Tip when using "wall wart" power supplies

[rxanand]

I am currently installing IR sensors (of my own design) on my layout. When I tested my sensors on my workbench, they worked flawlessly but when I installed them on my layout,I was seeing intermittent errors. After much concern and debugging, I finally discovered that the actual problem was that the DC power supply I was using to power the sensors was far too noisy. I then salvaged a 470 mfd electrolytic capacitor from one of my kid's broken toys and wired in parallel with the output and everything started working perfectly! The power supply in question is an old 5V/2A wall wart. I suppose I should have been suspicious since it was very light - indicating that it was a switched mode power supply. Such power supplies can be noisy. So I guess the moral of the story is to be careful when using these kinds of power supplies. Anand

[richg1998]

 Yes, you are correct about the switch mode power packs. I have run into this with DIY electronic projects. Some of these power packs are quite noisy. You did come up with a quick fix though.

I know some people have modified computer power supplies for model railroad and general electronic experimenting but those supplies are better examples of good switch mode supplies.

Many would not be aware of this issue.

Now someone is going to ask, What is a switch mode power supply? Rich

[locoi1sa]

Now someone is going to ask, What is a switch mode power supply? Rich

   I couldn't help it!!
     Pete

[maxman]

locoi1sa

What is a switch mode power supply? Rich

So, okay, I'm a mechanical guy.  What is it and how do I recognize it?  More importantly, if I use it to provide auxiliary power to my NCE cab bus, will there be any detrimental effect?

[richg1998]

 Many wall warts are transformer type. They will definitely weigh more that a switch mode wall wart of equal physical size. Usually cell phone chargers are switch mode types as are laptop wall warts.

I use the transformer types because I am very familiar with them. Radio Shack use to sell wall warts. A good online seller is All Electronics.

You can find them at tag sails sometimes. Usually the voltage and current capabilities are on the case. For powering a NCE cab bus, I have no idea.

All Electronics

http://www.allelectronics.com/make-a-store/category/815/Wall-Transformers/1.html

You might go to Yahoo.com and look for the NCE dcc forums. There is a lot of NCE specific information. I belong there and use the NCE Power Cab. Don't want to take you away from here, but DCC specific forums can be a great help when your DCC system begins to expand. You can come back here and help others.

Rich

[rrinker]

 A switch-mode power supply is a highly efficient method of generating power with low loss. A ommon one is the power supply in your PC. By using high frequencies internal, a switching power supply can generate a lot of power while being far lighter weight than a typical monolithic transformer. They are also more efficient than a linear regualtor (your typical 7800 series, ie 7805, 7812) which dissipate excess voltage as heat. Without good filtering, some of that high frequency can leak through in the form of ripple ont he low voltage outputs. On a number of computer enthusiast sites you can find reviews of various pwoer supplies, from economy ones to real high end ones, complete with scope views of the output. The good ones will have very little ripple on top of the DC, the cheapy ones will have a lot.

 Computer power supplies typically connect the ground through to the AC ground, so they aren't isolated like a monolithic SELV power supply. This can cause unexpected ground loops in railroad applications, particularly DCC where the command station might be powered by an SELV power supply and isolated from the house ground but an accessory decoder connected to the control bus is powered with a recycled computer power supply.

 This connection to grounds is not symptomatic of all switching power supplies, so simply dismissing any switch mode power supply as unsuitable is not really accurate.

 Most cheap wall-warts you would use for power to the cab bus aren't going to be switching power supplies. Most chepa wall-warts aren't even regulated, so under low load the voltage will be well above the rating, and it will fall as the load increases.

                                                        --Randy

 

[JSperan]

rrinker

Most cheap wall-warts you would use for power to the cab bus aren't going to be switching power supplies. Most cheap wall-warts aren't even regulated, so under low load the voltage will be well above the rating, and it will fall as the load increases.

 

Exactly!  I have one here that's rated, 12V @ 800ma but it is unregulated so without a load load it outputs 15.75 volts.

[ds137]

This thread came up at just the right time for me, as I was just looking at All electronics catalog and asking myself what the significance of what "switching" was and what I wanted for my NCE SB5 smart booster?  I also saw IC voltage regulators in the catalog and viewing this discussion wonder if I want to get one to go along with a switching power supply or will capacitor used as rxanand noted in his post be sufficient or even necessary?

Speaking of capacitors, which is actually why I came to this forum to begin with, is there any significant difference between ceramic and electrolytic capacitors when used in a  simple Capacitive discharge unit for Atlas switch machines?  I probably need to start a new thread for this question if answers get too technical.

Further related to the OP, I have an old 5 amp 12v battery charger..... Should I even dare to try and use it?  NO cost or shipping and handling involved it that choice....

[rrinker]

 Ceramic capacitors are usually only available in relatively low values. They are often used for local filtering around digital logic chips. Electrolytic capacitors have much hight capacitance values and are used for such things as power supply filtering and energy storage, like in a CD switch machine supply. You wn't find ceramic caps in a high enough value to be worthwhile as a CD power supply,

 As for DCC power supplies - 12 volts is probably too low, although a battery charger will actually be 13.8 volts or so. The problem I'd see with a battery charger is they are usually cheap and poorly regulated and filtered. Use a recommended power supply and avoid possible problems.

                                     --Randy

 

[richg1998]

 I have worked in electronics for many years so I like cobbling stuff together for a project.

Sharing with others could be a problem though. I hate to be considered responsible if someone hurt themselves.

Some years ago I used a 12 volt, five amp battery charger, added about 5,000 ufd electrolytic capacitor to the charger. Used some LM317 voltage regulators to get 5 volts DC. Two resistors with a LM317 and you have a voltage regulator. A 100ufd capacitor is hung on the output of the regulator. Clamp the regulator terminal to a piece of 1.4 inch aluminum for a heat sink. The LM313 is good for 3 amps. when on a heat sink. The regulator just shuts down if overloaded and waits for you to lighten the load.

For a twelve to 14 volt regulator, a 24 volt power supply would be needed.

Many different ways to do this.

If not comfortable working with components, a 13.8 volt, 6 amp regulated power supply would work just fine.

About four years ago I bought one from Circuit Specialist to operate three solid state short wave radios. I wanted rock solid regulated voltage for three  $600 radios.

http://webtronics.stores.yahoo.net/136ampdcrepo.html

Rich

 

[jeffrey-wimberly]

rxanand

I am currently installing IR sensors (of my own design) on my layout. When I tested my sensors on my workbench, they worked flawlessly but when I installed them on my layout,I was seeing intermittent errors. After much concern and debugging, I finally discovered that the actual problem was that the DC power supply I was using to power the sensors was far too noisy. I then salvaged a 470 mfd electrolytic capacitor from one of my kid's broken toys and wired in parallel with the output and everything started working perfectly! The power supply in question is an old 5V/2A wall wart. I suppose I should have been suspicious since it was very light - indicating that it was a switched mode power supply. Such power supplies can be noisy. So I guess the moral of the story is to be careful when using these kinds of power supplies. Anand

That's why I use an old computer power supply for most of my electrical needs. Very little, if any splatter (noise).

[richg1998]

JUst in case anyone is interested in using a PC power supply.

http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM

http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

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

Scroll down to

Using "AT" Type Computer Power Supplies

Click on 

AT Power Supplies information

 

Rich