Selectable output plug pack as adjustable bench supply

[kninja]

So I am new to DIY electronics. I'm finding out quickly its an expensive hobby... I need an adjustable voltage bench supply for testing but even on ebay all the units I found were quite pricey. However I did come across some 'universal' AC/DC plug packs that have a switch on them to select the output voltage...

like this

and it occurred to me that a cheap way to get an adjustable supply might be to just buy something like this, chop the end off and solder on some alligator clips...

but it sounds too simple and I'm sure I'm missing something... could this idea work?

 

[betwixt]

The link didn't show any power supplies but I thnk I know the gadgets you mean. Usually, they have a sliding switch marked 3-4.5-6-9-12 or something like that.
You can use these but normally they are unregulated. This means the voltage isn't actually stabilized to what it says on the switch, it is usually higher but drops to the quoted voltage under a particular load. If the load increases,the voltage continues to drop so if you plan to experiment with ICs. particularly digital ones with their fussy voltage demands, it probably wouldn't work. A regulated supply is one that holds the voltage steady from no load up to the maximum it is rated at.

Suggestion: If you get one of these unregulated supplies and add a simple voltage regulator circuit after it, it should do what you want. I suggest you look at the data sheet for the LM317 IC, with just a few inexpensive components you can make your own regulator to go after the unregulated one.

Brian.

 

[kninja]

The link didn't show any power supplies but I thnk I know the gadgets you mean. Usually, they have a sliding switch marked 3-4.5-6-9-12 or something like that.
You can use these but normally they are unregulated. This means the voltage isn't actually stabilized to what it says on the switch, it is usually higher but drops to the quoted voltage under a particular load. If the load increases,the voltage continues to drop so if you plan to experiment with ICs. particularly digital ones with their fussy voltage demands, it probably wouldn't work. A regulated supply is one that holds the voltage steady from no load up to the maximum it is rated at.

Suggestion: If you get one of these unregulated supplies and add a simple voltage regulator circuit after it, it should do what you want. I suggest you look at the data sheet for the LM317 IC, with just a few inexpensive components you can make your own regulator to go after the unregulated one.

Brian.

Gah stupid web site... well yes that is what I am talking about, altho the actual product header on the site is "Regulated International Voltage Power Adaptor", so I dont know if that means it is regulated like you say or in some other respect...? (if you search for power supply they will come up near the top of the list).

I have seen the lots of small kit type things which can make a variable power supply out of the LM317's so I might end up doing that... i just was wondering if I could get away with something simpler... if i was to make a circuit out of one of those kits, can I just hook the mains up to it? or does it have to be DC or something?

 

[betwixt]

Found it! Yes, it should work but I would urge caution. It doesn't specify how accurately it is regulated, for digital ICs the limit is usually +/- 5% or better. I would also be concerned about safety, you should really use a power supply which is either double isolated (bearing a symbol that looks like a square inside another square) or has an earth connection. Some of these power supplies, which invariably come from China, are downright dangerous and a fault could make your project dangerous to touch.
For example, I have some very sensitive radio equipment here and something was causing bad interference, I tracked it down to a power unit feeding a USB hub. It DID have all the safety marks on it and a certificate quoting it passed all the statutory tests but I opened it up and found it had absolutely no interference suppression and was not double isolated. In fact the circuit board was loose inside the plastic case and could easily have touched the back of the plug pins.
If you go the route I suggested, look for a power adapter that uses a transformer, they are a bit bigger and quite a lot heavier than the cheap ones but MUCH safer. You could also go the whole hog and buy a transformer then use the LM317 regulator after it, the cost would still be much less than the lab supplies on the web site.
Which country are you in, I might be able to suggest suppliers.
Brian.

 

[kninja]

Which country are you in, I might be able to suggest suppliers.
Brian.

I'm in australia... thats half of why everything is so expensive... I am concerned about damaging the components so if that is an issue I would think twice... can get one of those lm317 kits pretty cheap tho so might just buy a single voltage plug pack and hard wire it onto something like that... I am trying to build a x0xb0x, if that helps, but yeah, this is my first project so I'm kinda learning as i go.

 

[betwixt]

My other half, Pat, has a daughter that specializes in supplying components to Australia and SE Asia !! Unfortunately, she supplies large orders to manufacturers and isn't set up to deal with individuals.
Looking at the x0xb0x web pages I would thing you needed a well regulated supply for it so steer away from the "wall wart" types and make your own. Quite aside from the performance issues, I would worry about trusting something as complicated as the x0xb0x to a power unit of 'suspicious' origins.

I buy most of my components from a company called Farnell, they operate World wide and the Australian web site is at "http://au.farnell.com", they take credit card payments but they may have a minimum charge so be careful to read their terms and conditions. They are not the cheapest supplier but their product range is huge and you can be sure it comes from reliable sources. I'm in a remote location but they still manage to deliver next day.
Brian.