# Powering 1000 LEDs in a grid

Thread Starter

#### Jana Janous

Joined Sep 9, 2018
3
Hello, I am making a light installation of 1000 leds in a grid 20x50 leds. I want to connect 20 leds in a series and then connect each series in parallel (50 lines in total). Each led is about 3.1V 20mA. Can anyone help me with what adapter I would use to power this? (I have a regular EU plug 230V 16A). And do I need to put resistors? And where? Final question, because of the design restrictions I can fit max of 24awg cables in the series part of the installation, but I can put any size of a cable between the lines - for the parallel connection. What size of a cable would I use? I hope someone finds this possible and challenging enough to help me out.

#### wayneh

Joined Sep 9, 2010
17,160
You need a DC power supply capable of nearly 70v and rated to over 1A. (Direct-to-mains LED projects are a forbidden topic here.). Once you have that, you can place one current-limiting resistor on each series string.

This assumes you’re powering the whole thing all at once. If you expect to control individual lights or strings, that will require more thought.

#### Dodgydave

Joined Jun 22, 2012
10,137
Ok you have 20 leds @ 3.1V in series ,thats 62V, with a 20mA source , you're going to need a 70V DC supply with a 470 ohms 1/4W resistor in series with each strip, should give you approx 17mA.

Then you need a psu with 1A minimum .

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#### -live wire-

Joined Dec 22, 2017
927
You need a DC power supply capable of nearly 70v and rated to over 1A. (Direct-to-mains LED projects are a forbidden topic here.). Once you have that, you can place one current-limiting resistor on each series string.

This assumes you’re powering the whole thing all at once. If you expect to control individual lights or strings, that will require more thought.
You could just have a schematic with an isolation transformer, but if it really doesn't need isolation, the TS could not use one.

The forward voltage of an LED can vary a lot, based on temperature and other factors. So it is generally recommended to use constant current in large arrays. Also, have you considered something like this (in the color you're using)? It would probably be more compact and about as bright. You would just need to make sure to get a good heatsink, with a fan.

#### oz93666

Joined Sep 7, 2010
737
Error in your figures !! ... the current (max) per line is 20mA not 400mA ... they are in series

That makes a total current of 1A, at about 70 V ..

It's always best to underdrive leds they will last a lot longer , perhaps about 15mA ...

No need to put a resistor in each strip .... Just on the connection to the power supply.

You need an Ammeter , measure the total current entering the whole array and adjust the resistor value to get a current of about 0.75A

You could do this by connecting directly to the mains . Mains voltage does vary slightly during the day , to avoid change in light intensity put 40 in series (140V) , connect these groups of 40 in parallel and a resistor to get the correct current (0.35A)... a full bridge rectifier then a capacitor to eliminate the flicker.

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#### wayneh

Joined Sep 9, 2010
17,160
You could do this by connecting directly to the mains .
I’ll be surprised if a mod fails to come along and comment on this! Read the terms of service.

#### -live wire-

Joined Dec 22, 2017
927
Try a simple constant current switching power supply. Maybe like this.

fuse --> Isolation transformer (if necessary) --> FULL BRIDGE RECTIFIER --> capacitor --> HV MOSFET + inductor and capacitor in buck converter configuration --> shunt and voltage divider --> switching microcontroller or IC ^^^ --> LEDs.

#### oz93666

Joined Sep 7, 2010
737
I’ll be surprised if a mod fails to come along and comment on this! Read the terms of service.
Do I really need to say DON'T TOUCH WIRES CONNECTED TO THE MAINS ...

I did this sometime ago , connected 8 100w (32V ) leds in series to 240AC NO RESISTOR,,, there was an annoying flicker ...( these leds start to light at about 25V , max current at about 32V )

I just tried again with a 3W ...3.2V led in series with a 67K Ohm put straight into the mains ... no flicker ... why should that be???

Thread Starter

#### Jana Janous

Joined Sep 9, 2018
3
Thank you for the quick responses. Would a LED Driver such as this work?

