thing with lots of drilled holes that the 5mm fit into. Led strips won't line up correctly/fit.

There is a potential that i will change the Led's which will have a much larger current draw hence i wanted to go the constant current route, the version i am working on currently is just the test.
Though i suppose i could always just use larger resistors the dissipate the heat.

I was mainly curious if there was anything available on the constant current route rather than just resistors.

Wiring 300 of those LEDs will take days of work, including tracking down the one or two that ended up backwards. Then there will be a bad solder joint and then one of the smallest wires will break while you are moving it to find the other problems.

Buy them in strips. For WS2812 LEDs it is always cheaper to buy them on strips instead of in bulk. I don't know about single red LED strips but if your time is worth more than 5 cents per hour, strips will be cheaper.

Madmonkeyrtu:
I suppose i could get a buck boost converter to get that regulated supply and then use the resistors.

If you are going to do that I suggest a boost converter to, say, 24V and put enough LEDs in series to get to about 22V drop, then an appropriate resistor for the required current. You could even go to a higher voltage and have more LEDs in series.

I was going to suggest that, but I went to bed instead - too tired! :grinning:

Actually, what I was going to say, is that you need a "boost" converter with a current-controlled output, so that you do not waste power in resistors. Of course, you then need one per chain, so as many LEDs in the chain as possible.

Wonder what they use in those high-power 12 V floodlights?

If OP was using COB leds for that mysterious "floodlight", then e.g. a CC module with a PT4115 would be ok.
But for 5mm/20mA indicator LEDs that don't get hot, nah. Resistor will be fine.

What is the 12volt source.
Leo..

Wawa:
But for 5mm/20mA indicator LEDs that don't get hot, nah. Resistor will be fine.

Not necessarily.

A resistor requires a substantial "burden" to account for variations in the supply voltage and temperature-related variations in the LED voltage drops. Using more LEDs in series does not make this less critical at all; the temperature-related variation in the LED voltage drops is multiplied according to the number of LEDs, so you require a proportionately higher "burden" voltage across the resistor.

Using an active current limiter such as this allows you to minimise the "burden" voltage to a couple of volts more than the difference between the maximum voltage of the led chain and the (minimum) regulator output.


This allows you to reduce the wasted heat/ power, but is not as good as a switchmode current driver.

Paul__B:
A resistor requires a substantial "burden" to account for variations in the supply voltage and temperature-related variations in the LED voltage drops. Using more LEDs in series does not make this less critical at all; the temperature-related variation in the LED voltage drops is multiplied according to the number of LEDs, so you require a proportionately higher "burden" voltage across the resistor.

On top of that, if you use more LEDs in series, the voltage across the resistor gets smaller, so you need a smaller resistance to set the same current. This means that the slope of the load line gets steeper, resulting in much higher sensitivity to both supply voltages changes, and changes in forward voltage.

PieterP:
On top of that, if you use more LEDs in series, the voltage across the resistor gets smaller, so you need a smaller resistance to set the same current. This means that the slope of the load line gets steeper, resulting in much higher sensitivity to both supply voltages changes, and changes in forward voltage.

Well, that was my point.

The voltage across the resistor does not get smaller at all, quite the opposite because you need to have proportionately more voltage across it which is to say a proportionately higher resistance in order to compensate for those factors. :roll_eyes:

Paul__B:
The voltage across the resistor does not get smaller at all, quite the opposite because you need to have proportionately more voltage across it which is to say a proportionately higher resistance in order to compensate for those factors. :roll_eyes:

If you only change the number of LEDs and the resistor value, with the same supply voltage and operating current, the voltage across the resistor gets smaller.

The solid red lines are the load lines (IV curves of the resistors) for one LED (bottom) and for four LEDs in series (top). The resistor values (R1 and R4 respectively) are selected to set the current to the rated forward current of the LEDs (If).
The blue lines are the IV curves for one (left) or four LEDs in series (right).
The operating point of the LEDs is the intersection between the IV characteristics of the LEDs and the resistor.

If the supply voltage increases from Vs,1 to Vs,2, the load lines shift to the right (red dotted lines). Their slope doesn't change, because the resistance is constant. This moves the operating points to a higher current.
In the case of four LEDs in series, the change in current is much more significant than in the case of only a single LED.

PieterP:
If you only change the number of LEDs and the resistor value, with the same supply voltage and operating current, the voltage across the resistor gets smaller.

But that is not what we are discussing here. :astonished:

Wawa:
What is the 12volt source.
Leo..

Just a lead acid battery

Using the PT4115 you are meant to only drive a single string of Led's. I wonder if i could get away with 'testing' with some parallel strings, i was looking at current mirrors and they seem to be worth trying out.

No, you can't put LEDs in parallel. They are so highly non-linear that the tiniest mismatch will burn one string more than the other.

But you can use a small resistor like 1 Ohm on each string in parallel, with a controlled power supply.

MorganS:
But you can use a small resistor like 1 Ohm on each string in parallel, with a controlled power supply.

I think more than 1ohm is needed to ballance 20mA LEDs.
A drop of ~10% of Vf of the LEDs is more realistic (4*0.25volt).
Then there is little overhead left for current regulation.

I still wonder why OP thinks a simple CL resistor can't be used.
Lead/acid battery voltage doesn't vary a lot during discharge, and 5mm indicator LEDs don't get hot.
And why that many indicator LEDs are used instead of a COB light.
Leo..

MorganS:
No, you can't put LEDs in parallel. They are so highly non-linear that the tiniest mismatch will burn one string more than the other.

So are you saying this as a general rule or specifically when using a step down converter? I have seen numerous examples of people running leds in parallel.

To try and balance the strings was why i was looking at current mirror transistor linked at the end.

This example has the current mirror and uses different led types and amounts.

Wawa:
I still wonder why OP thinks a simple CL resistor can't be used.
Lead/acid battery voltage doesn't vary a lot during discharge, and 5mm indicator LEDs don't get hot.
And why that many indicator LEDs are used instead of a COB light.
Leo..

With regards to the 20ma led's i 100% see a resistor at the beginning of each of the strings would be the easiest solutions with a buck boost converter of some sort to up the voltage.

That many indicator leds because there are that many 5mm holes to put them in.

As a general rule. Try it with two LEDs. One will be brighter than the other. With only 2 it may be difficult to see the effect. Two strings of 6 will make it obvious.

"Numerous examples" might be people who don't care their strings are different brightnesses. Or maybe they were lucky and the accumulated resistance of the wires gives them just enough 'flex' in the system.

So i have added below as an edit to the beginning of the post, i will put it here also.

So i was potentially looking at changing the led's at a future date to high power ones which will have a much larger current draw and i do not think it is efficient to just drive them with a resistor.
https://www.diodes.com/assets/Datasheets/AL3066.pdf
The AL3066 was something along the lines i was thinking off. A constant current device with four channels.

Madmonkeyrtu:
That many indicator leds because there are that many 5mm holes to put them in.

Yep, that's a good enough reason.