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Author Topic: resistor to convert 12v led to 24v led clearance light  (Read 4860 times)
bevans6
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« on: September 12, 2010, 02:35:23 PM »

I have to do this tomorrow, add a resistor to drop the voltage so that I can make an emergency repair by installing a 12v LED clearance light on a 24v bus.  does anyone have a feel for the correct size range of resistor that I should pick up?

Thanks, Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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gumpy
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« Reply #1 on: September 12, 2010, 07:12:52 PM »

Is this on your bus?  Don't you have a toad converter installed?  How about connecting the 12v clearance light to the 12v output of the toad converter?
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Craig Shepard
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Nusa
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« Reply #2 on: September 12, 2010, 07:18:48 PM »

Buy a variety pack of resistors, they're pretty cheap...it'll likely include values from 100 ohms to 1 million ohms, several of each. Then you can experiment until you find the required value to get close to 12V measured across the light. Start high and go down, rather than low and go up...that way you won't blow the light with too much voltage.
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HighTechRedneck
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« Reply #3 on: September 12, 2010, 08:33:48 PM »

I moved this thread from the Spare Tire to the All Topics board.  I think it will get more answers here.
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Sean
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« Reply #4 on: September 12, 2010, 09:11:34 PM »

I have to do this tomorrow, add a resistor to drop the voltage so that I can make an emergency repair by installing a 12v LED clearance light on a 24v bus.  does anyone have a feel for the correct size range of resistor that I should pick up?

Brian, these lights are all over the map.  The amount they draw depends on brand, number of LEDs, internal construction, and specific LED part type.

Really the only way to know is to buy the light, then hook it to a 12-volt supply with an ammeter in-line and measure the current.

"Typical" would be around 200 milliamps at 13-14 volts, which would mean a resistor around 70 ohms or so.

Remember that these are high-power resistors; they will have metal bodies with heat-sink fins and will need to be exposed to cool air to dissipate the heat.  A 70-ohm resistor dropping 13.5 volts will need to be rated for at least three watts.

If you are in a jam, it is often quicker and easier just to buy an identical pair of 12-volt LEDs, and wire them in series -- even if one of them is buried inside the fender.  Just pick two-wire models as opposed to frame-ground items.

Buy a variety pack of resistors, they're pretty cheap...it'll likely include values from 100 ohms to 1 million ohms, several of each.

Umm, not in 5-watt ratings it won't.  They won't be that cheap either... I am guessing you are thinking of a hobby pack of resistors from, for example, Radio Shack, which are rated at only 0.25 watt each.  The dropping resistor for an automotive LED has to dissipate over ten times that amount of power.

Generally speaking, 5-watt resistors are ordered from specialty electronics houses, not retail stores like the Shack.  I think they might have a couple of values in 10-watt wire-wound types, but those are not really suitable for an automotive application -- you really want the metal heat sinks.

-Sean
http://OurOdyssey.BlogSpot.com
« Last Edit: September 12, 2010, 09:14:07 PM by Sean » Logged

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stevet903
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« Reply #5 on: September 12, 2010, 09:25:08 PM »

This page is helpful to calculate LED resistors for various circuit configurations.

http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator


Steve
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Sean
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« Reply #6 on: September 12, 2010, 09:57:07 PM »

Steve,

That's a great page for anyone trying to build LED fixtures.  It's not really needed for the OP, though.  This page is for figuring the current-limiting resistor for one or more individual LEDs; the automotive fixtures Brian is talking about already have current-limiting resistors built in.  What Brian needs is a simple dropping resistor to balance the voltage drop between the resistor and the fixture.  That's a simpler calculation, and the formula is simply R=V/I.  Figuring "I" is the hard part, and really, he just has to measure it or get the specs from the fixture manufacturer.

-Sean
http://OurOdyssey.BlogSpot.com
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bevans6
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« Reply #7 on: September 13, 2010, 05:08:19 AM »

Thank you for moving the post - I was browsing around and I guess I posted while still in the for sale forum!  Current spec is .06 amps. which suggests a 1 watt 220 ohm resistor might work.  The light in question is in the middle of the roof, so running a wire is a bit problematic.  I was doing the pre-trip yesterday, noticed that light was out, and when I got up there to look the bulb socket had completely disintegrated, I have no idea how it worked up until now.  Cracked lens let water in.  If I think about it, I may well switch to all 12v LED's for all of the clearance lights, then I can just do the wiring change once at the main panel, feed the circuit from 12v before the switch.  But not today...

