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Old 03-07-2019, 09:54 AM   #1
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Default The right resistor ???

Hey guys, I'm trying to use the COB LEDs out of one of these

For my rock lights. It comes wired to a switch and 4-AAA batteries. There is no resistor or anything other then a switch between batteries and LEDs. When you add all the batteries voltage you get 6v. I tried running a set on 6v from my Rx and smoked them in about 20 seconds. I'm thinking they need to be more like 4v or 5v. Can anyone help me figure out what size resistor I should pick up? Here's a few pics of the boards.

On one end it has NT64 The other end has 10YQ. I haven't been able to locate these boards anywhere to get specs on them. So I'm kinda guessing. From the LED calculator it's saying a 1.7w 2.7k ohm resistor. Does that sound right? I'm new to the electronics world as far as this. Any and all help is appreciated! Thanks for any help!
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Old 03-07-2019, 11:42 AM   #2
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Default Re: The right resistor ???

Sounds like you're probably on the right track by stepping the volts down to about 4v, that tends to be a pretty common value in the LED world. But 2.7k ohms seems pretty high.

In my (admittedly limited) experience with AA & AAA batteries, they tend to actually output about 1.2v even though they say 1.5v.

In order to get a better resistor value calculated you need to know the draw (amps) as well as the in & out volts..

To get the values you need, I'd turn it on in it's "stock" setup and check what volts/amps the cob is receiving and go from there.

Or just do some generic googling and see what you can up with in relation to cob lights that are about the same size. But you're basically down to the good old guess and check method at that point..

Last edited by QuesoDelDiablo; 03-07-2019 at 11:45 AM.
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Old 03-07-2019, 11:55 AM   #3
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Default Re: The right resistor ???

I tinkered with the same idea using that type of switched light from Harbor Freight Tools... after I burnt the second strip, I looked closer and saw "1.2v" printed right there on the strip....
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Old 03-07-2019, 12:11 PM   #4
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Default Re: The right resistor ???

Quote:
Originally Posted by QuesoDelDiablo View Post
Sounds like you're probably on the right track by stepping the volts down to about 4v, that tends to be a pretty common value in the LED world. But 2.7k ohms seems pretty high.

In my (admittedly limited) experience with AA & AAA batteries, they tend to actually output about 1.2v even though they say 1.5v.

In order to get a better resistor value calculated you need to know the draw (amps) as well as the in & out volts..

To get the values you need, I'd turn it on in it's "stock" setup and check what volts/amps the cob is receiving and go from there.

Or just do some generic googling and see what you can up with in relation to cob lights that are about the same size. But you're basically down to the good old guess and check method at that point..


I will bust out my multi meter when I get a chance here in a bit. Probably 2 hrs from now. Then I will post what I find out. Think an amp clamp on my multi meter will measure the amps? I have found some that are close to the same https://www.aliexpress.com/item/-/32826528937.html all tho they do not have the same markings.

Letter J, these ones have only the NT64 And 10YQ markings on them. Glad to see I'm not the only one who has ran into this.

Thank you for your help guys. I will post once I get my multi meter in hand.
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Old 03-07-2019, 12:41 PM   #5
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Default Re: The right resistor ???

Let us know what ya find Animal. Good, economical alternative for rock lights!
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Old 03-07-2019, 02:34 PM   #6
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Default Re: The right resistor ???

Ok now that the blue bar in my eyes is starting to go away. Maybe I can post this lmao!

Hooked the multi meter to boards. They are running at 2.889v. I'm getting 6v out of the batteries but the way the boards are hooked up makes it 2.889v

I get that same reading on all points on the boards. As for amperage it must be too low for my amp clamp to read because I'm getting nothing.

So my next question is. What size resistor should I be looking for to drop the 6.2v from my Rx to 3v???
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Old 03-07-2019, 03:02 PM   #7
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Default Re: The right resistor ???

Well, I had a long explanatory answer with links and formulas and calculations but I got logged out before I posted it and lost it all.

Short answer, 3.3ohm resistor that can handle 3.3watts.

For 4V at the lamp, 2.2ohm resistor that can handle 2.2watts
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Old 03-07-2019, 03:21 PM   #8
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Default Re: The right resistor ???

Quote:
Originally Posted by QuesoDelDiablo View Post
Well, I had a long explanatory answer with links and formulas and calculations but I got logged out before I posted it and lost it all.

Short answer, 3.3ohm resistor that can handle 3.3watts.

