Over voltages from wall wart power supplies a problem?

[CharlieBedard]

Folks,

 I have a box of old "wall warts" that I was planning on using for powering Tortoises and control panel LEDs. Good use of old phone or PC power supplies, right?

Well, I have 3 or 4 12 VDC supplies that I was planning on using for Tortoises. As a quick check, I tested the output voltage with my VOM and was surprised to see the DC voltage on each of them well beyond their 12 VDC rating. One of them showed 19 Volts!

 OK, so they are cheap, unregulated supplies, but that seems like way too much voltage. Do they drop down to their rating at full current load but deliver higher voltages if under current?

I am uncomfortable driving my Tortoises at 18VDC although they seem to not complain.

What are other people's experience with wall wart supplies? Should I invest in a substantial 12 VDC regulated supply for all my electronics and abandon my desire to recycle those old supplies?

I am wondering what other folks do.

Also, for powering LEDs, I wanted to use some old 9VDC supplies with the same over voltage issue. I calculated all the current limiting resistors for 9V but find that the wall wart is giving me 12 or 13 Volts instead. Maybe I shouldn't care?

Charlie 

[cacole]

Instead of fooling around with several cheap unregulated wall-wart supplies, I use a computer switching power supply.  They are much better regulated and give you 12 and 5 volts -- 12 volts for the Tortoise switch motors and 5 volts for LEDs.  One supply can do it all at a cost of $15 or $20 if you know where to shop.  I recently purchased a 650 Watt supply for use on a large HO scale club layout that was priced at $14.95 plus shipping from Computer Geeks in California.

 

[jeffrey-wimberly]

 As Cacole says, a computer power supply is the way to go if you want steady regulated power. I use an old AT type for powering my structure lights. Since my switch machines are all AC it doesn't help me in that respect.

[rrinker]

 If they are typical 300-500ma wal-warts, just a few tortoises hooked to them will drop the voltage to a more reasonable level. However a 9V wal-wart is plenty for a Tortoise, they run quieter with slightly less than 12 volts, which they would get from a 9V unregualted wal-wart.

 You only have to worry if you plan on driving a very light load (say one 12V bulb rated at 15ma from a '12V' 500ma wal-wart - 15ma load probably wouldn;t drop the output voltage much. But 30-40% load will get it down closer to the rated voltage.

 

                    --Randy

 

[jrbernier]

  As you have found, most 'wall warts' are unregulated.  If your 12VDC/300ma(for example) wall wart has a 'load' on it(like 12-15 Tortoise motors) - the voltage will be about 12VDC.  The Tortoise motor uses about 18 ma in the 'stall' position.  Look at the ma output rating and divide by 18 - this should give you the number of Tortoise motors required to 'load' the wall wart.

  I have a wall wart that outputs 18.5VAC/850ma - I used the dual voltage scheme on the Circuitron instructions and get about 8VDC to the Tortoise - they move slower, and it will handle almost 50 Tortoises.  Of course, I only have 12 Tortoises in the staging tracks....

Jim

[CharlieBedard]

Thanks to you all for the feedback.

 It appears that the idea of using a large regulated supply makes sense.

 Since I am already running a DCC power bus and DCC control bus around the layout, I see no reason I can't run a regulated 12VDC bus for all the Tortoises.

 Thanks, again.

Charlie

[Seamonster]

CharlieBedard

Thanks to you all for the feedback.

 It appears that the idea of using a large regulated supply makes sense.

 Since I am already running a DCC power bus and DCC control bus around the layout, I see no reason I can't run a regulated 12VDC bus for all the Tortoises.

 Thanks, again.

Charlie

I think you've come up with the best solution. That's what I'm doing. I have a 12 volt regulated bus for accessories, turnout controls, etc. No problems with over voltages, under voltages, sucking power from throttles (I run DC), etc. I have a separate 10 volt bus for structure lighting. That makes the bulbs last longer and shine at a more realistic brightness and colour inside the buildings.

[richg1998]

  For some years I have used the LM317 voltage regulator, TO-220 case, with two resistors and one capacitor to make a voltage regulator module to hang on a wall wart or unregulated DC power supply. Hang a one inch square piece of aluminum for a heat sink. All the regulator does is shut down if it gets too warm and waits for you to remove the load.

One resistor is 240 ohms, the other is selected to obtain the working voltage I like. I think I hang a 100 ufd electrolytic cap on the output of the regulator. This all assumes the unregulated power pack has a filter capacitor.

In the Model Railroader magazine in the past had typical resistor values for this regulator but right now I cannot remember which issues. I think it was in the electronics section back in the 1980s or 1990s.

I do this to obtain 12 volts, 9 volts and 5 volts DC depending on what I am powering. You can do a Google search for LM317 voltage regulator circuit.

You can also select say, 1.4 volts DC to keep 1.5 volt light bulbs happy for a long time.

With a wall wart, the voltage will drop as you had more items on it. If worried about the voltage varying when you switch things in and out, I would consider a voltage regulator.

Rich

I did some looking and I use a 220 ohm to 240 ohm resistor and trimmed the second resistor for what I wanted. I fed all the regulators with 24 volts DC, unregulated. For a 12 volt regulator, the input has to be at least 16 volts I believe. 24 volt supplies where easy to obtain some years ago.

I use a 1 ufd cap on the output and a 0.i ufd ceramic cap on the input as the unregulated supply had a hefty electrolytic.

By the way, for 1.5 volts DC, a 220 ohm resistor and  47 ohm resistor. Lower the 47 ohm value for say, 1.4 volts.

For those who do not like to Google search, look at the link below.

http://home.cogeco.ca/~rpaisley4/PSupply.html