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LED circuit, resistors!

Blake Shaffer

Jul 1, 2014
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Hi, I am very new to electronics but am trying to make a simple circuit. I have 16 3.2v .02a blue leds I want to power, I wire them in parallel and provide 2 9v batteries to power them, I use 2 100 ohm resistors which should be 50 ohms in parallel. After using ohms law I concluded that the resistance I needed was 46 ohms, 50 being almost 10% greater. When I tested the circuit out, all of my leds got shot, and the resistors didn't do anything, what do I do!! Please help, thanks!
 

Gryd3

Jun 25, 2014
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Can you please draw a picture / diagram or take a photo of the project?
LEDs should not be connected directly in parallel with each other. They can be if each LED has it's own resistor.
LEDs can also be connected in series, in which case one resistor can manage the entire string. Multiple string can then be connected in parallel.

If your LEDs have a 3.2V forward voltage drop and require 20mA, you will require a 290Ω for one LED when connected to a 9V.
You would require 130Ω if you have 2 LEDs connected in series to form a string.
Using standard resistor values you should have 8 130Ω resistors in total. One resistor and two LEDs in series to form a string, the connected in parallel to the remaining strings.
If you want to simply connect them all in parallel, you will need 16, 300Ω resistors. One for each LED.
This is assuming you have not connected the two 9V batteries in series to create an 18V source.

Please show your work when you do math as it will help us locate any problems.
 

Harald Kapp

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LEDs are different from light bulbs.
Connecting LEDs in parallel is not the best way (note the undestatement). LEDs should be connected in series (if voltage allows) or have their own current limiting resistor each to account for variations in forward voltage.
Have you read the tutorial on LEDs?
 

Blake Shaffer

Jul 1, 2014
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Can you please draw a picture / diagram or take a photo of the project?
LEDs should not be connected directly in parallel with each other. They can be if each LED has it's own resistor.
LEDs can also be connected in series, in which case one resistor can manage the entire string. Multiple string can then be connected in parallel.

If your LEDs have a 3.2V forward voltage drop and require 20mA, you will require a 290Ω for one LED when connected to a 9V.
You would require 130Ω if you have 2 LEDs connected in series to form a string.
Using standard resistor values you should have 8 130Ω resistors in total. One resistor and two LEDs in series to form a string, the connected in parallel to the remaining strings.
If you want to simply connect them all in parallel, you will need 16, 300Ω resistors. One for each LED.
This is assuming you have not connected the two 9V batteries in series to create an 18V source.

Please show your work when you do math as it will help us locate any problems.


I need to power 16 LEDs at once with my 18v source (sorry, I forgot to mention I wired the batteries in series) so wiring the LEDs in series was not an option considering the combined forward voltage would exceed the18v source, so wiring in parallel seemed to be a fix, so you are saying I need to wire a resistor to each individual LED when in parallel?
 

Gryd3

Jun 25, 2014
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I need to power 16 LEDs at once with my 18v source (sorry, I forgot to mention I wired the batteries in series) so wiring the LEDs in series was not an option considering the combined forward voltage would exceed the18v source, so wiring in parallel seemed to be a fix, so you are saying I need to wire a resistor to each individual LED when in parallel?
Yes, but there is another trick you can employ.
You said you can't wire them ALL in series due to the forward voltage drop.
You can however, make 2 or more smaller string of LEDs in series with one resistor per string.
You can then connect all of the strings of LEDs to the battery source in parallel.
You can connect as many string as you want. Each string must be setup with a proper resistor for the LEDs connected to it. The strings do not need to be identical!
 

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KrisBlueNZ

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I guess you didn't read the tutorial Harald linked you to.

You need to group your LEDs into series strings, each with one series resistor, and put those strings in parallel.

If each LED has a nominal forward voltage of 3.2V, that works out to 4 LEDs per series string for a total of (3.2 × 4V) = 12.8V per string. Then the resistor drops (18V - 12.8V) = 5.2V so the resistor should be (5.2V / 0.02A) = 260 Ω. You can use 270Ω.

Then you make four of these series strings, each with a 270Ω series resistor, and connect each string across the supply voltage, i.e. connect the strings in parallel.

The total load current will be (4 × 20 mA) = 80 mA and a pair of 9V PP3 batteries will last about half a day.
 

Gryd3

Jun 25, 2014
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I guess you didn't read the tutorial Harald linked you to.

You need to group your LEDs into series strings, each with one series resistor, and put those strings in parallel.

If each LED has a nominal forward voltage of 3.2V, that works out to 4 LEDs per series string for a total of (3.2 × 4V) = 12.8V per string. Then the resistor drops (18V - 12.8V) = 5.2V so the resistor should be (5.2V / 0.02A) = 260 Ω. You can use 270Ω.

Then you make four of these series strings, each with a 270Ω series resistor, and connect each string across the supply voltage, i.e. connect the strings in parallel.