For 4V at the lamp, 2.2ohm resistor that can handle 2.2watts


All the resistors I find are 1/4,1/2 or 1w. What I came up through a calulator is a 22ohm 1w resistor. Man this shit is so confusing to me. 3.3ohm with 3.3 wats on Google brings up some big resistors. 60mm long. Don't think those are what I'm after
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Old 03-07-2019, 04:18 PM   #9
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Default Re: The right resistor ???

What about a dc step down converter?
And clamp meters only measure a.c. amps, not dc.

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Old 03-07-2019, 04:51 PM   #10
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Default Re: The right resistor ???

Are you going to use both lights on the truck? If so, you could just keep them wired in series and they will get half of whatever your BEC is outputting...

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Old 03-07-2019, 04:55 PM   #11
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Default Re: The right resistor ???

I ran into once. Not understanding how resistors and all that works. I simply bought a mix pack of 12V leds with wire attached. I did my TRX4 with 16 individual LEDs all wired together and it just worked. No resistors or flicking or anything like that. This is what I used.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
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Old 03-08-2019, 06:29 AM   #12
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Default Re: The right resistor ???

Quote:
Originally Posted by Steve g View Post
What about a dc step down converter?
That is what I was thinking, something like this is small, covers a range of output voltages, and only costs $1.50.

https://www.ebay.com/itm/DC-Buck-Step-down-Converter-1-25V-1-5V-1-8V-2-5V-3-3V-5V-3A-Power-Supply-Module/122049334019?hash=item1c6ab50b03:gLUAAOSwqfNXjO2 u
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Old 03-08-2019, 06:46 AM   #13
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Default Re: The right resistor ???

svt923..... That will work.

I also found this too.
A 3.3 ohns @ 3 watts should be fine for what looks to be 1.00$ ..... don't sweat it.

https://www.mouser.com/Passive-Compo...z0wszxZ1z0x6uk

or

https://www.amazon.com/s?k=3watt+3+o...ref=nb_sb_noss

Last edited by Tim3; 03-08-2019 at 06:55 AM.
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Old 03-08-2019, 08:10 AM   #14
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Quote:
Originally Posted by Steve g View Post
What about a dc step down converter?
And clamp meters only measure a.c. amps, not dc.
I have not thought about one of those only cause I'm already limited on space. Installing a 8ch Rx, BEC, Rx bypass loop, light controller. Might have to soak the BEC in conformal coating and leave it on the chassis somewhere. I was unaware if it would read it or not. Had never tried using an amp clamp on DC power before yesterday

Quote:
Originally Posted by JSterrett View Post
Are you going to use both lights on the truck? If so, you could just keep them wired in series and they will get half of whatever your BEC is outputting...
Yes I'm going to use one in each fender. Total of 4. What are the benefits of running them series over the other option? Less power usage? More efficient? Wouldn't I have to loop the whole system around the chassis?

Quote:
Originally Posted by BIGW0RM View Post
I ran into once. Not understanding how resistors and all that works. I simply bought a mix pack of 12V leds with wire attached. I did my TRX4 with 16 individual LEDs all wired together and it just worked. No resistors or flicking or anything like that.
I'm using 12v headlights and taillights. On my last DIY rock lights I ran 12v strips for simplicity, but this time around I wanted to try something different. Also want to use a servo board light controller that I'm building to run my rock lights and light bar. Which is 6v Max. I also like to run smaller batteries 2200&2650mah and looking for a bit more efficient lighting. Hopefully they end up more efficient.


Thank you guys for digging into this with me! You have no idea how much I appreciate all of your help! I just found a local electronics repair shop that carries a wide variety of resistors. I'm going to try to make it there today. I will let you guys know what I come up with and how it all works out. Once I get everything wired up and working I will do another DIY build thread for these and one for my DIY 12v headlight and taillights. Thank you again everyone for your input!!!!

Last edited by AnimalHippie; 03-08-2019 at 08:51 AM. Reason: Typo
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Old 03-08-2019, 11:12 AM   #15
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Default Re: The right resistor ???

Score!!!

Looks like I'll be doing some soldering this weekend. Picked up a 3.3ohm 3watt & 2.2ohm 2watt for the rock lights. Might try them at 4v and see how hot they get. Grabbed the 2.2ohm 1/4watt for replacing the pot on my servo board light controller. I'll keep you guys up to date on my progress. Super stoked!!!
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Old 03-09-2019, 11:52 AM   #16
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Default Re: The right resistor ???

