| Author |
Message |
Richard Jones
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 30, 2006 - 6:22 pm: | |
I want to use LED lights on my tow vehicle. I bought some that are 12V and will need to put a dropping resistor in series as I have a 24v system in the bus. There are 12 LEDs per light assembly. So my question is how many ohms of resistance and wattage size will I need. I don't a meter that will measure the current draw of the lights.
Thanks in advance.
Richard Jones |
Jerry Liebler (Jerry_liebler)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 30, 2006 - 6:31 pm: | |
Richard,
Beg, or borrow or buy a meter to measure the current when the lights are powered from 12 volts. Or if you know the rated curent use it. Then the resistance you need is calculated by dividing 12 by the current. The wattage of the resistor is 12 times the curent times about 2 for safety.
Regards
Jerry 4107 1120 |
Craig (Ceieio)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 30, 2006 - 7:12 pm: | |
Richard - if the physical packaging will allow, put two led lights in series per circuit; each light will only see 12v that way and you know they are the same resistance and power capability. This would work if you are mag-mounting some lights, not so good if you are trying to use the vehicle lights.
Craig - MC7 Oregon |
Gary Stadler (Boogiethecat)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 30, 2006 - 7:53 pm: | |
Richard,
Because the LED's in your lights are likely internally hooked in a series/parallel arrangement, there is no way to second guess what current the lamp draws, hence there is no real way to guess what resistor you need without actually measuring the current draw of the lamp when running on 12 volts.
If you're not in a hurry and can't get a meter, you could mail a light to me and I'd be happy to both measure it and send you the right resistors...
Alternately you could do it this way... run the lamp on 12 volts and get an idea of how bright it is. Then stick a 20 ohm 1/2 watt resistor in series with it and try it on 24 volts. If it's about the right brightness, there you are. If it's too dim, stick another 20 ohm resistor across the one you just tried, this will give you an effective 10 ohms... try it again and keep adding resistors until the brightness is right.
Or if it's too bright, stick another 20 ohm resistor in series with the first one, giving you 40 ohms, etc
1/2 watt resistors are probably going to heat up too much to use permanently... I suggest you use them for the experimentation because you can buy a lot of them cheap.
You can keep messing with it this way and eventually you'll end up with the right resistance.... then if the resistors don't get hot at all you can just use the "wad" of 1/2 watt resistors that you ended up with. If they do get hot, or if you then want to physically clean it up a bit, you'd have to measure or calculate what value resistance you ended up with and finally purchase a higher wattage resistor of that ohm value.
My guess is that ultimately a 5 watt resistor will be plenty enough, maybe you'd get away with a 2 watt or smaller if you find they don't get hot...
Cheers |
Richard Jones
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 30, 2006 - 8:03 pm: | |
Checked the mfg tech sheet and at 14v the lamp will draw .42amps. So doing the math it appears that a 33 ohm resistor, 5 watt is needed.
Thanks for the help.
Richard Jones |
Pete/RTS Daytona (Pete_rtsdaytona)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 12:13 am: | |
radio shack wirewound 10 watts
271-135 100 ohm (2 pack)
three 100 ohms in parallel = 33 ohms
6 times more watts then is needed - by that's ok
Pete RTS/Daytona |
Bryce Gaston (Busted_knuckle)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 1:27 am: | |
Hey guys you've hit on something I've been need'n some help on for a while now I've got a full set of LED Tail/stop/& Turn lights for an MCI 102 but they are 12 volt we have found out they pull around 742 milliamps each so what is the best way to go about wiring these up? I'm not an electronics master by any means an that is why they been sitt'n on a shelf while I was waitin to find this out I think if I've done my math right I need about 17 ohms resistance is this correct? And I guess the proper way would be to make a harness with a resistor on each wire ? I appreciate any help I can get as electronics are not my specialty! But I love the looks of the LED's and with $130 invested so far it beats the heck out of MCI wanting $800 for the same 6 lights (4 red & 2 amber) in a 24 volt kit! Thanks Busted Knuckle aka Bryce |
Rich International Bus & Parts
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 10:21 am: | |
We use a (40ohm) resistor, hooked to the ground. You can hook up to (6) "like functioning lamps" to one resistor.
