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Setting resistors for a 12 LED lights parallel power supply

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Tim2255

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I have 12 LED lights running to a 12A toggle swith then to 4-AA batteries. The LED are 3.5V. Do I need to put a resitor to reduce power so I don't blow the lights or damage anything? If so what size resitor would I need? I would like to put the resitor between the battery and toggle switch, so it is just one resitor.
 

Re: Conversion Help

Technically speaking each diode should have it's own resistor, but if you intend to connect the LEDs in parallel then this is how you can estimate the value of resistor ..

4X-AA @ 1.5V => 6V
12 x LED @ 10mA => 120mA

{6V – U[LED] } / 120mA => {6V – 3.5V}/120mA => 21Ω

P = R * I² = 21Ω * (12mA)² ≈ 0.3W
The resistor should be rated for at least 0.5W ..

IanP
:D
 

Re: Conversion Help

Thanks for the information. I am not sure what you mean by parrell for connecting the leds, but what I do have is two wires coming off each led. They all come into the toggle switch. Then one wire comes from the toggle switch to the battery. Is this parrell. You say each light should have its own resistor, what will happen if I just use one as stated above. Is that going to be OK or will I have problems. Also with that type of power running, is it ok to run this whole thing using 24 awg? Thanks
 

Re: Conversion Help

Take a look at the attached pictures .. now both configurations and their limitations should become clear ..

24AWG wire is not an issue..

IanP
:D
 

Re: Conversion Help

Awesome thanks for the help. Each LED is identical, so I think I can get away with one Resistor of .5. Thanks for all your help.
 

Re: Conversion Help

OK I put a .5 Risistor in the line and when I turn the lights on, they are very very dim. I took the resistor out and the lights are very bright, but I notice the batteries get extremly hot. Any thoughts.
 


Re: Conversion Help

I am still a little lost as to what I need. All the information is great, but I am not to savy in this stuff.

I have 12 LED lights that are all are hooked up with a positive and negative. They all come back to a 12V switch. From that 12V toggle switch, I have two wires going to 4 AA batteries. The LED are 5mm, 3.6V max. When I hook a 1.5K Ohm Resistor, the lights are very dim. When I remove the resistor, the batteries get hot. Am I using the wrong resistor?
 

Conversion Help

Even if it is dim, what do you straightaway remove the resistors and connecting it directly to battery, instead of reducing the resistance value step by step..?? It might even spoil your LED's too.

why did you choose 1.5K by the way
IanP has clearly suggested you something. have you tried with 22Ohms??
 

Re: Conversion Help

I pipcked 1.5 Ohms because thats what the guy at radio shack told me to get. I can go back tomorrow and get a 220 ohm resistor, but if 1.5 ohms dimmed the lights, would 220 ohm resist way too much.. You I know I am not brighht at this. Thanks for all your help in helping me understand all this.
 

Re: Conversion Help

Please note, I told 22 Ohms and not 220 Ohms..

okay.. i think you are getting confused.
make a note of this:-
first you said 1.5 K and now you are sayin 1.5 Ohms.There is a big difference between 1.5K and 1.5 Ohms..

This is how it goes..

1 Ohm
1.5 Ohms
10 Ohms
22 Ohms
100 Ohms
220 Ohms
1000 Ohms = 1 K Ohms
1500 Ohms = 1.5 K Ohms
4700 Ohms = 4.7 K Ohms
10,000 Ohms = 10 K Ohms

1000 K Ohms = 1 M Ohm

The values marked in bold are the values in the discussion now. And you can see where it stands.

And next time when you go to shop, buy a trim pot (or potentiometer) of the value 1K.That is cheap. And instead of the resistor you can use this and vary the resistance and get the right value you are looking for. This is will be much more easy and effective.
 

Re: Conversion Help

Thanks for the information. The 4-AA batteries still get very hot and wear out rather quickly. I was told I should be using C batteries which has the same Volts, but more current or something along that line. I bought and hooked up 2-C batteries. I don't have a resisotr in the line, but the lights are very dim. Is that because the lights are 3.6V and I am only running 3V with the C-batteries? Should I get two more C batteries and then attach a resisitor? If so what size resisitor would I need? Thanks in advance. I just checked the batteries and they are very hot. I have the toggle switch off, but the batteries are hot. Does thise mean a line or something is crossed? When the switch is off, nothing lights up, but when I flick the switch on, all the lights come on.
 

Re: Conversion Help

Tim2255 said:
Is that because the lights are 3.6V and I am only running 3V with the C-batteries?
Click to expand...
That's rather obvious ..

Tim2255 said:
Should I get two more C batteries and then attach a resisitor? If so what size resisitor would I need? Thanks in advance.
Click to expand...
One more will be enough ..
All what that resistor has to do (with 3 batteries) is to drop the voltage from 4.5V to 3.5V ..
R = 1V / I[LEDs]

IanP
:D
 

Re: Conversion Help

OK I think I figured everything out and the problem was with the switch. I had a three prong switch and when I went to a two prong switch, everything worked out fine. I do have two questions.

1. With a three prong toggle switch should I have wired one hot from the battery to one prong, the hot to the led lights to another prong and then all the negatives to the ground?

2. As I stated, i think I have everything set up where it works now, but I notice my resisotr gets very hot when the lights are out, is this normal?

Thanks.
 

Re: Conversion Help

Tim2255 said:
1. With a three prong toggle switch should I have wired one hot from the battery to one prong, the hot to the led lights to another prong and then all the negatives to the ground?
Click to expand...
That sounds good ..

Tim2255 said:
2. As I stated, i think I have everything set up where it works now, but I notice my resisotr gets very hot when the lights are out, is this normal?
Click to expand...
No, it's not, looks like the switch shortens the battery through resistor ..
Leave the third "prong" not connected ..

IanP
:|
 

Re: Conversion Help

On point number 2, I should have mentioned that is a different toggle switch and only has twp prongs. The three prong switch I returned because I couldn't get it to work. No matter which way I hook up the positive and negative, the resistor gets hot when the lights are off.

Might this be because I have all the positives wired to the positive on the switch, then a wire coming from the switch to the battery. Since all the positives are connected, but the toggle switch is in the off position, does this still draw a current, but doesn't complete the circuit because the toggle switch is off and that is what is causing the heat or doesn't it matter that all the positives are hooked together as long as the switch is off, the circuit is not completed, therefore not drawing any current from the battery.
 

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