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# Help Needed Calculating Resistance

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Hi Everyone,

Would someone please give me some advice for the following problem? I'm not an electronics person so please forgive any incorrect descriptions.

I want to calculate what value resistor to use when connecting LED's in series.

In all cases the supply voltage is 12V.

I need the LED's to burn as brightly as possible but I also need to reduce the risk of them burning out.

The voltage (VF) of the LED's is listed as  3.0 - 3.4  20mA and I don't know which voltage to use in the calculation.

The set up I have now uses 3V LED's and is configured as follows.

The first case I have is a panel of 22 LED's connected in series 3x3x3x3x3x3x3x1

Each series of three LED's has a 100 ohm resistor.
Each series of two LED's has a 270 ohm resistor.
Each single LED has a 430 ohm resistor.

There is one 1.1V diode connected to each panel of 22 leds.

The other set ups are a panel of 16 LED's and a panel of 11 LED's connected as described above.

I hope I've given you enough info and described this correctly.

Thanks.

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Hi Demon,
Welcome to our forum. ;D
Your LEDs have a huge range of voltage rating and your current-limiting resistor has a low voltage across it with 3 LEDs in series. I assume that the 1.1V diode is in series with the array reducing the voltage to 10.9V.
If the LEDs are 3V, then their current is 19mA.
If they are 3.4V then their current is only 7mA.
Measure and sort them so their voltages are nearly the same, then re-calculate a current-limiting resistor for 20mA or 25mA. ;D

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Hi audioguru,

Thanks for the reply and the welcome. I really don't understand much about this as you can probably tell from my description of the problem.

I think you're correct in assuming the 1.1V diode is to reduce the voltage thus protecting the LED's. (or I think that's the purpose)

The LED's I buy are all sold that way, within a range, in this case 3.0V - 3.4V, which is why I'm confused as to which resistor to use. In your answer you suggested testing the LED's and sorting them into the various voltages. Firstly, I'm not sure how to go about that and secondly, there's quite a few thousand of these. In the near future I'll be getting these particular PCB's made, and for each set there'll be about 20,000 LED's so I'm fairly sure they won't test each individual LED. The LED's are supplied to me from a manufacturer and I've asked them to supply me with LED's all of the same voltage. For some reason they are unable to supply LED's of the same voltage except in small numbers. From your answer, I presume now it's because they have to test each individual LED.

Audioguru, is there any other way around this problem?

Demon

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I think you're correct in assuming the 1.1V diode is to reduce the voltage thus protecting the LED's. (or I think that's the purpose)

I think the series diode stops damage to the LEDs if the power to the circuit is connected backwards.

The LED's I buy are all sold that way, within a range, in this case 3.0V - 3.4V, ...
Audioguru, is there any other way around this problem?

Then I suggest using a higher input voltage or fewer LEDs in series so the current-limiting resistor has more voltage across it and the LEDs operate more with a constant current if their voltages are different.
A two-transistor/2-resistor constant current sink or source for each series string of 3 LEDs would be perfect, then the current-limiting resistor for the series strings of 1 and 2 LEDs could be averaged to have the same current:

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Hi Audioguru,

Thanks for your help, it's very much appreciated.

Demon

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• 3 weeks later...

well i make it a golden rule:
current in an LED should not exceed 14mA and normally 5k in series with a single LED is enough........
u can calculate it easily for any circuit...

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14mA thru 5K is 70V. Is that what you meant to say??

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14mA thru 5K is 70V. Is that what you meant to say??

With a 9V battery and a 2V red LED, 5k reduces the current to a very low 1.4mA.
My new Sound Level Indicator project operates its 1.8V red LEDs at 26mA and they are blinding.
Its brightness switch reduces their current to only 2mA and I still get complaints from my wifey that it is too bright. :'(
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hi audioguru!
do u really have 1.8V LEDs?
i mean how do u have a voltage above 0.7V in forward bias condition?

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from this topic i used bicolor LED once in my lifetime.
i guess u also remember the same.can u guess?

it was in electronic stethoscope---the fuzzzzzzzzy project......

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electronic stethoscope---the fuzzzzzzzzy project

That was the 1st schematic I drew on a pc. On my old 486 pc.

i didn't know that JPG schematic files look so baaaaad. ;D

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