# Desperate help needed...How do I power 8 LEDs?

#### MisterBill2

Joined Jan 23, 2018
14,756
See my last post. Just wire them all in parallel and use a 3V AC Adapter. It has to have enough power for all your leds. With 14 leds (or however many you end up with) and using a typical led current of 15mA, you will need a supply more than 14 x 0.015 or 0.6A. A 1A supply would work.

Instead of 14 of those expensive batteries you can use two AA batteries in series to give about 3 volts. and wire all of the 3 volt leds in parallel. That will take care of most of the lighting. They will not all be the same brightness, which may or not matter. If the batteries are not fresh than their internal resistance will limit the current quite well.

#### Tonyr1084

Joined Sep 24, 2015
7,358
What happens if batteries are fresh?

#### Tonyr1084

Joined Sep 24, 2015
7,358
The types of LED's you're wanting to use come in various colors. Each color type has a different forward voltage ( Vf ). ALL LED's are current based components. The data sheet will tell you what the Vf is and what is the max recommended current. I have a stock of four different color LED's. The Vf for "MINE" are:
Blue • • • 2.82Vf (average)
Green • • 2.92Vf (average)
Yellow • 2.01Vf (average)
Red • • • 2.95Vf (average)

Depending on which color and the actual Vf for YOUR LED's - you will need to calculate the necessary resistance.

#### Tonyr1084

Joined Sep 24, 2015
7,358
Suppose I have a 6V supply (batteries) and want a Green LED powered at 10mA. Take the 6V and subtract the 2.9Vf (MY LED's, not necessarily yours). That equals 3.1V. To calculate the needed resistor at 10mA, 3.1V ÷ 10mA (0.01A) = 310Ω The nearest standard resistor is 330Ω. That means 3.1V ÷ 330Ω = 9.4mA. Close enough. When you use two LED's in series you have to subtract ALL the Vf's. Again, suppose you have 6V and a target of 10mA with TWO Yellow LED's (Vf = 2.01). (6V - 4Vf) ÷ 0.01 = 200Ω. That would give me 10mA through the two LED's (as in the diagram below). Notice that there are four groups of "2 LED's in series". Those groups are in parallel and each group has its own resistor. You MUST use resistors for each group because using different color LED's will result in some non-functioning LED's. The ones with the lowest Vf will draw all the current. There's more to that story, but for now just accept that you have to put a resistor with each group. And each group - depending on the color - will have its own calculated resistance. This may seem hard, but it's not.

The difference in the two diagrams is the one on the left has only one switch for all LED's. The one on the right has a switch for each group of LED's.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
Aside: @Tonyr1084 Reminds me of a joke: What;s the difference between a teepee and wigwam? Two tents.

Good point. Your right, in series, they act like barriers.

#### BobTPH

Joined Jun 5, 2013
6,546
@BobTPH True, but likely the LED will appear at different brightness based on color and Vf.
Except that the TS was happy with each powered by a coin cell which certainly had more variation than my scheme would.

I cannot fathom why everyone wants a random additional voltage when we already have a 24 supply.

Bob

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
Except that the TS was happy with each powered by a coin cell
Have you ever priced a coin cell? I get Sony or maxell for $0.89 ea, qty 20, free postage. Locally, they are about$2.50 to %6.00 each. For panasonic, if I want to pay postage, they are \$0.32 each. Prices in USD.

#### Audioguru again

Joined Oct 21, 2019
5,829
The types of LED's you're wanting to use come in various colors. Each color type has a different forward voltage ( Vf ). ALL LED's are current based components. The data sheet will tell you what the Vf is and what is the max recommended current. I have a stock of four different color LED's. The Vf for "MINE" are:
Blue • • • 2.82Vf (average)
Green • • 2.92Vf (average)
Yellow • 2.01Vf (average)
Red • • • 2.95Vf (average)

Depending on which color and the actual Vf for YOUR LED's - you will need to calculate the necessary resistance.
The voltages of your LEDs do not match any list shown in Google.

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#### MisterBill2

Joined Jan 23, 2018
14,756
I suggested using AA cells because mostly they are a lot cheaper, and mostly more power available.And as for on-line, will it arrive by the time you need it?? Usually NOT!!

#### BobTPH

Joined Jun 5, 2013
6,546
The voltages of your LEDs do not match any list shown in Google.
Well, I wouldn't go by Ebay. I looked up green 3mm 20ma through hold LED on Mouser.

Out of 125 items for sale, 116 of them had a forward voltage of 1.9, 2.0, or 2.1 V, which agrees with my assessment that generic 20ma LED for red, yellow, and green are about 2V.

Now, green high power LEDs tend to be high 2's or low 3's of Volts, but I don't think that is what the TS has. Unfortunately, he has not been able to give us any info about them.

Bob

#### Audioguru again

Joined Oct 21, 2019
5,829
A white LED is a 3.4V blue one with a yellow-looking phosphor in top. A modern bright green LED uses the same 3.4V chemistry as a blue or white one.

#### Audioguru again

Joined Oct 21, 2019
5,829
The LED modules shown have 3 LEDs plus a resistor. They are powered from the controller in a 3s to 6s model.
3s is 11.1V nominal or 12.6V fully charged. 6s is 22.2V or 25.2V. A larger LED module with 6 LEDs is shown marked 2s-3s and 4s-6s so has two groups of 3 LEDs. Maybe a 6s controller limits the voltage to 12.6V?

#### Tonyr1084

Joined Sep 24, 2015
7,358
The voltages of your LEDs do not match any list shown in Google.
I got these from Chinaville. No data sheet, no information of any kind. I tested all 100 of each at 5V with 470Ω and measured the Vf that way. Then did an average and a standard deviation calculation and came up with numbers that showed most of them are very close in Vf. I did a video using a single resistor and putting LED's in parallel to demonstrate that the lower Vf LED's would hog all the current. But when the Vf's were very very close you could stack in parallel - I think the video I did six. But those numbers I got are from physical testing of the LED's. I've used those numbers in any projects with LED's.

I was surprised at how vastly different they are from the normal numbers. My numbers are actual testing numbers. Maybe I did something wrong, but like I said, I used 5V and 470Ω. I did it that way because I knew I couldn't over stress the LED's. I suppose with a higher current the numbers would be different. Higher current means higher temperatures, which will change the Vf measurement. But this is how I came up with my numbers.

For instance: my clear white super bright LED's have an average of 2.97Vf. 2.92Vf minimum and 3.01Vf max with a standard deviation of 3mV. That's pretty darn close.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
A white LED is a 3.4V blue one with a yellow-looking phosphor in top. A modern bright green LED uses the same 3.4V chemistry as a blue or white one.
that's pretty close to my calculated value of 3.5V.

#### Audioguru again

Joined Oct 21, 2019
5,829
How can you calculate 3.5V when some of them can be 3.2V and others can be 3.6V? 3.4V in the center of that range of voltages.

#### BobTPH

Joined Jun 5, 2013
6,546
Meanwhile, the TS is totally confused and probably given up on us as we argue about the forward voltage of LEDs we no nothing about other than their diameter and colors.

Bob

#### Audioguru again

Joined Oct 21, 2019
5,829
In post #5 he said he has some LED modules and posted the link to 3 LEDs plus a resistor. The link says 3s which is 12.6V, not the 7.4V the TS says. We know nothing about the single LEDs he has.