Proto 2000 and White LED Headlights

I'd like to put a White LED headlight in my FA1 Proto2000 locomotive. The circuit board provides 1.5 volts for a headlight, but the White LEDs I have
want 3 volts.
How can I go about doing this? Are there reasonable sources for 1.5V White LEDs, or does someone make an inexpensive kit to support this?
Puckdropper
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Puckdropper skriver:

You can not get 1,5volt white LED's.
How is the 1,5 volts created ? I guess that it is diodes in series with the motor, adding more diodes will cause a higher voltage drop and higher voltage for your diodes.
Du you run digital or analog ?
Klaus
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Are you saying the white LED you have will not conduct or the light is not bright enough for you. I have white LEDs that will light with 1-1/2 volts.
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The LED will not visibly light up. I know it works, as I've tested it in a different circuit.
Puckdropper
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Well as someone suggested you can add some more diodes or go to DCC <GRIN>!
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Puckdropper spake thus:

Yes, there is another, better way to go: use a current regulator.
This is actually a superior way to drive LEDs, as it limits the current through the device. The current reg is a cheap, small-package semiconductor. You also need a small resistor. I can't recall offhand what type it is; I have a bunch here, but don't want to get up to check just now. I think others here can supply that info if I don't get a round tuit soon.
I intalled one in my Bachmann Spectrum Mountain, and it works fine. Easy to put in.
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David Nebenzahl skriver:

LM317
Klaus
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Klaus D. Mikkelsen spake thus:

No, that's not it: the 317 is a voltage regulator. I'm talking about a *current* regulator (constant current source): different critter.
It's the LM334Z: http://www.national.com/pf/LM/LM334.html
They come in a TO-92 package (little bitty transistor size), so fit nicely in any loco.
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David Nebenzahl skriver:

Belive Me, the LM317 is both voltage and current regualtor, for using it at current regulator it only requires one resistor, just like here:
http://home6.inet.tele.dk/moppe/Konstant/lm317.gif

Just like the LM317
Klaus
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Klaus D. Mikkelsen spake thus:

Nope, the 317 is in the larger TO-220 package (unless you get the surface-mount varieties). That's the one with the heat-sink tab.
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David Nebenzahl skriver:

Very odd, what is the the LM317 in to92 housing I have in My garage ?
Try to google LM317 to92
Klaus
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Klaus D. Mikkelsen spake thus:

Odd indeed: National Semicircular* doesn't list that package in their availability chart: http://www.national.com/pf/LM/LM317.html
I'll take your word that you have one in your garage, though.
* Someone I knew who used to work @ National called it that, long time ago.
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David Nebenzahl skriver:

Perhaps they have removed it from production. At least Texas makes it as TL317 http://focus.ti.com/lit/ds/symlink/tl317.pdf
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Whether currently produced or not, both Fairchild and National currently show LM317 in a TO-92 package in their data sheets.
Here's the URL for the National datasheet:
www.national.com/ds/LM/LM317L.pdf
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In article
snipped-for-privacy@given.com (Joe) wrote:

time ago.

A correction:
LM317L is the low power version of the LM317.
LM317L "guaranteed" outputs 100mA which makes it ideal for LED current regulation. I have used it to plug various types of LEDs (red, white, yellow, green...) into a breadboard with the LM317L wired for 15mA output current, without having to change components to adjust for the various forward voltages of the LEDs.
Another nice feature of using the LM317L in constant-current mode is that LEDs can be wired in series, as long as the input voltage is high enough.
--- Joe
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Joe spake thus:

So would you recommend using the LM317L (as a constant current source instead of as a voltage regulator) over the LM334?
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Can't say that I would. I haven't used an LM334. Jon Miller apparently has, and quite successfully. The fact that a reverse connection to the input of the LM334 is well tolerated may actually convince me that the LM334 is a better choice.
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I should have replied this way first as I use this circuit all the time. Go to; http://www.pollensoftware.com/railroad/index.html
I use a SMR between the two legs and it makes for a small package.
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That's nice and small, looks like it'd be a good fit. Thanks.
Puckdropper
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Puckdropper wrote:

You should not think of LED's as having a voltage,like incandescent lamps do. Think of them as semiconductor diodes that just happen to emit light when they conduct. Once a diode is forward biased enough to conduct at all, it switches ON and looks like a closed switch. A closed switch connected from plus to minus of a voltage source is a short circuit. It conducts and conducts until something lets go, a fuse, the diode itself, an over current protection circuit in your power pak. You have to have a separate resistor in series with a LED to limit the current to 20 milliamps (mA) or less. Most ordinary LED's will glow brightly with 20 mA and glow brightly enough for model railroading purposes with as little as 10 mA. Plain old silicon diodes need 0.7 VDC of forward bias to switch them on. LED's need a bit more, anything from 1.2 to 3.something, depending upon the individual diode. But once ON, the LED MUST have a resistor in series with it to avoid destruction of the LED. For your Proto FA-1's (nice locomotive, I have an ABBA set of FA2's) you want a 560 ohm resistor in series with each LED. That will give full brightness at full speed (12 volts) and dimmer as you reduce voltage. That's the bare minimum to make the LED's work at all. To compute the LED series resistor, you can ignore the LED forward bias voltage. Just divide your source voltage by 0.02 amps to get the needed resistance. This will give a resistor value close enough for model railroading work. Then you have to get the LED polarity right, so the LED is forward biased when the locomotive is moving forward, otherwise the LED will be reverse biased (dark) when going ahead and forward biased (ON) when going in reverse. LED's don't like reverse bias. Many of them will only withstand 6 volts reverse bias, which means you need to protect the LED with an ordinary diode in series to take the reverse bias. Ordinary diodes will take 50 to 500 volts reverse, much more than they will ever see on a model railroad track. For extra credit, you can go for a constant brightness circuit that gives a nice bright headlamp for all track voltages high enough to move the locomotive. This needs a full wave bridge rectifier, and a current limiter circuit. The current limiter can be as simple as a single transistor, or some IC regulator chip. I've done tings like this, using parts from my electronic scrapbox, each circuit is a little bit different depending upon what parts I happen to have at the moment.
I used to have a wonderful book by Peter Thorne with all the model railroading circuits you could ask for, but it seems to be missing in action right now. There are some railroading web sites with circuits as well.
Final thought. I prefer incandescent lamps for headlights because the color is realistic. I have yet to see a LED headlamp that wasn't terribly blue.
David Starr
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