12.5volt Power Supply ... ?

[CSX_road_slug]

I just bought a few Tomar B&O color-position-light signals.   I'm really pleased with their appearance, but since I'm not an electrical engineer, I can't figure out where to get the various resistors I'll need.  Tomar sells some of their own, but those require me to use a 12.5-volt power supply.  Most of my power pack accessory circuits put out 16 to 20 vDC.  My question is: Who manufactures that kind of [12.5v] power supply, and where can I get one?

T-I-A

 

[retsignalmtr]

i'm using atlas signals on my n scale layout that use led's. they require 5 volts dc to light them but each color uses a different resistor so that they all light with the same brilliance. i went online and found all electronics www.allelectronics.com had the resistors that i need plus a 5 volt 3 1/2 amp dc wall wart power supply. i still have to get a 12 volt dc supply that i need for my signal relays when i send in an order next month. check them out.

[djb39]

I am using an All Electronics PS-1236 12VDC 300 MA power supply for my Tortise switch machines, works great, only $5.05 
http://www.allelectronics.com/make-a-store/item/PS-1236/12VDC-300MA-SWITCHING-POWER-SUPPLY/-/1.html  Many other powersupplies available.

All Electronics has good selection of LEDs, resistors, etc.
http://www.allelectronics.com/

 

[Seamonster]

You don't need to be an electrical engineer to figure that out.  You just need to know one formula, called Ohm's Law and be able to do a little basic arithmetic.  Ohm's Law says that if you know two of these values, you can calculate the third one.  The values are voltage, resistance and current.  To calculate voltage, multiply resistance and current.  To calculate current, divide resistance into voltage.  To calculate resistance, divide current into voltage.  Each has to be in their basic units, that is volts (not millivolts), ohms (not kilohms or megohms) and amperes (not milliamps).  To convert milli-anything to the base value, just divide by 1,000.  Thus 100 milliamps (mA) for instance becomes 0.1 amps.

To apply this to your specific case, you will need to know how many milliamps your signal lights draw.  Divide this by 1,000 to get amps.  Now find out the difference in voltage between the voltage your supply puts out and the voltage you need.  That would be 16 volts minus 12.5 volts equals 3.5 volts.  Divide 3.5 by the amps and that is the value of the resistor you will need.  It likely won't come out to any standard resistor value, so use the next highest stndard resistor value.

To plug some numbers into that, let's suppose each of your lamps draws 50 mA of current.  That will be 0.05 amps.  You need to drop 3.5 volts in the resistor.  3.5 divided by 0.05 equals 70 ohms.  The next largest standard resistor value is 82 ohms.  For a lamp drawing 50 mA, a 1/4 watt resistor will be okay.  If the lamp draws more than 50 mA use a 1/2 watt resistor.

You might be able to find suitable resistors at Radio Shack.  But there are many electronic distributors who sell on-line, and All Electronics, which someone already mentioned, is one of them.

I hope this has helped you.

 

[CSX_road_slug]

Thank you all for your replies, gents!   Especially that link to the 12v PS from AllElectronics - looks like exactly what I need.

Bob, I think I erred in using the phrase "electrical engineer" - I think contractor would have been a more applicable word.  They are the ones who know where to get the specialty electrical parts. The math isn't the problem, it's the Radio Shack stores in my area that have a crappy selection of component parts for us do-it-yerselfers.  And when I ask their salespeople exactly where I can find the type of resistors I need, they usually say "I dunno". But now that I have the AllElectronics link, I'll just order from them online. 

Thanx again