Connecting 50 LED diodes into parallel - power requirement

Thread Starter

Marin Sebesic

Joined Dec 14, 2017
15
Hello folks,

I just joined to ask this.
If I want to connect 25 5mm Transparent Red and 25 5mm transparent White LED diodes.
I want to connect them in parallel, in case one of them gives up, the whole row won't stop working.
Anyways, my question is, how much power do I need, also do I need any resistors?
The adapters I've got are:

5V 0.8A
7V 300mA 2.1VA
6V 300mA 1.8VA
9.5V 300mA

So, which one should I use?

Thanks in advance!
 

LesJones

Joined Jan 8, 2017
2,667
If you connect the LEDs in parallel with just one current limiting resistor the white LEDs will not light as they need about 3.3 volts and the red LEDs only need about 1.8 volts. So the red LEDs will limit the voltage. The best way is to have a current limiting resistor in series with each LED. If you use any of the 300 mA units you will need to calculate your current limiting resistors for no more than about 5 mA. With the 800 mA unit you could go upto about 15 mA for each LED. (Assuming that is within the ratings of the LEDs.

Les.
 

dl324

Joined Mar 30, 2015
10,985
Welcome to AAC!
I want to connect them in parallel, in case one of them gives up, the whole row won't stop working.
LED lifetimes are measured in decades when they're not abused.
Anyways, my question is, how much power do I need, also do I need any resistors?
You need resistors. Operating LEDs in parallel without ballast resistors in each leg is abusing them. One will hog current and die. That is likely to cause a cascading failure.
The adapters I've got are:
You don't give sufficient details, but even if you operated them at 10mA, you'd need more than 15W. If you used all of the supplies you mentioned, you still wouldn't have enough power. EDIT: calculations off by a decimal place.
 
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Bernard

Joined Aug 7, 2008
5,558
White LED @ 10 mA + R with 5 V supply = 50 mW X 25 = 1.25 W.
Red LED, 2 in series @ 10 mA + R with 5 V supply X 12 = .6 W.
!.25 W + .6 W + .05 W = 1.9 W , 5 V @ .8 A = 4 W power supply.
LED night light has been on for over 20 years, not to worry, spare LED on hand.
 

Audioguru

Joined Dec 20, 2007
11,249
Cheap no-name-brand Chinese LEDs bought on ebay last about 1 week if they are operated at 1/10th their exaggerated current rating. Good LEDs bought locally last forever when operated below their truthfull current rating.

Every datasheet for LEDs shows a range of voltages even if the LEDs have the same part number. I use a certain American red LED with a voltage range that is from 1.5V to 2.4V. If I buy some then the voltage can be anything in that range.
If I connect a 1.5V LED in parallel with a 2.4V LED then the 2.4V LED will not light and the lower voltage LED gets all the current and burns out, then the higher voltage lights and burns out.
Chinese flashlights have many LEDs in parallel because they hire a kid to test and group thousands of them into piles with the same voltage. Do you want to buy thousands of LEDs and test and group them?

Connect a few LEDs in series and with a series current-limiting resistor. This is called a string and the strings can all be powered from the same power supply.
 

philba

Joined Aug 17, 2017
960
You will be far better off figuring out how to drive series strings of your LEDs. The biggest advantage is you can reduce current requirements but you can also reduce the number of resistors you need.

In your original design, you will need 50 resistors, one for each LED. If you create 5 strings of 5 for each color, you only need one resistor per string if you drive with constant voltage. With 20 mA current per LED, you would draw 200 mA. You'd need around 18V to drive the white ones. Though, you need to do the calculations using the Vf of the LEDs.
 

Thread Starter

Marin Sebesic

Joined Dec 14, 2017
15
Ah, so, I see.
So, I see the biggest suggestion here is that I should connect them in series.
Anyways, if I want to connect them is such a way such as "red led, then the white, red, white,red, etc", does that change anything, also, would I still need only one resistor, because it's only one string?
I'll forget about the parallel connection...
Thanks!
 

dl324

Joined Mar 30, 2015
10,985
nyways, if I want to connect them is such a way such as "red led, then the white, red, white,red, etc", does that change anything, also, would I still need only one resistor, because it's only one string?
How you compose the parallel legs depends on the voltage you have to work with.

If I were doing this and only had what you have on hand to work with, I'd use the 9.5VDC@300mA supply.

I pulled a random datasheet for a white LED (Osram LW3333). At 10mA, it states forward voltage is 2.7-3.7V, with 3.1V being typical. For a random red (Osram LS3341), forward voltage is 2.0V(typical) to 2.6V max (they didn't give min for that LED). For a conservative design, you'd use the maximum voltages.

With a 9.5V supply, you could do combinations of 3 red, 2 white, or 2 red/1 white. I'd check that power supply to see what it's output voltage is with a 300mA load. If you operate the LEDs at 10mA (both of the LEDs I referenced are spec'ed at 10mA), you could run up to 30 strings; assuming the supply is good enough quality to provide 9.5V@300mA.

How you drive the LEDs and how you arrange them physically are unrelated; though some combinations may make assembly easier.

