LED Series Setup Dilemma

  • Thread starter Pkagos
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  • #1
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I have tried doing a thorough search through multiple forums as well as the ever popular http://www.google.com" [Broken], however my lack of experience has led me to create a post and plead to the experts in such a "noob" fashion about how I would go about setting this up.

I am looking to put the finishing touches on an RC body I have been working on and to do so requires an LED configuration that I just cannot seem to understand. I (ideally) would like to set up 14 LEDs to one battery source. The list of lights I would be using is as follows:
  1. 2x Red LED (1.7v Typical, 2.4v Max / 20mA Max)
  2. 2x Yellow LED (2.1v Typical, 3.0v Max / 40mA Max)
  3. 4x Green LED (2.1v Typical, 2.8v Max / 30mA Max)
  4. 6x (High-Brightness) White LED (3.3v Typical, 3.6v Max / 25mA Max)
There may be a chance that I will only be using 2 of the white LEDs, but would like to prepare for the full setup just in case and can always transfer over the information if need be for the fewer lights. The main predicament that I am having is not all of the LEDs are the same voltage and such as you can see, and I am unable to find much documentation on this circumstance. I was able to track down http://ledcalc.com/" [Broken] page still be applicable in my circumstance.[*]What battery/resistor/LED configuration would work best.[/list]Below is a rough sketch of what I believe the best setup would be, but I am by far an expert on this and before I waste the time and money and fry anything unnecessarily I would like to get any advise I can about this project. For my rough example the batteries are 2 9 volts in a series.

http://www.graidiandesigns.com/images/crude.jpg [Broken]
Resistor List
  1. 220Ohm 1/8W (192 Exact)
  2. 470Ohm 1/2W (456 Exact)
  3. 390Ohm 1W (345 Exact)
  4. 820Ohm 1W (730 Exact)
  5. 470Ohm 1/2W (460 Exact)
I know I am more than likely over thinking this, I just want to make sure it is done right, I spent so much time on the rest of the project with airbrushing and modification that I would hate for this to be the weak link of it all.

Thank you so much in advance.
 
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Answers and Replies

  • #2
vk6kro
Science Advisor
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The setup you describe is going to be pretty hungry for power.

You can give the LEDs less than their maximum current and possibly settle on something like 15 mA for each LED.
However, you will have to give each of them the voltage they require to start up.

For example, if you have several white LEDs you want to be able to turn on and off, each will need its own resistor and a switch.

LED voltages should add up to about 70% of the supply voltage with the remaining 30% appearing across the series resistor. You could possibly run all your green lights off one resistor if your supply was 9 volts, but what happens when the battery voltage drops below 9 volts? So it would be better to have just 3 green LEDs on one resistor.

The diagram shown in the question would not work if that is 2.9 volts total. The supply has to be greater than the combined turn-on voltages of the LEDs in series. For example, 4 green LEDs in series HAVE to have 8.4 volts to turn on plus a voltage for the series resistor plus a safety margin for when the battery starts to go flat.

The easy way is to get some LEDs and work out what voltage they need to turn on. Subtract this voltage from the power supply voltage. This is the voltage across a series resistor. Calculate what size resistor will allow 15 mA to flow.

eg Voltage across LED = 3 Volts
Supply = 9 Volts so resistor drops 6 volts.
R= E / I so resistor value = 6 volts / 15 mA = 400 ohms so you could use a 390 ohm R.
But it isn't critical. You could use 330 or 470 and it would just affect the brightness of the LED.

If you had two 3 Volt LEDs, the voltage across the resistor would then be 3 volts and the resistor would then be 200 ohms.

Just never put a LED straight across a source of power without a series resistor.
 
  • #3
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Not sure if it makes any difference, but the calculations were all done with the power source being 18V...
For my rough example the batteries are 2 9 volts in a series
Again, I am sure this is wrong, which is why I came here.:wink:
 
  • #4
Redbelly98
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You're setup would work, but could probably be done with 3 strings of resistors rather than 5. What I am thinking is:

String 1: 4 white LED's.
String 2: 2 white, 2 red, 1 green LED.
String 3: 2 yellow, 3 green LED's.​

Do a test run to get about 10-20 mA through each LED. Then you can measure the actual LED voltages, and fine-tune the resistor values to get say 15 mA.

How long do you need the batteries to last? A 9V alkaline is something like 500 mA-hrs. With 3 strings at 15 mA each, that's 45 mA total so it will last roughly

(500 mA*hours) / (45 mA) = 11 hours​
 
  • #5
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Well, ideally the setup will be powered by a rechargeable 9v (or multiple 9v batteries) when it is all said and done, and since it is for an alternate RC body configuration as long as it lasts for a half hour or so on each charge it will be more than sufficient. I have decided to do a temp setup involving far less LED but I still want to find the optimal setup for this as I will be swapping out the temp setup for the full fledged setup as soon as I can.
 
  • #6
Redbelly98
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Oh, if you just need it to run for 1/2 hour, you could also try 7 strings of 2 LED's and use just one 9V battery, which would help save on weight.

Total current would be 7x15 mA = 105 mA, which would last about 5 hours for a 500 mA-hr battery.

Regards,

Mark
 
  • #7
vk6kro
Science Advisor
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If you could use flashing LEDs in your project, there are lots of Google references to running LEDs off 1.5 volts by means of voltage doublers. Type "LED flasher 1.5 volt" in Google.

They can run a LED this way for months apparently off a single (possibly D cell ) battery.

The circuitry is more complex, but the battery savings and running convenience would be a lot better.
 

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