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Old 11-07-2010, 09:30 PM   #1 (permalink)
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Anyone here good with LED's ?

Need some expert LED advice. I am getting some yellow red and white LED's to replace all my navigation lights on my metro with LED's

Found a great deal on some nice 1watt LED's

anyway I have to wire them. I need 3.5 v for the white ones and 2 to 2.5 volts for the yellow and red led's

anyway I found ledcalculator.net

the result I got was this

* You will need 1 x 30 ohm 5 watt resistor.
* The 30 ohm resistor is color coded: Orange, Black, Black, Gold.
* Each 30 ohm resistor consumes 4800 milliwatt.
* Total power consumed by the resistors is 4800 milliwatt.
* Total power consumed by the LEDs is 800 milliwatt.
* Total power consumed by the circuit is 5600 milliwatt.
* Total current drawn by the circuit is 400 milliampere.
* The resistor values are calculated based on the common ±5% tolerance resistors.
* Make sure to wire the LEDs in the correct direction as shown below.
* Always leave some space for the resistors to breathe. They might get hot.

am I reading this right? I am going to be dumping 5 watts into a resistor? if that is true there is no real point to going led since with 5 watts going into the resistor and 1 watt going into the led thats not much lower than just using the incan.

am I reading that wrong? if not how can I get the correct voltage and current without dumping so much power into the control circuitry?

I found some 2v and 3.3v voltage zener diodes. are they what I think they are? wire one into 13.8v car power and it will change it to 2.0 and 3.3 v ? do they consume a lot of power? DO they work like I think they do?

any help would be greatly appreciated!

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Old 11-07-2010, 09:35 PM   #2 (permalink)
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Do the LEDs all use the same amount of current? If the amp rating is the same for each, you could put them in parallel and then sum the voltages and provide that voltage with a voltage regulator.

Look up LM317, it is a voltage regulator.

Also, see this:

Low cost LED lights - EcoRenovator
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Old 11-07-2010, 09:42 PM   #3 (permalink)
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I would prefer to wire each separate so one failing won't effect the others.

I will go over that thread you linked too. Still not sure what the zener diode does.
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Old 11-08-2010, 02:07 AM   #4 (permalink)
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Hi Nerys,
You are correct in your assumptions, you will be dumping wattage across your resistor if you choose to do it that way.
There are ways to get around the loss in the resistor which are
- as suggested already, linking several LEDs together so that the voltage drop of all of them amounts up to say 10 V then the resistor drops a comparatively small amount. Note: changes in your battery voltage (when charging while running the car) will dramatically change the current if you do it this way, however the voltage regulator or a current regulator will fix this issue.
- You can also do zeners but they are just as good as resistors, they will still consume the same amount of power
- The ideal way is to use a LED driver which pulses the power into the LED (sorry im dumbing it down a bit, i can explain in much more detail if anyone wants). These systems can be purchased usually from electronic shops however they are pricey well will likely cost more than the LED however they may be around 80 or 90% efficient which means if your LED uses 1 Watt these systems may take 1.11 Watt from your battery. You can also build these yourself however you will need to make a circuit board and get components so i wouldn't recommend this method for anyone that isn't an electrical engineer or electronics hobbyist.

hope that gives you a bit of a guide,
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Old 11-08-2010, 09:59 AM   #5 (permalink)
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what about voltage regulators? are they more efficient?

if I drop the voltage first to 2.0 and 3.3v then I only need a 1 ohm 1/4 watt resistor and it only consumes a few milliwatts in stead of thousands of milliwatts since it only has to control current now instead of current and voltage.

how much power typically do voltage regs consume? IE this one is for 2 volts.

300361405417 (ebay) would that be a lot more efficient than a resistor to drop the voltage? (though its rated for 2 amps I would only be pushing maybe 300-400 milliamps through it) and would use my led heat sink as its heat sink as well.

I would design the circuit for 13.8v so when its at 12.6-12.9 volts (engine off) its safer for the LED's not worse.

I might reduce the current even more to make the LED's dimmer and just use 2 instead of 1 so I have less heat dissipation issues.

I plan to put a "plate" of copper behind the entire light fixture and solder fins to it so that it acts as one giant "heat sink" for all the LED's

I want to have an individual resistor and regulator for each emitter so that if one fails for any reason it does not take out the others and this would make it modular IE easy replacement. (well easier)

Plus I won't be using ENOUGH led's to gang them up. it would take 7 of the yellow LED's to get to 14v and I only plan to use ONE. Two at most. (they put out 50lumens yellow 60 lumens red and 80 lumens white so more than enough without ganging them up)

I wanted to use drivers since they are far more efficient but I have yet to find any drivers that go from 13.8 volts to 2v and 3.3v

I found some 110v drivers but then I would have to run an inverter and that can't be too efficient or reliable.
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Old 11-08-2010, 04:26 PM   #6 (permalink)
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The voltage regulators you are looking at are referred to as linear regulators and they consume the same wattage as resistors they just change their resistance to ensure that the voltage output is steady. You can basically think of them as a changing resistor.