The total load current will be (4 × 20 mA) = 80 mA and a pair of 9V PP3 batteries will last about half a day.
Your so nice, I left all the values out of my drawing to get him to do some math ;)
 

Blake Shaffer

Jul 1, 2014
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Yes, but there is another trick you can employ.
You said you can't wire them ALL in series due to the forward voltage drop.
You can however, make 2 or more smaller string of LEDs in series with one resistor per string.
You can then connect all of the strings of LEDs to the battery source in parallel.
You can connect as many string as you want. Each string must be setup with a proper resistor for the LEDs connected to it. The strings do not need to be identical!

So when wiring, I connect all the series groups in parallel with the positive lead to a resistor then to the source, then connect the bare negative at the end of each strand in parallel to the negative lead directly?
 

Blake Shaffer

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I guess you didn't read the tutorial Harald linked you to.

You need to group your LEDs into series strings, each with one series resistor, and put those strings in parallel.

If each LED has a nominal forward voltage of 3.2V, that works out to 4 LEDs per series string for a total of (3.2 × 4V) = 12.8V per string. Then the resistor drops (18V - 12.8V) = 5.2V so the resistor should be (5.2V / 0.02A) = 260 Ω. You can use 270Ω.

Then you make four of these series strings, each with a 270Ω series resistor, and connect each string across the supply voltage, i.e. connect the strings in parallel.

The total load current will be (4 × 20 mA) = 80 mA and a pair of 9V PP3 batteries will last about half a day.


I just got home and was currently reading the post after I saw it, haha thank you!
 

KrisBlueNZ

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So when wiring, I connect all the series groups in parallel with the positive lead to a resistor then to the source, then connect the bare negative at the end of each strand in parallel to the negative lead directly?
Yes, just like the diagram in post 5 except (a) there is the same number of LEDs in each string, i.e. 4, and there are four strings, and (b) the LEDs need to be the right way round - anode (the triangle side) towards the positive supply.
 

Gryd3

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(b) the LEDs need to be the right way round - anode (the triangle side) towards the positive supply.
I did that in a hurry... didn't notice I put them in backwards... good catch, and I'll fix it!
 

Blake Shaffer

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Yes, just like the diagram in post 5 except (a) there is the same number of LEDs in each string, i.e. 4, and there are four strings, and (b) the LEDs need to be the right way round - anode (the triangle side) towards the positive supply.


Thank you thank you so so much! I really appreciate you using your time to help me out, I will create the new circuit as soon as I can; and what did you use to create that schematic? Have a great day!
 

Blake Shaffer

Jul 1, 2014
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I guess you didn't read the tutorial Harald linked you to.

You need to group your LEDs into series strings, each with one series resistor, and put those strings in parallel.

If each LED has a nominal forward voltage of 3.2V, that works out to 4 LEDs per series string for a total of (3.2 × 4V) = 12.8V per string. Then the resistor drops (18V - 12.8V) = 5.2V so the resistor should be (5.2V / 0.02A) = 260 Ω. You can use 270Ω.

Then you make four of these series strings, each with a 270Ω series resistor, and connect each string across the supply voltage, i.e. connect the strings in parallel.

The total load current will be (4 × 20 mA) = 80 mA and a pair of 9V PP3 batteries will last about half a day.


You also said it should last about half a day? Is their anyway to make it last a possibly longer period of time?
 

Gryd3

Jun 25, 2014
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You also said it should last about half a day? Is their anyway to make it last a possibly longer period of time?
Each string of LEDs will draw current. This current drains your battery so you have 3 options that I can think of.
Remove a string, increase the value of the resistor, add more batteries in parallel.
(Increasing the resistor value will dim your LEDs)
 

Blake Shaffer

Jul 1, 2014
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Each string of LEDs will draw current. This current drains your battery so you have 3 options that I can think of.
Remove a string, increase the value of the resistor, add more batteries in parallel.
(Increasing the resistor value will dim your LEDs)


How difficult is it to connect it to a wall outlet?
 

Gryd3

Jun 25, 2014
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How difficult is it to connect it to a wall outlet?
No more difficult than a battery ;)

Some things to keep in mind:
-Cheap wall adaptors may not regulate the voltage properly, which means the 18V you expect could be in the 20s.
-Make sure you use a multimeter to figure out polarity (+ / -)
-Make sure the wall adaptor is rated for enough current to handle your LEDs.
(Each string will draw approx 20mA... so if you are running 4 strings make sure the adaptor is at LEAST 80mA. 20 for each string.)
-If the adaptor is rated for a different voltage, you will need to re-do your math to find out how many LEDs per string you can handle, and what size resistor is needed.
-Buy a small barrel jack from RadioShack or similar so you don't need to hack the end off*
-Buy a small inline fuse to protect the wall adaptor*

*optional but not a bad idea ;)
 
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