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Old 03-09-2019, 04:37 PM   #21
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Default Re: The right resistor ???

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Originally Posted by AnimalHippie View Post
Thank you again bro! Just tried my resistors with shitty luck. Two big ones 3.3/3w and 2.2/2w both got super hot super quick and was bright as all get up. The 2.2k 1/4 barely turned them on. So back to the drawing board. Maybe I can run by that shop tomorrow if I get the size figured out
Depends in how bright you want it. If you put a multimeter across the led when you have the 2.2k in there it will tell you the drop of the led and then you can use the equation above.

Otherwise stick a 100 ohm , 1 watt resistor in there and it will get you in the right ballpark.

EDIT: i overlooked that you measured the drop earlier in the post.. if the leds are dropping 2.9 v and you want to limit the current to 150ma then you would want a 6-2.9 = 3.1v drop . (v=ir) 3.1v / 0.150ma = 20.1 Ohm resistor . The wattage that this resistor is burning would be 0.465 watts so you could get by with a cheaper half watt resistor. The resistor would still get warm / maybe hot as it will have to put off a half watt of heat.

A 100 ohm resistor in there would limit the current to 3.1 / 100 = 30ma. It would be not so bright but the resistor would be not warm at all. Anywhere between the two will be good safe choices.. good luck!

Last edited by Alka; 03-10-2019 at 05:58 AM.
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Old 03-10-2019, 10:58 AM   #22
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Default Re: The right resistor ???

So I made it to the electronics store. Picked up a few different sizes. If I get a chance today I will do some test runs to see what's gonna work out.

Went with the 1w resistors just cause I wanna keep the temps down.
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Old 03-12-2019, 07:53 AM   #23
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Default Re: The right resistor ???

Got a few mins with the soldering iron on Sunday. Tried the 100ohm 1w resistor. Results were not very bright. Since it really didn't do much going from the 2.2k to the 100. I went ahead and tried the 22 ohm 1w resistor. It does light the boards up pretty good. I think I could get a little more out of them tho. They don't seem to be as bright as they were hooked to original supply. So I might try another resistor. I left them on for about 10 mins to check the temps. The LEDs stayed cool. Not even warm at all. The resistor on the other hand was hot. Too hot to touch kinda hot. So I guess I'm going to get a few more selections of resistors probably get 5,8,12,15 ohm just so I have some more range. Good thing they are cheap! I will have a nice collection once I'm done with this project. Now wattage. Does higher watt mean it would run a lower temp? Even through all this I'm still a little confused about how this works. I plan on running 4 of these boards. Is that going to change what resistor I should be using? I could run a resistor for each pair if need be. Guess I need to take some electronics courses at least I finally got my soldering skills dialed in
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Old 03-12-2019, 09:40 AM   #24
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Default Re: The right resistor ???

The problem with the resistor heating up is going to be tricky. The wattage rating of the resistor is how much it can handle but they will all waste as much heat at the same resistance. A 1/4 watt resistor would just get hot and burn out where a 1 watt one would get hot and survive. If 20 ohms gives you the brightness you need then two 40 ohm resistors in parallel would heat up much less. A lower value resistor will just heat up more.

When driving high power leds , this is the problem with current limiting resistors. The ideal device to drive leds are called constant current drivers. They only allow a certain current to flow.
Something like this:

https://www.digikey.com/product-deta...06-ND/7704763&

Using a series resistor is going to be tricky for brightness because unfortunately you can't have it both ways. For the led to be bright you must have a decent current but when you pass a higher current through a resistor it will get hot! So that's why constant current drivers are a thing..

The get around that originally by using AA batteries that you can't draw a high current from anyway. 4 in series would probably be only able to output 300-500ma or so.

The other thing you could do if you are driving more than one of these lights, put the leds in series so they have a 5.8v drop. Then you can use your 6 volt source with very little current limiting needed.

Last edited by Alka; 03-12-2019 at 09:43 AM.
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Old 03-13-2019, 09:13 AM   #25
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Default Re: The right resistor ???