LED Driver 35W 70V 500mA APC-35-500 Meanwell AC-DC Switching Power Supply APC-35 Series MEAN WELL C.C Power Supply

With a current-limiting resistor on each series string.

I mean I know it's 500mA and not 1A. But 10mA per LED would be enough, I tried, and it's very bright. Would the current distribute evenly? (sorry if this is a silly question).

#### oz93666

Joined Sep 7, 2010
737
that's a strange reply from dodgydave !!! That power supply seems ideal perfect current , just what you want it will keep the current at the level required , no need for any other resistors OF ANY KIND . I use these devices , they are designed for running leds.. There is no need for current limiting resistors anywhere Why would there be??? The current will be divided evenly (within about 3%) through each line of 20 leds

As for will the current distribute evenly ?? almost certainly . there are minor differences in leds , but if selected randomly the differences will even out and each line of 20 will have about the same back voltage .

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#### Sensacell

Joined Jun 19, 2012
2,929
You will experience much less pain if you run this setup from 12 Volts.
Buy a 600 Watt, 12V power supply, that's double what you need.

- One current limiting resistor for each LED - 20 ma.

- Make all of this a parallel branch circuit.

- 50 branches of 400 ma is only 20A, your #6 AWG is more than hefty enough.

#### oz93666

Joined Sep 7, 2010
737
Those are Constant Current devices, the one he selected is 500mA with 25-35V output, i don't think the led will last very long with 500mA zapped through it!

Yes they will work, but he needs 20mA.
Jeezz Dave !! you've made 7k posts and have 1k likes but your advice is very dodgy!!

He spelled it out quite clearly with a diagram... he has 50 lines of leds in parallel each consuming 20mA .... that's 1A max !
And the voltage is 70 not 25-35

As for the moderators post above !! words fail me!

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#### digsys

Joined Jun 27, 2018
27
Agree with oz .. constant current is by far the preferred method ! Takes V drift into account. I use it for near all my high power led projects.
I'd make ONE change. I'd add a small resistor - say 0.5V - 1.0V drop (1W ish) per string. It works like an emitter resistor in the power supplies
ie helps equalize the currents in each string. The total voltage drop per string will always be slightly different, and this helps a LOT.

#### Dodgydave

Joined Jun 22, 2012
10,137
@oz936 i see your reasoning , 500mA over 50 strings is 10mA per string, but you can't guarantee each string will have the same current.

Then what happens if a string or two go down, all the current goes in the other strings of leds.

#### WBahn

Joined Mar 31, 2012
26,398
Do I really need to say DON'T TOUCH WIRES CONNECTED TO THE MAINS ...

I did this sometime ago , connected 8 100w (32V ) leds in series to 240AC NO RESISTOR,,, there was an annoying flicker ...( these leds start to light at about 25V , max current at about 32V )

I just tried again with a 3W ...3.2V led in series with a 67K Ohm put straight into the mains ... no flicker ... why should that be???
The Terms of Service you agreed to abide by are very explicit -- discussion of LEDs to mains is a prohibited topic. You don't have to agree with it, but if you want to remain a member here, you do have to abide by it.

One of the reasons is that many people (including yourself, apparently) have very little grasp of what you are playing with and the risk of working on what you think is a safe circuit is actually placing you at high risk of getting tangled with the mains.

To answer your question, without the current limiting resistors the voltage across the LEDs is spiking enormously. A 240 VAC power line has a peak voltage of about 340 V, which means that the voltage across each LED is peaking at around 42 V resulting in excessive current for a very brief period of time each cycle. However, since they start to light at 25 V (taking your word for that), the voltage has to rise above 200 V before they even start to turn on. Thus they are off for about 2/3 of the cycle with very high excessive currents for a significant portion of the time that they are on.

In the second case, you have a current that starts flowing at 3.2 V, or less than 1% of the peak voltage and then varies as a half sinusoid up to about 5 mA peak (340 V / 67 kΩ) over nearly the entire half period that the diode is forward biased. The human eye can smooth that out very easily.

