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dc power supply

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things

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hi,
amazingly (even though i do play with electronics alot) i do not have a variable power supply:( . i have looked around and the prices are very high:( . i was wondering what the best way to make a variable power supply would be eg;pwm. also i don't muck around with 240 volts so is it possible to make one run off 12 volts or so?
thanks
 
What you might do is decide what you need and share that with us in terms of voltage and current requirements. Ripple/noise are important too but defining what you actually need might be tough. Many designs are good enough. If your current requirements are low enough (1/2 amp or so) you might find a common wall wart to be a way of safely getting down to a lower voltage - maybe 24 vac or 24 vdc or less - then build your own from there. I have some power supplies that are 1 amp, 24 volts or so that aren't wall warts but are intended to function the same - very short cord from mains to supply - then lower voltage from there to the appliance - a printer or some other device.

If your voltage source is higher than what you need by 3 volts or so you can use a common linear regulator. Low dropout devices allow you to come closer.

If your voltage source does not have the headroom or if the output must be equal to or higher then a switched mode supply is a solution.

Additionally you might find that you'll adjust your requirements based on what you have available to you. An example - you might decide that a 10 amp supply would be nice to have but find an easy to build 3 amp design that will take care of 90% of your needs.
 
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Have you considered buying a kit from Dick Smith, JayCar or Altronics?

240 Volt is quite safe if you use a good transformer, wire it properly and insulate the high voltage wires well. Also keep the high voltages away from the low voltages.
 
thanks for the reply's. basically i just want a power supply that can supply about 3-12 volts at no more than 1 amp. kit... hmmm is there a kit, i will have a look:)
 
things said:
thanks for the reply's. basically i just want a power supply that can supply about 3-12 volts at no more than 1 amp. kit... hmmm is there a kit, i will have a look:)

An adjustable switchmode power supply would be a fun and educational project. But it sounds like you probably want a quicker and easier solution.

If you start with a wall-wart that's 15v or more, and rated for 1 amp or more, you can VERY-easily construct such a power supply, without even buying a kit.

I recommend that you right-click on the links below and select "Save Target As" (if using Windows), and save the datasheets for the LM117/317 and the LM337 adjustable linear regulators.

**broken link removed**

and

http://www.st.com/stonline/products/literature/ds/2167/lm337.pdf

Those datasheets have extremely-simple schematics for adjustable power supplies, just like you are wanting. They require only a few resistors and capacitors, and a potentiometer for adjustment.

And if you use a lower input voltage than they specify in the schematics, you just won't have as large a range for the adjustable output.

You can also look for "Application Notes" about linear and/or LDO linear regulators, as well as datasheets for othe ICs, on those and other IC manufacturers' websites, and find many more circuits.

I included the second one (the LM337), which is for a negative adjustable regulator, because it has a schematic for a DUAL-output (positive AND negative) power supply (using both an LM317 and an LM337), which would be very handy if you want to work with dual-supply op amps, for example. (Also, if you're going to make or buy some kind of enclosure for it, it makes economic sense to put more into that enclosure. Of course, the negative half could also be added later.)

For a dual supply, you could just use two identical wall-warts, for the inputs. The two halves (i.e. + and -) are completely independent, too, meaning that you could use only one wall-wart at a time, if you needed only one of the output voltages.

Also, if you go to national.com, for example, you can probably even order free samples of the regulators. But they're extremely cheap, anyway, usually, and very widely available. Also, any other similar adjustable linear regulators should work in nearly the same way.

Remember to use something as a heatsink, for the regulators. If you mount everything in a small metal box, the box itself might work well-enough as a heatsink, if you bolt the regulators to the inside of it. If you then found that you needed more heat-sinking, you could probably just bolt an additional heatsink onto the outside of the box, using the same bolts.

Wall-wart power sources are so common that you can probably get them almost free, if you're at-all resourceful (think "garage sales", or "salvage from old equipment"). But you can also buy new wall-warts, at places like Mall-Wart, er, I mean, WalMart, or Radio Shack, in the USA at least. There's also a good selection of them on line, with reasonable prices, along with the panel-mountable DC jacks for any of them, at places like http://www.mouser.com , and http://www.jameco.com . Another often-dirt-cheap source for the parts you would need is ebay.com, as long as you watch out for the shipping and handling costs. You might also want to look at places like **broken link removed** . Google should be able to find many more similar sources, for everything you need.

Good luck!

Tom Gootee

**broken link removed**

-
 
thanks for the very informative and helpful answers and links.i'm going to get all the parts tommorow:)
 
LM317T is one of my favorites in voltage control circuits. You can refer to the National data sheet it contains number of applications.

Recently I made a NIMH charger from that. Working perfectly.

I’m planning to make a kind of switch mode power supply from that like output voltages are selecting from digital inputs……………
 
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