I may go the resistor route, depending on what Radio Shack has, or I may buy two  of the tiny little running lights and wire them in series.  I can special order 24v lights, but that takes a couple of weeks and I am leaving in a few days on vacation.

Thanks for the help.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
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1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
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« Reply #8 on: September 13, 2010, 07:50:28 PM »

Just put two 12v lights in series, no need to try to figure out the voltage drop for a resistor.

Resistors generate heat and do nothing.

If one goes out they both will go out but LEDs are so long lived that probably isn't much of a problem.
« Last Edit: September 13, 2010, 07:52:15 PM by gus » Logged

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bevans6
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« Reply #9 on: September 14, 2010, 05:54:53 AM »

I got two 12v LED lights and put them in series.  They are the 1" by 4" ones that clip into a base.  The base only has one wire, it is supposed to ground to the chassis through a mounting screw.  I had to make a base out of lexan (the only heavy non-conductive plastic I had in the shop), mount the light bases to it, connect the two ground lugs together so the lights were in series, then mount the whole thing on the bus.  I didn't want to drill a bunch of new holes in the roof of the bus, so i was able to rivet on a couple of 10-32 aircraft nut plates that I keep on hand using existing holes that were used to mount a backing plate  for the stock light and I screwed the new monstrosity onto those nut-plates.  Copious amounts of silicone to seal, and done.  Given that it's 10 feet in the air, in the middle of the bus on the curb-side, it looks OK.  I will probably get some new MCI clearance lights and change out some, and figure out a conversion to LED's but I will wait on that.  With 14 lights to change and MCI lights at around $40 each, this is not an inexpensive project...

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
Busted Knuckle
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« Reply #10 on: September 14, 2010, 09:49:55 AM »

Setra of North America sells 24 V led clearance lights for $25.00 ea! I have them on all my coaches and they are awesome and really stand out and can be seen from a long way!
Setra N. America # is 800-882-8054 ask for parts. All of them in parts do a great job and can help you, but Amy is my favorite she's purtyer than Bryce, Daniel, Howard, & Roger! Wink
They can have them to you overnight for the cost of shipping, or free ground shipping in a couple days w/order over $100 (4 lights)
Grin  BK  Grin

OH yeah be sure to tell them you need the style for the 200 series coaches!


OH and one last note they are DOT approved! Wink
« Last Edit: September 14, 2010, 09:53:49 AM by Busted Knuckle » Logged

Busted Knuckle aka Bryce Gaston
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rwc
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« Reply #11 on: September 14, 2010, 04:35:22 PM »

BK do those just plug in on an MCI as replacements?
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Busted Knuckle
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« Reply #12 on: September 14, 2010, 05:05:55 PM »

BK do those just plug in on an MCI as replacements?

No they don't just "plug in" but they have the 2 wires coming out the back and we just cut the old ones off and use the heat shrink covered crimp connectors and hook 'm up and screw 'm on! They are a direct replacement for the original lights on the 200 series (S215 & S217 models)  Setra's which are the SAME as the MCI lights. I'll go out and take a picture of one of the buses with them lit up in a minute! As a matter of fact I'll take one of the LED's and the last bus we haven't installed them on yet! (have them, but not installed yet!)
Grin  BK  Grin
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Busted Knuckle aka Bryce Gaston
KY Lakeside Travel's Busted Knuckle Garage
Huntingdon, TN 12 minutes N of I-40 @ exit 108
www.kylakesidetravel.net

Grin Keep SMILING it makes people wonder what yer up to! Grin (at least thats what momma always told me! Grin)
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« Reply #13 on: September 14, 2010, 06:40:06 PM »

I'll post the pics after I figure out why my laptop keeps disconnecting in the middle of trying to post them!
« Last Edit: September 14, 2010, 06:41:58 PM by Busted Knuckle » Logged

Busted Knuckle aka Bryce Gaston
KY Lakeside Travel's Busted Knuckle Garage
Huntingdon, TN 12 minutes N of I-40 @ exit 108
www.kylakesidetravel.net

Grin Keep SMILING it makes people wonder what yer up to! Grin (at least thats what momma always told me! Grin)
cody
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« Reply #14 on: September 14, 2010, 06:42:58 PM »

wow bk, I can't tell them apart lol
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