Rich
|
Pete/RTS Daytona (Pete_rtsdaytona)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 1:09 pm: | |
Busted
radio shack 271-132 (10 ohms / 10 watts)
2 in series will give you 20 ohms
or about 11 volts across the led lights
Pete RTS/Daytona |
Bryce Gaston (Busted_knuckle)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 1:19 pm: | |
Thanks I wasn't sure! Did ya by any chance check and see if I figured it right? I did some of this figuring stuff in electronics class as a sophmore in high school about 24 yrs ago and haven't used it since! For some odd reason owning and driving trucks there wasn't much call for it! (LOL) But having given up on trucking after 15 yrs back in 2000 and going back to wrenching I occasionally find some of the stuff I can't remeber usefull! Thanks again! Busted Knuckle aka Bryce Gaston |
Tim Strommen (Tim_strommen)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 2:25 pm: | |
Hello,
I went into detail on this topic about a week ago. Check out this post:
http://www.busnut.com/bbs/messages/11/12184.html?1136416030
If you can't find the specs posted by the manufacturer, try finding a bench power supply with voltage and current limiting. Set it for voltage limiting and the dial to the correct operating voltage for the fixture. The power supply's meter should tell you how much current is being drawn.
One other thing to remember is that diodes are current devices - so they will always use the same ammount of voltage. This is why you lose .7 volts when you use a battery isolator, but the current doesn't get clamped by the diode (this is also why "thermal runaway" is a problem - silicon diode's resistance goes down the hotter they get, so the ammount of heat they produce will accelarate until the diode burns up if it is overloaded).
Cheers!
-Tim |
Bryce Gaston (Busted_knuckle)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 10:10 pm: | |
Tim, I'm sorry but I didn't even register on this board for the first time until 01/15/06 and after you pointed it out I went back and read what you posted on 01/04/06. So ya see I wasn/t aware "that you went into detail on this 27 days ago" and since it was already mentioned I asked for help with it! I do appreciate the help on this from all of you! When you are a "newbee" it's hard to know what has been already discused in detail! And also sometimes it takes several times before it sinks in for those of us that are sorta thick headed when it comes to stuff we don't quite understand right away. (at least it does me!) Thanks to all for all the help! BRYCE aka Busted Knuckle |
Tim Strommen (Tim_strommen)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 01, 2006 - 1:11 am: | |
Gee, was it that long ago? (golly time flies on this board!)
Anyway, welcome to the madness!
Since you're new hear I'd love to point out the "search" feature to your left that Ian has kindly provided to us. I've been using it for quite some time and can say that if it aint' in there it ain't been asked (or someone shanghi'd the thread with a cock fight and it got nuked...)
Even if it has already been asked - if you don't understand something please ask us to elaborate (we always do!)
Again, welcome!
Cheers!
-Tim |
Bryce Gaston (Busted_knuckle)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 01, 2006 - 1:08 pm: | |
Thanks Tim, and yes you can bet I'll ask I always have trouble understanding all that technical garbo and don't really grasp new things until I have hands on experince, I guess you could say my hands are smarting than my mind! LOL but it's true I learn faster and easier by doing than reading, re-reading and re-reading and re-reading and uh well you get the idea! Thanks again Bryce aka Busted Knuckle |
james dean boggs (Jd_boggs)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 08, 2006 - 11:56 pm: | |
If you want to use LED lamps designed for 12 vdc on a 24 vdc system then you need a 480 ohm,
1.2 watt resistor. This is based on my calculation of a 12 LED cluster lamp from superbrightleds.com
They say that an 12 LED cluster uses 50-75 (depends on LED color) milliamps on a 12 volt system. This less than one tenth of an amp (100 milliamps is .1 amp)
So, you can use a resitor value from about 400 to 600 ohm 2 to 5 watts without damage.
If you overdrive the LED (too much current) the result is that the LED gets brighter and shortens its life from 20 years of constant ON to about 15 years. No big deal. |