I am interested in buying a used benchtop power supply. I am a newbie, just starting out with simple circuits. I want to play around with programing some AVR mcu's and moving on toward building some simple robots. Folks have recommended looking on eBay, craigslist etc.

I read some stuff on building you own (not that I am ready to do that), most of which was chock full of warnings and disclaimers telling you how you were hooking up to mains and could electrocute yourself if you didn't know what you were doing! Well, how does one confirm that a used one (or , I guess new one, for that matter) isn't faulty? My question is, how do I make sure that a used unit is safe?

Also, what about this unit - http://www.circuitspecialists.com/prod.itml/icOid/8238 ?

 

I just wrote a short piece about rolling your own, using a wall wart. This gets rid of the hazardous part of the project.

Bill's Index

Basic Bench Top Power Supplies

 

or you can get a Computer power supply, ATX one would work, has +12,+5,+3.3, -12, -5 outputs, no need to make your own.....


My .02

 

for a single output i recommend this...

http://cgi.ebay.co.uk/THURLBY-THAND...emQQptZLH_DefaultDomain_0?hash=item48382d421b

or a duel, i have a pl330qmd. It is brilliant.

 

I just wrote a short piece about rolling your own, using a wall wart. This gets rid of the hazardous part of the project.

Bill's Index

Basic Bench Top Power Supplies

Bill, I had read your article, and found it very interesting.

I have been toying with the notion of connecting a wall wart directly into my bread board since reading these tutorials on SparkFun Electronics and Instructables. But your idea takes it much further, making more a elaborate psu...I really like it!

I know that you say the wall warts are safe...and when I asked about the SparksFun or Instructable ideas on other forums the general response has always been that they are safe. But there has always been a one or two naysayers that suggest it might be dangerous - that wall warts may not be isolated, etc. That has made me nervous.

That is what kind of got me thinking about the question in my original post - - how do you know a used (or new for that matter) psu or wall wart is safe? Made correctly and/or not damages?

for a single output i recommend this...

http://cgi.ebay.co.uk/THURLBY-THANDA...item48382d421b

or a duel, i have a pl330qmd. It is brilliant.

Thanks, bluebrakes...but with shipping that is > $250US...more than I can spend for used equipment.

 

The only way to *truly* know about what is inside a wallwart, is to open it up. But most of the time, if what you read on you meter matches what is says on the wart, you have a unit that is good to go.

If the wart says 12v and your reading 18v on your meter, don't use it. Better yet, dissect it for usable parts. Old equipment is a good source for filling your parts bins.

 

If the wart says 12v and your reading 18v on your meter, don't use it. Better yet, dissect it for usable parts. Old equipment is a good source for filling your parts bins.

I understood that unregulated wall warts would show a high voltage if read without a load. That seemed to a consistent point in much of the research that I have done. For example, see this from Bill's article (linked above):

I'm going to assume a simple DC model, or a switching unit that is highly regulated. The simple units will have non-replaceable fuses inside, if you blow them you are expected to throw them away. The unregulated models do not have stable DC voltages either, a 12VDC 1A can have as much as 18VDC out with no load, and it can be worse. You will need to check this before you use it.

 

The only way to *truly* know about what is inside a wallwart, is to open it up. But most of the time, if what you read on you meter matches what is says on the wart, you have a unit that is good to go.

If the wart says 12v and your reading 18v on your meter, don't use it. Better yet, dissect it for usable parts. Old equipment is a good source for filling your parts bins.

Uh, no. All it means is you don't have the Wall Wart loaded to it's rated current. This is normal behavior for a unregulated power supply, which happens to be a wall wart in this case.

You really don't have to know what is inside, it is a black box after all. The whole point of using one is safety, which is not what opening one would be.

 

You should make your own variable power supply from a Wallwart and a few resistors an a pot, filter capacitors, an LM317 regulator and perhaps diodes if the wallwart output is not rectified already. The datasheet for the LM317 has a lot of information on basic setups. You can get outputs from about 1.2 volts up to about 35 volts I think. With a heat sink attached you should be able to supply about 1 amp depending on the current rating of the wallwart. This is a very good simple project.
One of my first projects. I still use it. Super cheap. You do need a meter to set the output.

 

or you can get a Computer power supply, ATX one would work, has +12,+5,+3.3, -12, -5 outputs, no need to make your own.....

Sparkfun even sells a breakout board for it.
http://www.sparkfun.com/commerce/product_info.php?products_id=9558