You still haven't provided part numbers or datasheets for the LEDs you want to use. or how you want to arrange them. If you provide better information, you'll get better suggestions. If you keep on the present path, we're going to still be here after dozens of posts trying pull details from you.
 

Bernard

Joined Aug 7, 2008
5,558
Series parallel: white , 2 in series X 12 strings = 120 mA; 4 red in series X 6 strings = 60 mA
total 180 mA + 20 ma on 9.5 v @ 300 mA supply ?
When I'm making series strings, I measure all LED's & put them in stacks of = V, with 100 mV separation of stacks. So in a string of 4 , 2 average 1 low & 1 high etc. so that all strings will almost be =. It takes me about 4 sec. / LED to measure V.
 
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shteii01

Joined Feb 19, 2010
4,644
What is Vf for red Light Emitting DIODE?
What is maximum current for red Light Emitting DIODE?


What is Vf for white Light Emitting DIODE?
What is maximum current for white Light Emitting DIODE?
 

Audioguru

Joined Dec 20, 2007
11,249
What is Vf for red Light Emitting DIODE?
What is maximum current for red Light Emitting DIODE?
What is Vf for white Light Emitting DIODE?
What is maximum current for white Light Emitting DIODE?
The datasheet for the LED shows its range of forward voltage because even if they have the same part number their voltages are all a little different. The voltage is different for different colors. The current skyrockets or stops if you feed an LED a voltage so you feed it with a current, usually from a series resistor. The maximum allowed current is listed in the datasheet and all detailed specs use a lower current, usually 20mA for 5mm LEDs.
 

shteii01

Joined Feb 19, 2010
4,644
The datasheet for the LED shows its range of forward voltage because even if they have the same part number their voltages are all a little different. The voltage is different for different colors. The current skyrockets or stops if you feed an LED a voltage so you feed it with a current, usually from a series resistor. The maximum allowed current is listed in the datasheet and all detailed specs use a lower current, usually 20mA for 5mm LEDs.
tell it to op
 

Audioguru

Joined Dec 20, 2007
11,249
The OP does not know the voltage of the LEDs and where ever he bought them (ebay?) also do not know so he asks us. It is simple but it will take time for him to test and group them all.
 

Thread Starter

Marin Sebesic

Joined Dec 14, 2017
15
I would like to provide all the info folks, but I can't. I'm trying to buy it from a site from my country, since there's nothing I can do.
The seller didn't provide nothing, and I really mean nothing.
Price per one is 0.06 usd, if that means anything to you.
 

dl324

Joined Mar 30, 2015
10,985
The seller didn't provide nothing, and I really mean nothing.
Then determine the range of forward voltages by measuring them and pick a current you want to operate at.

If the maximum continuous current rating isn't known, you should use something around 10mA (haven't seen any LEDs with a lower maximum). 20mA is common, but you do that at your own risk.

Arrange the strings of LEDs as I mentioned above, using the maximum forward voltage of a string to determine the value for the current limiting resistor.
 

Audioguru

Joined Dec 20, 2007
11,249
You never revealed your country. Go to http://www.farnell.com/ where they have a list of the 44 countries they have offices and warehouses. They sell Western Name-Brand reliable electronic parts at reasonable prices not cheap Chinese junk made in somebody's basement. Farnell even has the datasheets available on their sites. If I order something before 8:00PM then it is delivered to me the next morning.
 

philba

Joined Aug 17, 2017
960
You can figure out the forward voltage (Vf) by making a test circuit. The resistor values aren't super critical. Measure a number of them to get the average.
vf.png
 

neonstrobe

Joined May 15, 2009
81
5mm LEDS generally work around 10mA, though not all. You could assume a worst case voltage of 3.5V for white and 2.5 for red, not knowing the actual components. However, older white LEDs sometimes had a max. Vf of 4V. Generally, LEDs are always preferably operated in strings, The only decision is how many. For 50 LEDs 5 strings of 10 LEDs might be a good choice: that would allow 5 white+5 red per string, totalling a forward voltage of 5x(2.5+3.5) worst case or 30V. However, the more LEDs in a string, the worse is the temperature coefficient of the string: LEDs generally have the same negative slope as all diodes, about -2mV/C, but may be higher. The series resistor needs to be high enough to compensate not only for Vf changes but also temperature effects. If your LEDs are typical with Vf's of say 2 and 3V, the typical string voltage would be 25V. To keep the current within 25% of each other will need a voltage drop across the resistor of 5/2.5=2k. At 10mA that needs 20V and will dissipate 200mW, but pushes your power supply voltage up to 50V, which is then close to the maximum allowed for low voltage wiring without special precautions. Alternatively, you might consider using a transistor current source for each string, which would keep the additional voltage burden to a minimum (e.g. just over 30V, say 33-36V) and tolerate different Vf's, temperature drift etc.
 

dl324

Joined Mar 30, 2015
10,985
Here's a simple 10mA current source you can use to measure the forward voltage of your batch of LEDs:
upload_2017-12-15_10-52-8.png
Current is set by R2. I assumed a forward voltage for LED1 of about 2.5V so 180 ohms will give about 10mA.

You can use some sort of socket to insert the LED being tested between the collector and ground. You can place your DVM between those same points so you just need to keep swapping LEDs and binning them by forward voltage.
 
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