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Author Topic: Exactly 12 volts  (Read 1542 times)
jerryl

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« on: May 27, 2009, 03:22:19 PM »

  A friend needs to have a power supply with exactly 12 volts DC to control flashing crossbucks.   Any suggestions without getting an expensive regulated power supply?   I suggested wiring 8 alkaline D cells together. Any other suggestions?  Thanks
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pdlethbridge
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« Reply #1 on: May 27, 2009, 03:27:19 PM »

there are several here including one down the page for $5.50

http://www.allelectronics.com/make-a-store/category/480/Power-Supplies/1.html
« Last Edit: May 27, 2009, 03:29:33 PM by pdlethbridge » Logged
rustyrails
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« Reply #2 on: May 27, 2009, 03:40:16 PM »

Electornics stores sell wall wart adjustable AC and DC power supplies for audio devices.  Move the little switch to 12 V and off you go.  A more expensive solution would be to buy the most inexpensive power pack you can find, hook it up with a volt meter in the circuit, crank up the rheostat until the volt meter reads 12 volts, and let 'er rip.  Batteries will give you 12 volts only briefly.  Most circuits that say they require a certain voltage really mean a certain voltage plus or minus some percentage so precision with consumer stuff is seldom required.

Rusty




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Jim Banner

Enjoying electric model railroading since 1950.


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« Reply #3 on: May 27, 2009, 05:10:10 PM »

jerryl, how "exactly 12 volts" does the power supply have to be?  A "wall wart" transformer type supply may be 15 or more volts if lightly loaded.  A computer power supply is usually between 11.75 and 12.25 volts, which is better but still not exact.  Electronic wall warts (the ones that are so light in weight that you think they are empty) are about the same as a computer power supply.  Occasionally you will run across electronic wall warts that are internally adjustable.  These can be set as close to 12 volts as your meter will allow.  Even with a low cost meter you should be able to set it between 11.98 and 12.02 volts.  This is quite good, but still not exact.  With a high cost, high precision lab supply calibrated against a traceable voltage standard, you might be able to set the voltage between 11.9999 and 12.0001 volts, but even that would not be exact.  "Exact" is a hard term to deal with.  If the manufacturer says something like "12 volts +/-  .25 volts" then we know how exact it has to be.

I would suspect that an electronic wall wart would be exact enough for all model railroad electronics.  Just make sure the current rating is high enough.  Regular transformer type wall warts will withstand momentary overloads, such as light bulbs and charging capacitors, but the electronic ones will not.  Keep this in mind when buying one.

Jim 
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jerryl

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« Reply #4 on: May 27, 2009, 06:30:19 PM »

  I really don't know how accurate the voltage has to be, but a wall mounted adapter should do for him. The crossbucks will operate on 14 volts one time & overheat & shut down.  I emailed him the replys.  THANKS ,  Jerry
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RAM

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« Reply #5 on: May 27, 2009, 09:17:31 PM »

Why wouldn't a cheap train set power pack work.  Just adjust it to 12volts.
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Jim Banner

Enjoying electric model railroading since 1950.


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« Reply #6 on: May 27, 2009, 11:41:42 PM »

Unfortunately, a cheap power pack usually uses a rheostat as a speed control.  It works by putting more or less resistance in series with the output.  This means that as the current to the load changes, so does the output voltage.  So if the cross bucks draw more current when they are flashing than they do when they are off, then the voltage will depend on what they are doing at any particular point in time.

RAM's idea is good, but not with a cheap power pack.  A more expensive electronic power pack would likely work as long as it does not have pulsed output.

Jim
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grumpy

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« Reply #7 on: May 28, 2009, 01:08:25 AM »

I use the trackside of a Life Like controller and set the voltage as required.
Don
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Yampa Bob

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« Reply #8 on: May 28, 2009, 03:24:43 AM »

Over the years I've collected a drawer full of 6 and 12 volt wall warts left over from various electronics,  ranging from 75ma to 600ma. I'm glad now I saved them.
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