what is the easiest/cheapest (preferably free) way to get 12v 1 amp or close (12-24v, 0.9-2a is ok range) from 120vac mains? i have some things i could use to make something, like various transformers, diodes, rectifier, wire, solder, multimeter, variable resistors, etc.
2007-09-29 15:58:53 · 5 answers · asked by Alex 3 in Science & Mathematics ➔ Engineering
I would use a step-down transformer that is rated at 2A. You can probably find one at radio shack or Ebay. If you need DC then add a full-wave rectifier and capacitor. I would also add a fuse on the mains side that allows for 2-3A on the secondary side. So, if you are stepping down by 1/10th then your fuse on mains should be about 0.3A. It will be higher to account for the magnitization current. You will need an amp meter to truly measure the primary current when the secondary is loaded with 2A to properly select the fuse you need.
2007-09-29 16:04:52 · answer #1 · answered by J S 2 · 1⤊ 0⤋
tkquestion has a pretty good answer, but you're gonna beat the hell out of that '7812. Even with a big heatsink it's gonna run real warm. Just make sure that you get the CK version (the one in a TO-3 package) because the smaller package (TO-220) will go into thermal overload. And put it on a --BIG-- heatsink.
And you might think about using a 12V transformer into a full-wave bridge. That'll give you 12√2 - 2 diode drops = about 15.7 volts instead of the 21.5 volts that a 16 volt transformer would supply. That's about 6 watts less the '7812 has to dissipate.
Doug
2007-09-29 16:22:16 · answer #2 · answered by doug_donaghue 7 · 1⤊ 0⤋
Well... the easiest way, perhaps, is to go to a junk store or dig though your junk box and find a power pack (aka. wall warts) that has the desired output.
Short of this, perhaps, you can do the following:
Take a transformer with output of about 16 volts
Put its output through a bridge rectifier then into 1000 microF or higher filter capacitor.
Then put that into a 3 terminal regulator, such as 7812. Be sure to decouple in/out port to ground with a capacitor with good high frequency response, such as ceramic or tantalium capacitors. You will also need to heatsink the regulator.
If you google "7812 regulator", you should be able to find a sample circuit.
-doug-
you are right about the RMS vs peak voltage after rectification, but if you pull a full load off it, the output of the filter tends to fall far below peak, and settle around RMS. I *recall* 7812 needs about 1.5 volts of delta anyway, so that will offer no regulation at all. I would say, minimum of 14 volt output from a transformer would be required. Isn't there a 7812 that can do 1.5amp?? (in TO-3 package)
2007-09-29 16:04:00 · answer #3 · answered by tkquestion 7 · 1⤊ 0⤋
Use a variac like this one to reduce the voltage to 12V. Hook up a 12Ω resistor and you'll have 1 amp.
http://s223.photobucket.com/albums/dd78/floodtl/?action=view¤t=SmallVariac.jpg
2007-09-29 16:06:58 · answer #4 · answered by Anonymous · 1⤊ 0⤋
The CHEAPEST (virtually free) method is that you go to the city dump and get a discarded PC.
And then you follow these steps:
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
2007-09-30 03:20:46 · answer #5 · answered by Marianna 6 · 1⤊ 0⤋