LED series/parallel connection can be connected this way? what are the pros and cons?

Thread Starter

Arul7

Joined Apr 20, 2022
12
Hello Everyone,

I have 11 strings of 3 LEDs in a series connection (Power LED from Lumiled Z series). I have shorted all the cathodes together so to avoid a series of LED failing issues.
If one LED fails in a string and rest will remain functioning with a slight change in current (500mA to 600mA). The LED's forward voltage is 2.8VDC at 500mA.

I have simulated the circuit and got the expected result in LTSpice (attached for reference). Can someone guide me or verify the circuit works fine in real life? I'm going to use a constant current LED driver and not going to use a resistor in series with LEDs.

The LED has a built-in TVS diode. Is there any other circuit to be included to protect this circuit?

Please share your valuable suggestions.
Thanks,
Arul
 

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Audioguru again

Joined Oct 21, 2019
6,688
An LED part number shows a range of forward voltage. Yours might be from 2.4V to 3.2V.
1) Did you buy thousands of LEDs, test them and select only the ones with an identical forward voltage?
2) Did you select a power supply voltage to be at the same voltage as 3 times the LED forward voltage?
3) Did yo make a temperature sensor that controls the power supply voltage change to match the voltage changed by the LED temperature?

LTspice does not know about the wide range of LED forward voltage or about the forward voltage change caused by the temperature.
 

BobTPH

Joined Jun 5, 2013
8,932
I would certainly not advise that arrangement.

What you have is thee sets of parallel LEDs put in series.

There is no guarantee that the current in each parallel LED is the same.

And there is nothing to control the overall current.

Why are you worrying about LEDs burning out? They pretty much last forever. I have never seen an LED burn out except when I am playing with them and apply too much current.
The LED has a built-in TVS diode. Is there any other circuit to be included to protect this circuit?
And what does this mean? I have never heard of such a thing. Please provide a link to the datasheet.
 

Thread Starter

Arul7

Joined Apr 20, 2022
12
I would certainly not advise that arrangement.

What you have is thee sets of parallel LEDs put in series.

There is no guarantee that the current in each parallel LED is the same.

And there is nothing to control the overall current.

Why are you worrying about LEDs burning out? They pretty much last forever. I have never seen an LED burn out except when I am playing with them and apply too much current.

And what does this mean? I have never heard of such a thing. Please provide a link to the datasheet.
Thanks for prompt response.
Do you mean LT spice result is wrong? The run and removed few LEDs, there is a slight change in the LED string. I do not find any other issue there.

Here is the link for LED spec https://lumileds.com/products/high-power-leds/luxeon-z/
 

Thread Starter

Arul7

Joined Apr 20, 2022
12
An LED part number shows a range of forward voltage. Yours might be from 2.4V to 3.2V.
1) Did you buy thousands of LEDs, test them and select only the ones with an identical forward voltage?
2) Did you select a power supply voltage to be at the same voltage as 3 times the LED forward voltage?
3) Did yo make a temperature sensor that controls the power supply voltage change to match the voltage changed by the LED temperature?

LTspice does not know about the wide range of LED forward voltage or about the forward voltage change caused by the temperature.
Thanks for your prompt response.
1. Yes, I agree with your 1st point. LEDs has to be tested for Vf.
2. Yes, I have a constant current supply for this range.
3. I haven't made the sensor part and have no idea about this :) you mean to say, temp. sensor should be part of the circuit and that controls power supply voltage?
What happens if Vf increases due to temperature change? We are still supplying the required current. Does it cause any issue?
 

Audioguru again

Joined Oct 21, 2019
6,688
I see that the "6" does not say 6A, I wrongly assumed 6V.
Yes the circuit will work with a 6A current source and will not need a temperature sensor. The LEDs need heatsinking for 6A/11 x 3.2V= 1.7W each.
 

tonyStewart

Joined May 8, 2012
131
This will fail from thermal runaway with a very high probability in less than 1 minute.

1. You may need to balance current using ~ Rs>=0.5 to 1 ohm power resistor in each string, with only 3S or 3-series.
2. Each LED will need at least 1.5 sq.in. of Alum-clad-PCB preferably attached to a large heatsink
3. I suggest instead of 11P3S you make them 3P10S and have 1 spare with 30V/1.25A. Test each one at 29V then match with Rs if necessary.

I have done 8P4S without any Rs before except I used long speaker wire between each 4S outdoors on a fence using 14V. A 20 meter feed wire dropped the Voltage enough. The wire between each provided small additional resistance and it worked great. The Vf on my LEDs were already fairly well-matched built already on MCPCB and designed for ambulances. I got about $1k worth in a big box from a scrap metal yard for $20 . They were rejects, I guessed from ATE made by a major contract manufacturer(Celestica) in Toronto. Most of them worked fine but may have been off-spec, some for colour , brightness and built as 18S strips. I cut them down to 4S on a bandsaw and made a cedar cover for them.fence LEDs winter.jpg
 
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