#### WBahn

Joined Mar 31, 2012
26,398
that's a strange reply from dodgydave !!! That power supply seems ideal perfect current , just what you want it will keep the current at the level required , no need for any other resistors OF ANY KIND . I use these devices , they are designed for running leds.. There is no need for current limiting resistors anywhere Why would there be??? The current will be divided evenly (within about 3%) through each line of 20 leds

As for will the current distribute evenly ?? almost certainly . there are minor differences in leds , but if selected randomly the differences will even out and each line of 20 will have about the same back voltage .
Putting LEDs, particularly power LEDs, in parallel without ballast resistors is asking for trouble.

Even if you get them perfectly matched (which is impossible), they will not dissipate power equally well and hence some strings will heat up slightly more than others. The ones that don't cool as effectively will get warmer, which means that their voltage at the same current will drop because they have a negative thermal coefficient. Since they are all in parallel and thus have the same voltage impressed across them, the warm ones will take more current leaving less for the cooler ones. But that results in the warm ones heating up more and the cool ones cooling down, thus making the situation even worse. This is known as thermal runaway. Unless the LED has a significant resistive component in its I-V characterstic, the result will be that one string will end up hogging nearly all of the current and, if it fails as a result, another will then hog the current. In the end, you have a cascading failure that starts of slow and then accelerates -- which can be rather impressive to watch. The more strings you have in parallel, the more susceptible your circuit is to this phenomenon - you do not get an averaging out effect because the behavior is dictated by the extreme string and, with more strings, the more excess power is available to drive into that string, particularly with a constant current source supply.

#### olphart

Joined Sep 22, 2012
100
My rough rule: if at the supply voltage, my current limiter is under 20 ohms, I'll up the supply voltage or take 1 LED out of each series string, and that's with a very stable power supply. WBahn is spot on about LED behavior. They're diodes, voltages above conduction threshold make currents climb nearly exponentially.

That current limiting resistor is like a spring / shock suspension damper. You don't off-road the Baja with 2" of suspension travel, nor do Formula 1 cars have 12". "Right" sizing that resistor for the worst case supply variation provides best life. Any lower is less protection, higher is wasted power.

Another reason (besides safety) direct to mains is verboten is line spikes can ruin your whole day.

#### Sensacell

Joined Jun 19, 2012
2,929
Series / parallel setups are very hard to keep balanced.

One LED opens up and the entire string goes dark, while the others now have to share the extra current.

12 V power supplies are ubiquitous and dirt cheap.

Thermal runaway failure is always a possibility without bulletproof current control.

Practicality and Reliability are my design criteria.

Thread Starter

#### Jana Janous

Joined Sep 9, 2018
3
You will experience much less pain if you run this setup from 12 Volts.
Buy a 600 Watt, 12V power supply, that's double what you need.

- One current limiting resistor for each LED - 20 ma.

- Make all of this a parallel branch circuit.

- 50 branches of 400 ma is only 20A, your #6 AWG is more than hefty enough.

View attachment 159633

I mean this would be great. And was my initial thought since thats how i saw led christmas lights i ordered from china set up. They are all in parallel. always two cables in and two cables out of each led except the last one.

And I like I could use 12V/20A power source. But yes, there seems to be some concerns about this that I still dont understand. What is the issue with the current not being under control in this solution? Do I need resistors or other parts in this circuit? (I am talking about the parallel-branched circuit suggested by Sensacell above.) Thank you.

#### dendad

Joined Feb 20, 2016
3,982
You do need resistors to limit the current. Either external to the power supply or having a constant current supply FOR EACH STRING.
You are asking for trouble using a constant current supply on parallel strings, as has been mentioned above.
For example, say you use a constant current supply of 1Amp.
Then have 50 strings of LEDS to run at 20mA each. (assuming current sharing works)
Someone trips over your wires and disconnects 49 strings, Then the last remaining string has the full 1A driven through it. SMOKE!

Just use a fixed voltage supply, say 12 or 24V. Then, select each string current setting resistor to give the required current. Now you have no trouble with how many strings you run, up to the current limit of the supply. And you can have different colours and numbers of LEDs in each string (up to the voltage limit), just be selecting the appropriate resistor.

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