There are voltage regulators which are referred to as switching regulators, they work using the same principle as LED drivers so they are up to 90% efficient at reducing the voltage from your input voltage of 13.8v down to a voltage of say 5v then you would use a resistor from 5v to give your desired current on your LED.
this link might give you a bit of background into switching regulators
A beginner's guide to switching regulators

you mention you haven't found any drivers that go from 13.8 to 2v and 3v3, well here is the regulator for you,
Digi-Key - 102-1898-ND (Manufacturer - VLD24-300)
have a look at the datasheet there is a bit of info about the circuit / how to hook it up

if you want to search around more digikey has a lot of different regulators just search under "led driver"
Note digikey also have many switching regulators which will do your high efficiency voltage conversion if you want to use that method.
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Old 11-08-2010, 04:54 PM   #7 (permalink)
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hmm says output is 2 to 32 v how would you SET IT to 2v ? They appear to have other output current models so they have what I need to meet my needs and the cost is not bad at $10 a pop. not cheap but doable.

I do not know how to read schema but can gleam a little info but I fail to see HOW you set the output voltage. ie how do you set it to 2 volt 3.3 volt or 32 volt???

ahh that link with the guide has some VERY NICE and EASY to use regulators. Vin Grnd V out then I can just use resistors to control the current since the voltage is correct almost no power is lost in the resistor to control the current.

They are more money but super easy :-) only $5 more though.

question. These things are powerful enough to run ALL the lights in the car at the same time. I could just use TWO one for the red and yellow and one for the white led's

is there a SINGLE input power feed to these lights? IE instead of putting a v regulator on each bulb I could just make the WHOLE TRUNK 2v to run the lights (or would there be too much loss in that much wire at 2v ??)
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Old 11-09-2010, 03:11 AM   #8 (permalink)
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regarding your question about how to set the voltage, you do not set the voltage you set the current then the driver will generate whatever voltage is required to give your commanded current. This is probably what you want because LED's should be driven with a consistent current not a consistent voltage because their voltage drop will change over time.

You can work out if they are powerful enough by looking at their current rating and then look at how many LED's you are going to drive off one and how much current each LED will take.
You can run all of the LED's off the same output however they will not light evenly if you use the LED driver i have suggested unless the LEDs are in series. So if you want to drive them all at 2v in parallel then you will get uneven brightness because you can not set the current in each individual one. If you however decide to use a voltage regulator that goes down to say 3v then you use resistors on each individual LED to command the desired current off the 3v rail then that will work.
To work out if the current is too much for your wires you can do a voltage drop calculation, work out how long the wires are likely to be, work out how much current will go through them then you can find an online calculator or you can just look up the resistance per meter of the wire you are using and work it out that way.

hope that isn't too confusing.
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Old 11-24-2010, 11:19 AM   #9 (permalink)
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I think the easiest way would be to use resistors. Figure that the resistor is going to drop about 10 volts, the forward voltage of the LED is about 3.5 volts. I wouldn't use such a powerful LED either. Get some ones that use about 10-30ma.
Something like these: 5mm LED RL5-W8045 Specifications

Figuring 15ma and 10 volts you might choose a resistor around 600 ohms with a wattage of 1/4 watt or larger. I like using one resistor per LED.
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Last edited by Varn; 11-24-2010 at 11:28 AM..
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Old 11-24-2010, 11:25 AM   #10 (permalink)
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the problem is by the time you have enough LED's to be bright enough (around 1 watt) the resistor is not sucking down 5 watts IE your taking 6 watts

the incandescent takes 5-8 watts. IE you defeat one of the major reasons to use LED's

I have discovered some stepping regulators (90%+ efficient) that will do what I need but man they are expensive. $15 a pop and I need 1 for EACH LED.

I could gang up LED's till I am near 14v and then use resistors (the voltage drop is what sucks the amps) but now I am consuming FAR more watts than I need to I would need 7 yellow or 7 red led's for each one but now I am using 7 watts. too bright for the need as well and again back into the wattage range of the Incans. the point is to reduce power to reduce alternator load to increase fuel economy :-)

problem is I need 12 bulbs. so with single 1 watt LED's thats 12 watts or just slightly more power than a single nav light and almost HALF the power of a single brake or turn signal bulb. IE huge power reduction.

but they take 2v each :-)

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