It's not about dropping the voltage with the resistor but limiting the current. I would limit to maybe 120 ma for a cob like this which would be a 100 ohm resistor for a 12v input. If you were using the light on 6v then a 50 ohm resistor would five you the 120ma. The voltage that the led is fed does not matter so long as the current is limited. Feeding the led with too low a voltage will cause the led to not light regardless of current. To change brightness you alter current only not voltage. The voltage just has to be more than the voltage drop of the led ( plus a little bit).

That 1/4 watt resistor package says 2.2 k ohms . Not 2.2 ohms like you mention in the caption. That's a 1000 times more maybe that's what you wanted to get though for the pot.

Anyway the led is a voltage dropping device not a resistive device , you can put the thing on 4, 6, 12, 20 whatever volts you want . You need to pick the right resistor to keep the current low enough that it doesn't burn out.

Last edited by Alka; 03-09-2019 at 12:02 PM.
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Old 03-09-2019, 12:19 PM   #17
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Default Re: The right resistor ???

Quote:
Originally Posted by Alka View Post
It's not about dropping the voltage with the resistor but limiting the current. I would limit to maybe 120 ma for a cob like this which would be a 100 ohm resistor for a 12v input. If you were using the light on 6v then a 50 ohm resistor would five you the 120ma. The voltage that the led is fed does not matter so long as the current is limited. Feeding the led with too low a voltage will cause the led to not light regardless of current. To change brightness you alter current only not voltage. The voltage just has to be more than the voltage drop of the led ( plus a little bit).

That 1/4 watt resistor package says 2.2 k ohms . Not 2.2 ohms like you mention in the caption. That's a 1000 times more maybe that's what you wanted to get though for the pot.

Anyway the led is a voltage dropping device not a resistive device , you can put the thing on 4, 6, 12, 20 whatever volts you want . You need to pick the right resistor to keep the current low enough that it doesn't burn out.


Thank you for the input. Not knowing my arse from a hole in the ground when it comes to this type of thing. I'm just trying what others had suggested mixed with some of my own research. I used a calculator for resistors to come up with my purchase. I did 6v input drop to 3v at 150ma (didn't know exact ma for this LED but used a happy medium from other LEDs this size in the net) this where I got the 3.3ohm 3watt resistor for my results. I'm going to be messing with this here in a few mins and going to try the 2.2ohm 2w resistor first and see what kind of power/heat I get from that. If I don't like it I'll try the 3.3. The 2.2k 1/4 is for my servo board light controller. Thank you for your knowledge it is much appreciated. Ears are open for some more sir :-)
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Old 03-09-2019, 01:43 PM   #18
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Quote:
Originally Posted by AnimalHippie View Post
Thank you for the input. Not knowing my arse from a hole in the ground when it comes to this type of thing. I'm just trying what others had suggested mixed with some of my own research. I used a calculator for resistors to come up with my purchase. I did 6v input drop to 3v at 150ma (didn't know exact ma for this LED but used a happy medium from other LEDs this size in the net) this where I got the 3.3ohm 3watt resistor for my results. I'm going to be messing with this here in a few mins and going to try the 2.2ohm 2w resistor first and see what kind of power/heat I get from that. If I don't like it I'll try the 3.3. The 2.2k 1/4 is for my servo board light controller. Thank you for your knowledge it is much appreciated. Ears are open for some more sir :-)

LEDs are a strange thing. If you know the forward drop of the led then it's pretty easy. Trickier with a unknown led but say the led forward drop is 4v and you are giving an input of 9v. The resistor is just chosen using v=ir to limit the current. So 9-4 = 5 volt drop. 5/.150 amps is 33 ohms. This resistor would be burning (p=iv) 5 x .150 = 0.75 watts.
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Old 03-09-2019, 01:47 PM   #19
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Default Re: The right resistor ???

Quote:
Originally Posted by Alka View Post
LEDs are a strange thing. If you know the forward drop of the led then it's pretty easy. Trickier with a unknown led but say the led forward drop is 4v and you are giving an input of 9v. The resistor is just chosen using v=ir to limit the current. So 9-4 = 5 volt drop. 5/.150 amps is 33 ohms. This resistor would be burning (p=iv) 5 x .150 = 0.75 watts.


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
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Old 03-09-2019, 03:11 PM   #20
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Default Re: The right resistor ???

if you just double stick that to your skid as purchases that handy switch drags the ground and you get automatic reverse lights
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