# Powering an LED off of a 36volt power source.



## therealciviczc

I'm trying to get an LED powered on my 36 volt golf cart. I want to remove the reverse buzzer, but replace it with an LED on the dash that lights up so that I know when I'm in reverse. 

The problem is that everything on the cart is 36volts. I removed the buzzer, and put a volt meter on just to check, and when I put the cart in reverse, sure enough it is 36volts to the buzzer. Is there any realistic/cheap way to bleed that voltage away to a level that the LED can handle? Would I be better off using the 36 volts from the reverse switch to trigger a relay and then run the LED off of a lower level power source... perhaps even a few double AA batters or something since that would probably last about 2 years with the rarity of use. Any oppinions are appreciated.


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## Ken_McE

How about a 24 volt neon indicator light and a resistor?


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## bshanahan14rulz

that's just an excuse to use a bunch of white LEDs to help drop all that voltage. I'm thinking bad-*** reverse lights (plus one remote LED in the dash to tell you when your reverse lights are on) 
:welcome:


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## alpg88

resistor will drop v.


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## bshanahan14rulz

yeah, but that's gonna be a beefy resistor. 

Lets say you're driving LED at 1A and need to drop 32V (assuming Vf ridiculously high 4v)

V=IR
32V=1A(R)
R=32Ohms

well, gee, that's not so bad....

P=IV
P=(1A)(32V)
P=32W

hmm, a 32Ohm resistor rated at 32W.... 
most resistors are rated at 1/4W to 1/2W.


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## Coolricks

why use a high current LED?


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## bshanahan14rulz

True enough. 

single white 5mm
Vf is 3.2V
current we want to run through it: 20mA, or 0.02A.

So, our source voltage is 36V. Our LED drops 3.2V from that, leaving us with 32.8V left to drop.

Lets use a resistor:
Ohm's law: V=I*R (V=voltage dropped by device, I = current through device, R = resistance of device in Ohms)
32.8V=(0.02A)R
R=32.8/0.02
R=1640 Ohms, a 1.6k or 1.7k resistor.

Power used by this resistor:
P=I*V (P = Power this device dissipates, I = current through the device, V = voltage dropped by this device)
P=.02*32.8
P=0.66W. You will need a 1.6kOhm or 1.7kOhm resistor with a power rating of at least .66W (probably go with a 1W power resistor)


Basically, find the LED you want to use, and do what I did for it and you will figure out what resistor you need. The resistor will look like, ehh, almost like a lego brick (without the indents and bumps) and may get hot during use.


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## Illum

bshanahan14rulz said:


> The resistor will look like, ehh, almost like a lego brick (without the indents and bumps) and may get hot during use.



this is certainly true for power resistors...

while ordinary resistors would look something like this




Don't be surprised if what comes out of the package ends up looking like this




so...plan for space limitations before you start shopping for good deals:laughing:
Joking aside 30W power resistors will resemble like lego blocks, not anything as monstrous as the image depicts, but mount it to a heatsink of some sorts if your golf cart frame in plastic as to not melt a hole in the chassis


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## alpg88

why not just get incd. bulb, there are planty of places you can buy one, cheap (for a lot less than power resistor alone). many already come as an assembly with lens,reflector, plug, all in one. it dosn't have to be bright, as long as you see it is on.


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## bhvm

Why not make an array of 5mm Super White LEDs?

Just put 12 of them in series, and they take about 3 Volts each.

No resistors! No wastage, no Fuss!

You can wire up One of those LEDs on the Dash, Keep rest for reverse light!


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## Illum

I'd throw a resistor in for that array bhvm, while the voltage may be compensated using the array, there's nothing to limit current. As the LEDs heat up, the forward voltage requirement decreases for each LED, and increasing the current draw which increases heat dissipation even more...eventually those "20ma" LEDs will eat more than 200ma and eventually burns out.

A 1Ω 1/8W resistor is all you need after 10 LEDs in that 36V circuit
Also note that "36V" is a nominal voltage rating. A "12V battery is about 10.8V dead 14.3V full," so prepare for a overvoltage/surge when you use it [or about 42V].


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## bhvm

Illum said:


> I'd throw a resistor in for that array bhvm, while the voltage may be compensated using the array, there's nothing to limit current. As the LEDs heat up, the forward voltage requirement decreases for each LED, and increasing the current draw which increases heat dissipation even more...eventually those "20ma" LEDs will eat more than 200ma and eventually burns out.
> 
> A 1Ω 1/8W resistor is all you need after 10 LEDs in that 36V circuit
> Also note that "36V" is a nominal voltage rating. A "12V battery is about 10.8V dead 14.3V full," so prepare for a overvoltage/surge when you use it [or about 42V].



A resistor is always welcome.

As you're pulling only 3v (12 LEDs X3 =36v), you'd severely underdrive the 3.3v White ones. Hence the Chances of Thermal Runway is nullified.

I am using practically such under-driven arrays and they work like a charm.



By the way, Updates and Pics Please!!


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## etard

WOW, talk about a PITA!! Why not just use a DC to DC convertor to drop the voltage down to 12v and then you have your outlet for charging phones, flashlights, musica, whatever. All this series lights, resistors is nonsense. I'm really not a nerd, I just play one online. Actually I'm straight baller son!