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Originally Posted by Alka View Post
The problem with the resistor heating up is going to be tricky. The wattage rating of the resistor is how much it can handle but they will all waste as much heat at the same resistance. A 1/4 watt resistor would just get hot and burn out where a 1 watt one would get hot and survive. If 20 ohms gives you the brightness you need then two 40 ohm resistors in parallel would heat up much less. A lower value resistor will just heat up more.

When driving high power leds , this is the problem with current limiting resistors. The ideal device to drive leds are called constant current drivers. They only allow a certain current to flow.
Something like this:

https://www.digikey.com/product-deta...06-ND/7704763&

Using a series resistor is going to be tricky for brightness because unfortunately you can't have it both ways. For the led to be bright you must have a decent current but when you pass a higher current through a resistor it will get hot! So that's why constant current drivers are a thing..

The get around that originally by using AA batteries that you can't draw a high current from anyway. 4 in series would probably be only able to output 300-500ma or so.

The other thing you could do if you are driving more than one of these lights, put the leds in series so they have a 5.8v drop. Then you can use your 6 volt source with very little current limiting needed.
in reference to the red. I would like them to be a little brighter as I think the boards could handle it. I hooked all four LED boards together last night. Ran them for about 15 mins. The boards never even got warm in any way. The resistor on the other hand was too hot to handle. I wired the boards + in one side + & - between boards and the - coming off the end. Parallel correct? So if the 20ohm isn't bright enough. I could wire the 2-20ohm resistors together side by side not end to end and get a cooler operating 10ohm from doing this. Is that correct? If I ran the boards with + & - on one end would with the last board being the end of the run. Series correct? Would that make any difference? I can't see any visual difference in the light it makes either way it is wired. Thanks again for all your knowledge. I would have been to the electronics store 20 times if it wasn't for your help.
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Old 03-14-2019, 07:00 AM   #26
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Default Re: The right resistor ???

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Originally Posted by AnimalHippie View Post
in reference to the red. I would like them to be a little brighter as I think the boards could handle it. I hooked all four LED boards together last night. Ran them for about 15 mins. The boards never even got warm in any way. The resistor on the other hand was too hot to handle. I wired the boards + in one side + & - between boards and the - coming off the end. Parallel correct? So if the 20ohm isn't bright enough. I could wire the 2-20ohm resistors together side by side not end to end and get a cooler operating 10ohm from doing this. Is that correct? If I ran the boards with + & - on one end would with the last board being the end of the run. Series correct? Would that make any difference? I can't see any visual difference in the light it makes either way it is wired. Thanks again for all your knowledge. I would have been to the electronics store 20 times if it wasn't for your help.
yeah, sounds like you got it ! if you wired the LED's in series then the voltage drop across them would be higher.. That means the difference between the voltage you are supplying and the drop of the leds would be less so a smaller value resistor would be used and a higher current can be passed through the LEDS.

Wiring the LEDS in parallel will do the opposite, for the same brightness TWICE the current would be needed and also a higher value resistor would be needed to keep the current regulated as the voltage difference is greater.

If you wire too many in series the extra leds just wont light because the forward voltage drop of the leds is higher than what you are feeding. At that point you would either have to up your voltage or wire series and parallel to meet the drop and current requirements.

Now wiring two equal resistors in parallel halves the effective resistance but doubles the area to emit heat plus the current is also split now between both resistors.

The best practice is to wire as many LEDS in series as you can get away with for your power supply. Then the resistance needed would be less and the power dissipated by those resistors would be less. Then if you have too much heat in the resistors still you can parallel the resistors to decrease their heat ( keeping in mind parallel resistors have half the resistance so 10Ohm == 2x 20Ohms) .

Last edited by Alka; 03-14-2019 at 07:03 AM.
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Old 04-19-2019, 07:13 AM   #27
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Default Re: The right resistor ???

Finally got these lights finished up and installed. Will be putting up a video on this in the next week or so. Here's a few pics of the end result. The perfect amount of light to me. Not as bright as the other rock lights I have made, but should work perfectly. Thank you everyone for your help and information! I have allot better understanding of how this stuff works now. Have a great day!

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Old 04-19-2019, 11:28 AM   #28
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Default Re: The right resistor ???

Great job. I'm a little dense when it comes to figuring out electronics, so I look forward to your video!
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