Question about voltage regulator sizing with LED's

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Question about voltage regulator sizing with LED's

Post by Merlin83 » Thu Feb 23, 2012 5:27 pm


I need help with a couple questions regarding LED's and sizing a voltage regulator. Here's what I have in mind, I want to add a PCB board with LED lighting for my G scale trains (these are very large) - this would be inside the passenger trains. I have used Express PCB to do the schematic and then PCB design.I made an FTDI USB to RS-485 adapter - the purpose of that project was to use the USB to connect to Modbus devices. It worked all right, I used all surface mount devices, I had to use a couple (many actually) of mod wires because I made some mistakes with the traces. Anyway, this project is far simpler.

The train track voltage can vary from 5 volts to 24 volts (DC), and depending on the train direction the polarity of the tracks will change. So, using as an example the available LED's available on the Spark-fun website - can I or should I use a voltage regulator along with a small capacitor (for smoothing ) with the LED's ? If so how do I size my voltage regulator and capacitor ? And should I use a full wave bridge rectifier to allow the LED's to work regardless of track polarity ?

I tried to include a image of my PCB from Express PCB (it was very simple) but was unable to do so. Initially I plan to do this with simple thru-hole design, and then I would like to try using all surface mount components and add a small micro controller so I can control additional features (color, brightest, party mode (strobe of all colors) into the interior lighting. Also, are there bi-color LED's I can use that will simply cycle between there two respective colors without any need for a micro controller ?

Thank You

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Post by bmarvo » Fri Feb 24, 2012 1:52 pm

You have the right idea with the full wave rectifier. You just need to keep in mind the ratings for max forward and reverse voltage as well as the forward voltage drop Vf of the diodes. If the Vf of each diode is .6 then when your track is at 5 volts, you'll actually only be working with 3.8 volts after the rectifier.

5 volt regulators are out of the question then. The best option would be to go with an adjustable one like the LM317 Since the drop out voltage for that device is around 1.5 Volts and you need it to work down to 3.8 volts, the maximum voltage you can get out of it is 2.2 volts. Assuming you're just using standard 5mm LEDs and not anything special, this is fine.

The big issue here is power dissipation. If your track is at the max voltage of 24 volts, and you're pulling 500 mA with your LEDs then your vreg is dissipating (22.8-2.2)*.5 = 10.3 Watts. The total thermal resistance for this device is about 50 deg. C per Watt. So I don't need to multiply that out for you to see how ridiculous it is.

Heatsink time! This one: has a thermal resistance of 2.7 deg. C per Watt. So the new heat dissipation becomes 2.7*10.3 = 27.81 degrees C. Can't forget to add ambient temperature: 27.81 + about 22 = 49.81 or about 50 degrees Celsius which is within the operating specifications for the LM317.
Also dont forget to use this stuff:

and as for the change in input voltage effecting the output voltage. Short answer: yes it will but not by much so don't worry about it and just use the cap values suggested in the datasheet (vets correct me if im wrong).

Of course some of that can change if you're drawing more than 500 mA but you should be able to figure it out if thats the case. If you need more thermal relief, try using a heatsink with a fan.

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