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## bhvm

etard said:


> WOW, talk about a PITA!! Why not just use a DC to DC convertor to drop the voltage down to 12v and then you have your outlet for charging phones, flashlights, musica, whatever. All this series lights, resistors is nonsense. I'm really not a nerd, I just play one online. Actually I'm straight baller son!



Good idea, but it depends upon whether you'd need all that.

Plus, you will incurr losses on Down-conversion.
And yeah, you STILL need a resistor or a constant current source.


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## Illum

There's nothing inherently wrong with a 36V power supply, just the solutions may not fit your budget

EDIT: try this






A couple flavors, all 9-56VDC in, 2-52Vout, MeanWell LED CC Drivers
350ma, $5.95
500ma, $5.95
700ma, $5.95
1000ma, $11.52


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## lydon71

Perhaps I don't understand the question fully, but...

Why not just use a 36V LED bulb?

http://www.bulbtown.com/T5_5_36V_130V_WHITE_L_E_D_MINIATURE_BULB_E12_BASE_p/l8034.htm


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## recycledelectrons

Edit: My suggestion would work with powerful (several-watt) LEDs. For an indicator light, a resistor is probably best.

DX has 3 different step-down converters. They take up to 40V in. You set the voltage output that you want (maybe by turning a potentiometer) and it gives a constant voltage out. They are all rated at 3.0 Amps max, which I think means 3.0 Amps out. I would not run them at their max.

$3.50 http://dx.com/p/dc-4-40v-to-dc-1-5-35v-voltage-step-down-transformer-126108
$4.20 http://dx.com/p/mini-dc-4-40v-to-dc-1-5-35v-voltage-step-down-transformer-module-142488
$3.20 http://dx.com/p/dc-3-40v-to-dc-1-5-35v-voltage-step-down-transformer-module-154907

There are other voltage converters that work up to a max of 36V, but it's good to have a little leeway before the magic smoke comes out.

Now you just need to spec out the correct LEDs, and figure out how to mount them.


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## Johnny2Bad

lydon71 said:


> Perhaps I don't understand the question fully, but...
> 
> Why not just use a 36V LED bulb?
> 
> http://www.bulbtown.com/T5_5_36V_130V_WHITE_L_E_D_MINIATURE_BULB_E12_BASE_p/l8034.htm




A bulb like that simply incorporates it's own internal resistor array. It will work fine, but it's unnecessarily expensive in my opinion.

1W and 5W resistors are standard values, and cheap. You can also always increase a resistor power dissipation factor by heatsinking or forced air cooling; the rating is based on use without either. In this case, however, I would expect it's unnecessary.


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## ianfield

bshanahan14rulz said:


> yeah, but that's gonna be a beefy resistor.
> 
> Lets say you're driving LED at 1A and need to drop 32V (assuming Vf ridiculously high 4v)
> 
> V=IR
> 32V=1A(R)
> R=32Ohms
> 
> well, gee, that's not so bad....
> 
> P=IV
> P=(1A)(32V)
> P=32W
> 
> hmm, a 32Ohm resistor rated at 32W....
> most resistors are rated at 1/4W to 1/2W.




Typical LED draws about 0.02A so the calculation could be revised a bit - an array of multiple white LEDs to act as an actual reversing light can be stacked to take up a fair bit of the voltage at roughly 3.4V per LED.

A resistor is still reqired in the series circuit to limit the current within LED max limit at full battery voltage, but if its only dropping - say 10V at 0.02A it doesen't have to be 'weapons grade'.


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## krazy

Try Googling 'LED calculator'. This is a common problem/task.

To buy resistors or other electronic parts, check out Radio Shack (usually online) or Mouser or DigiKey.


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## Txluke

therealciviczc said:


> I'm trying to get an LED powered on my 36 volt golf cart. I want to remove the reverse buzzer, but replace it with an LED on the dash that lights up so that I know when I'm in reverse.
> 
> The problem is that everything on the cart is 36volts. I removed the buzzer, and put a volt meter on just to check, and when I put the cart in reverse, sure enough it is 36volts to the buzzer. Is there any realistic/cheap way to bleed that voltage away to a level that the LED can handle? Would I be better off using the 36 volts from the reverse switch to trigger a relay and then run the LED off of a lower level power source... perhaps even a few double AA batters or something since that would probably last about 2 years with the rarity of use. Any oppinions are appreciated.


Run the power from 2 of the batteries. I assume you have 6 x 6v batteries. Go to the last one in line and use negative off that one then positive off the next one in line. You now have 12v.


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## WeLight

Why not just get a 36V led
http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Arrays-NonDirectional/XLamp-CXA1304
small, bright. I would suggest a varistor or similar to clamp large surges common in auto systems


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## Illum

WeLight said:


> Why not just get a 36V led
> http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Arrays-NonDirectional/XLamp-CXA1304
> small, bright. I would suggest a varistor or similar to clamp large surges common in auto systems



I don't think the thread op needs 1000 lumens in his face coming from the steering wheel to tell him he is in reverse


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## Steve K

does the OP realize that the buzzer that he wants to replace isn't for his benefit? It's so people behind him are alerted to the fact that he's about to run them over. 

Of course, the OP hasn't checked back in since he started the thread, so we'll probably never know what happened.

btw, what's the record for threads like this coming back to life?


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