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LED array help for a novice

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Revdjem

New Member
Probably best to say what I want to achieve then what I've tried, and then ask if there is a better way to do it.
So. My daughter's restaurant got a Michelin Star this year and I want to make them a Michelin Star for the top of their Christmas tree. To make it a bit more lively I would like to add 49 5mm Red LEDs around the edge. See the photo which was the first attempt and has only 30 LEDs it is 210mm in diameter and the "outline" is between 10 and 12mm wide.
PXL_20211102_080654421.jpg

Using an online calculator it suggested making 6 stings of LEDs in series protected by a resistor and a 12v power supply. When I tried to create this and fit it into the back of the hollowed out star it just wouldn't fit (for me) So, my next idea after doing further research was to put all 49 LEDs in parallel on a homemade PCB being driven by a commercial LED driver designed to drive LED strip lights. This is where I'm up to. I've built the circuit on a breadboard, plugged it in and it all went pop! Hmmm. I've played with the circuit in Everycircuit and it seems to work.
Am I missing something? is there a better way to do this?
The commercial LED driver is an Aurora AU-LED16T 12v 1-16w constant voltage non-dimable LED Driver output current 1.4A Output voltage 12v volt-ampers 16w
So any help would be much appreciated.

Thank you
 

danadak

Well-Known Member
Most Helpful Member
Did you have series current limiting R's with each LED ? LEDs like to be current
driven. Note curves below show you that there is rapid change in LED current with
voltage, and you were driving them each with 12 V so its no wonder you fried
a lot of LEDs and/or the power supply.

So your R = Vled / Iled, red LEDs typically are ~2V s. So look at LED datasheet and
find the typical V at a given current then compute the series R you need to limit
their current. In your case R = (12V - 2V) / Iled. Note most LED datasheets do not
give you worst case values which is why professional LED drive circuits are driven
by current regulated power supplies.

Some designs, strings of leds, drive with V and get away with it because the stacked
LED V is ~ = supply V, and LEDs have a dynamic R that crudely limits V. But thats
cheap crap way of doing this unless string current is regulated, either by using a
series R in the string or a current source driving the string. But as you can see
a string means a lot less R's.

1635843057158.png






Regards, Dana.
 
Last edited:

rjenkinsgb

Well-Known Member
Most Helpful Member
The driver unit you list is still a 12V power supply.

Also, LEDs vary slight one to another and direct parallel connecting them without individual resistors tends to give uneven brightness.
The series strings of 6 LEDs, each string with its own current limiting resistor, is the simplest and most reliable approach.
 

eTech

Well-Known Member
Probably best to say what I want to achieve then what I've tried, and then ask if there is a better way to do it.
So. My daughter's restaurant got a Michelin Star this year and I want to make them a Michelin Star for the top of their Christmas tree. To make it a bit more lively I would like to add 49 5mm Red LEDs around the edge. See the photo which was the first attempt and has only 30 LEDs it is 210mm in diameter and the "outline" is between 10 and 12mm wide.
View attachment 134327
Using an online calculator it suggested making 6 stings of LEDs in series protected by a resistor and a 12v power supply. When I tried to create this and fit it into the back of the hollowed out star it just wouldn't fit (for me) So, my next idea after doing further research was to put all 49 LEDs in parallel on a homemade PCB being driven by a commercial LED driver designed to drive LED strip lights. This is where I'm up to. I've built the circuit on a breadboard, plugged it in and it all went pop! Hmmm. I've played with the circuit in Everycircuit and it seems to work.
Am I missing something? is there a better way to do this?
The commercial LED driver is an Aurora AU-LED16T 12v 1-16w constant voltage non-dimable LED Driver output current 1.4A Output voltage 12v volt-ampers 16w
So any help would be much appreciated.

Thank you
Hi

LED's are current driven devices. That is, if you connect an LED across a DC power supply, it will draw as much current as the supply can provide until the LED burns up. So the current needs to be limited by connecting a resistor in series with the LED. Since your LED's had no current limiting, some or all burned up. If you connect them in parallel, you'll need to use a limiting resistor connected in series with each LED.
Use the information in post #2 to compute the required value of the resistor.
 

Diver300

Well-Known Member
Most Helpful Member
The driver unit you list is still a 12V power supply.

Also, LEDs vary slight one to another and direct parallel connecting them without individual resistors tends to give uneven brightness.
The series strings of 6 LEDs, each string with its own current limiting resistor, is the simplest and most reliable approach.
I would not use as many as 6 LEDs in one string on a 12 V power supply. The LEDs are around 2 V each, so 6 of them leaves nearly no voltage for the resistor. If the voltage across the resistor is too small, its resistance will need to be very small, and then small changes in voltage, from inaccuracies in the power supply or variation between LEDs, will result in large changes in current.

You should go for 4 or 5 in series. That will leave 4 or 2 V across the resistor, and the current will be more stable.

A few years ago I wrote this:- https://mtrak.co.uk/led_calculator.html which tell you what resistors to use and how many LEDs you can have in series, depending on how accurately you have to control the current.
 

Diver300

Well-Known Member
Most Helpful Member
I suggest that you wire up the LEDs like this. You will need several strings of 4 (or 5) LEDs like this. Put the series resistor (rectangle in the diagram) on the back of the star. If you are using 4 LEDs in each string, the resistor will dissipate about twice as much heat as a single LED, but that won't be much.

This will only need three wires running along the back of each part of the star. You can put as many of these in line as you want.

This is how the flexible strips of LEDs are wired. They use white LEDs, which are about 3 V each, so they use three LEDs for each resistor.

LED connection.png
 

Revdjem

New Member
Thank you all very much. I am beginning to see how I can put this all together. Particular thanks to Drive for showing how to physically wire them up. I think I can now make a kind of PCB to solder them and their resistors to so that the wiring can all be hidden inside the star.
Brilliant. Much appreciated.
 
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