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Ac/dc Adapter (wall Wart) As Benchtop Power Supply, Cheap mans's power supply

Ac/dc Adapter (wall Wart) As Benchtop Power Supply, Cheap mans's power supply


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Old 02-17-08, 05:11 AM
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Ac/dc Adapter (wall Wart) As Benchtop Power Supply, Cheap mans's power supply

I am an absolute newbie.

I am just beginning to play around with some projects, trying to learn about electronics (been messing around with a mcu). I would like to make a power supply for my breadboard. A few weeks back I was reading a tutorial on building a bench top power supply that had a lot of cautionary info on how not to kill yourself. Being a newbie I heeded this advice and steered clear of that kind of project. Then I came across the wall wart idea (see this link http://www.sparkfun.com/commerce/pre...-1-PowerSupply), it seemed like a good approach.

A nearby store sells used AC/DC adapters for very little (some as cheap as a $1) - wide assortment of voltage and current output. I was thinking that could use the adapter as a power supply for my breadboard. Thought that the wall wart set up would save some battery use (currently using a 9 volt) and would be fun/instructional to build. Thought that power supply could be used for low voltage/low current circuits that would otherwise be using 9 volt battery. I researched this on the net and have found some projects that do just this...with voltage regulator, etc...(see for example http://www.instructables.com/id/STGS44CF3SYP07P/).

I thought it best to ask around first, however, and to get some advice on whether this was a safe thing to be messing with. My question is, is this safe to do. The idea of playing around with AC concerns me...I am hoping that it is relatively safe as I will be connecting the DC side to breadboard.
 
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Old 02-17-08, 05:48 AM
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Wall-warts are a great way to work with electronics. You probably don't need anything over a few hundred mA, so you don't need a real bench supply. Plus, as the output is only 9v at a few hundred mA, you can not worry about shocking yourself.

Note that 9v is still enough to fry ICs if connected backwards though! They are a great alternative to batteries and wiring up your own power supply.
 
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Old 02-17-08, 05:50 AM
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A DC power supply consists of four components:
-- A transformer that lowers the AC line voltage from 120V on the input, or primary side, to something near your desired DC voltage at the output, or secondary side.
-- A rectifier that changes AC to DC. This can be a single diode, a four-diode "full wave bridge", or a packaged, self-contained bridge rectifier.
-- Filters that smooth the AC "ripple" that rides on the DC. Usually one or two large capacitors.
-- A voltage regulator that holds the circuit to a specifc voltage.

All wall warts have at least the first two components. The benefit of a wall-wart is, you don't have to mess with any AC line voltage -- just plug it in. The downside is, the cheapest won't have a full-wave bridge and will not provide ripple-free DC.

If I were you, I'd opt for an AC wall wart, not a DC. An AC wall wart is just a transfomer. It plugs into the wall, which again keeps you safe from the AC line power. The output side is low voltage. Buy one that's close to or a bit higher than your desired DC voltage.

All of the other components can be added to your breadboard.

The main thing you need to pay attention to is the current capability of the entire supply. A fuse should be the first thing in the circuit, immediately after the transformer. Wear safety glasses when you work on it, too. If your hobby circuit draws too much power the components can pop. A shorted capacitor can explode with surprising force, even in very low-voltage circuits.
 
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Old 02-17-08, 06:14 AM
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Originally Posted by Rick Johnston View Post
If I were you, I'd opt for an AC wall wart, not a DC. An AC wall wart is just a transfomer. It plugs into the wall, which again keeps you safe from the AC line power. The output side is low voltage. Buy one that's close to or a bit higher than your desired DC voltage.
Thanks!

Is your suggestion to use an AC wall wart based on safety or quality of output considerations (or both)?

I assume that if I use an AC adapter that I should add a rectifier to the breadboard - to convert from AC to AD?
 
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Old 02-17-08, 08:37 AM
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Before you can really learn to "fool around" with electronics, you should learn about electricity. Get a book or take a class on basic AC and DC electricity.
 
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Old 02-17-08, 10:03 AM
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Grew up using telephone batteries (aka door bell batteries) for a power supply. Plenty of amps and you can string them together in series for the voltage you want. Do they even still sell them.?Anybody here but the old timers even seen one of these massive single cell batteries about six inches high and two inches in diameter? Had nice threaded terminals with a knurled nut on top for connecting.
 
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Old 02-18-08, 03:57 AM
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Originally Posted by gatoruss View Post
Is your suggestion to use an AC wall wart based on safety or quality of output considerations (or both)?
Safety. You won't have any exposed 120VAC wires.
I assume that if I use an AC adapter that I should add a rectifier to the breadboard - to convert from AC to AD?
AC to DC. All of the other components should be added to the breadboard. I'd listen to 594tough's advice, too.
 
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Old 02-18-08, 04:52 AM
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thanks to all...I am off to the library.
 
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Old 02-18-08, 05:00 AM
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Ray, remember the batteries, and used to fish with telephone ringer boxes, too. Wow, quartz piezo is tame compared to those puppies.
 
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Old 02-18-08, 12:18 PM
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I don’t know what you are running, but I just wanted to suggest that you could connect 6 AA, C or D batteries in series. This would give you 9V, with a longer life and more amperage. Radio shack sells (at least they used to) battery holders.

I used to make a "poor man's" battery holder (not recommended) by driving nails in a board, propping the batteries between them, then using wires from Christmas lights to connect them as needed.
 
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Old 02-19-08, 12:02 AM
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Here's a link for some Good e-books on electronics, their large.
http://openbookproject.net//electricCircuits/index.htm

For a voltage regulator you should search the web for "LM317"
 
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Old 02-19-08, 04:22 AM
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Originally Posted by GWIZ View Post
For a voltage regulator you should search the web for "LM317"
Another common v-reg is the 7809.
 
 

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