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Classification Lights constant supply

I am thinking of building a little Constant voltage supply to install on my O Scale engines to power the classification lights through micro LEDs. Typically, these LEDs require between

1.8 and 2.3 volts, depending on their color (red, green, white).

I was wondering if anybody has ever built such constant voltage supply with one of the micro voltage regulators offered by National Semiconductor LP295X series or other vendors. Apparently, these regulators are capable of delivering a constant 2.5 or 3.3 Volts under a low current to one LED or two LEDs linked in series. Micro light bulbs could be used as well.... This way, as soon as you apply some voltage to the rails, and even before the motor can start humming, the lights will be on.

One drawback though, is that all lights will be off when reversing the engine.....or you would have to install a dual system.

Any thoughs, ideas, schematics people may want to share?

Yves

Reply to
Yves
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The September 1991 Railroad Model Craftsman had an article on pp 62-3 about using the LM317T adjustable voltage regulator for constant lighting. There was also a correction to 3 of the circuit diagrams on pg 95 of the January

1992 RMC. If the LM317T is used without a resistor voltage divider, it produces a 1.25 volt output. Adding a pair of appropriately sized resistors allows setting any higher voltage desired. The LM317T is (or at least was) available from Radio Shack as 276-1778, and in bulk from the usual sources. The RMC article included diagrams for use for one headlight, for a pair of directional headlights, and for bi-directional car, marker, or number board lighting.

The LM317T comes in a TO-220 case, the kind with a metal tab for mounting in a heat sink. A heat sink is needed for larger loads (the chip will handle up to 1.5 amps - I used one in a simple throttle I use with one of those sealed auto starting portable batteries when I take trains outdoor displays or to schools where 120VAC may not be close by), but would not be needed for powering LEDs.

I've used the LM317T's in several HO, O and G locos with good results. I have provided scans of the articles to other readers of this group and can probably find the old files if I dig through my catch-all "MISC" directory. Would you like them? Geezer

Reply to
Geezer

You want to put a full wave bridge rectifier ahead of the voltage regulator, so the regulator always sees the proper polarity. Otherwise, reversing track polarity will bias the regulator backwards and likely destroy it. The voltage regulators are good for incandescent lamps, but won't work with LED's. LED's want a constant current of 10-20 mA. When connected to a voltage source LED's switch on and draw unlimited amounts of current until they destroy themselves. That's what the current limiting resistors are for, to limit the current. Some cruising of the semiconductor manufacturer's websites (Analog Devices, National Semi or Texas Instruments) should find a current limiting device intended for LED drive.

David Starr

Reply to
David Starr

Yves skriver:

Yes, fro LED's you need constant curren source instead of constant voltage. It can be achieved by a constant voltage supply and a resistor in series with the LED ot a constant current supply direct to the LED.

On my personal homepage there are som examples og constant current supplys

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unfortunnaly the page is only in danish, but the calcultaions for the LM317 should be "universal language".

Klaus

Reply to
Klaus D. Mikkelsen

There is a circuit where you use standard 7805 voltage regulators soldered with the ground lead to a negative voltage of the other one and visa versa - or so i remember but for the life of me i can't remember where i put that schematic. Would anyone of you'ns know it? It gives constant, directional voltage!

Reply to
Big Rich Soprano

While that might be true, I know that a LM317 regulator plus one resistor can be wired to be a constant current supply. I've used it for that purpose in the past. The circuit is shown in the data sheets for that IC (which I don't have near me at this point).

SO, I did the next best thing - Googled for "lm317 constant current source"

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Here are some results (all you have to figure out is the resistor value for your application):
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(look for Constant Current Battery Charger)

Peteski

Reply to
Peter W.

Big Rich Soprano wrote in news: snipped-for-privacy@4ax.com:

Try looking at mrollins.com (Sorry, I don't have my web browser open to give you a more specific link.)

Puckdropper

Reply to
Puckdropper

Thank you - on the way...

Reply to
Big Rich Soprano

Good sight but it didn't have the circuit i'm thinking of. Unfortunately the person who gave it to me has passed on. I'll have to ask his son...

Reply to
Big Rich Soprano

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