Small DC Power Supply

Well, it has probably been done quite a bit, but I think that it is so useful that I'm going to do one as well. This instructable will go through the basic steps of creating you own small power supply, much like those 'wall warts' in case you need one at a specific voltage, or need to replace one that you fried.

Step 1: Stepping Down the Wall Voltage

The first step of the creation of one of these types of power supplies is to step down the wall voltage to something in the neighbourhood of what you need. I happened to have bought a couple of surplus transformers that stepped 120Vac down to 12Vac, but you could really use any transformer that did enough voltage reduction from 120Vac.

These transformers that I bought did not really have any information provided about them, and I would guess that any scavenged parts would likewise be as much a mystery. The primary side was easily identifiable by the heavier gauge of wire. A scavenged plug was soldered onto the primary, and the connector was lopped off of the secondary, as I wouldn't use it anyways.

An important thing to thing about, if you get any say in the transformer that you are going to use, is how much current that you are going to draw from it. Size seems to be an indication of how much you can draw, but here, if it isn't listed somewhere, I usually go ahead with it and check to make sure it isn't heating up too badly at the end of it all.

Any Time You Are Plugging Anything Into the Wall, Be Extra Careful About Where You Put Your Hands, and What Conducts Electricity!!!

Comments

[timbit1985]

Most consumer electronics come with a fuse. Either in-line, as a IEC socket, or a one time use burried in the case somewhere. Don't forget, fusing is meant to protect the wiring from frying and causing a fire, not to protect against electrocution ;)

[raja26]

The regulator here looks like a NPN transistor (2N5294) that I have used for my joule thief, are they same?.

[KT Gadget]

NPN Transistors and voltage regulators are two separate electronic devices. NPNs act as a gate for larger currents to go through when the original output cannot handle the high currents while regulators step down the original voltage to a smaller voltage ( some are fixed voltages and some can be variable).

Like the 7805 regulator which is the 5 volt voltage regulator but there are other ones that can output a lower or higher voltage, but the input voltage has to be higher than the output.

[raja26]

Thanks, but actually I have typed it by mistake.

[pro2xy]

how do you calculate the voltage drop across a resistor?
V=IR??????? But when you cant predict the current???

[KT Gadget]

Take this equation: (original voltage in - the new voltage after the resistor) = (the current through the resistor) * (the resistor value in ohms)

[raja26]

The bridge rectifier setup was really useful.. thanks a lot.. By the way, tell us about the diode names or model that u have used here..

[Mudbud]

you might want to add a heatsink in with the regulator.
 

[Mudbud]

esspecially if your going to use it allot and work it hard. it lengthens its life span

[PoisonMondo]

would a 1000uf capacitor work?

[sara majble]

I want to aske this expremenit have data?

[coolguy]

all you need to add is a few more voltage regulators wired in parallel to specify a few common voltages. 5V, 12V, 15V, 24V

[Wesley666]

don't forget 1.5V, 3V and 9V. Those are used alot as well.

[brodyf]

Or, he could just use some switches and more resistors and still only need 1 regulator.

[thatonekid]

Ooooor, you could just use a variable regulator and a potentiometer to control it!

[brodyf]

The only reason I didn't suggest that was because one may want to have constant output voltages especially if there is no redout.

[amarquez]

'IMPORTANT!!! Usually the so-called "Step-Down" transformers have the THINNER wire on the PRIMARY side (120 V in the USA), and the heavier wire on the lower voltage secondary side. This is to be able to wind much more turns with the thinner wire on the higher voltage side, and since the secondary in this case only requires a tenth of voltage (12 V), is correspondingly needs only a tenth of the turns of the Primary; and also in that way, the secondary can carry more current. Transformers interchange voltage and current, thus a step-down transformer usually delivers a higher secondary current at a lower voltage. the RATIO of primary to secondary turns dictates the voltage ratio of the transformer.
AM Claussen, Mexico City.

[geeklord]

At first glance I thought your rectifier was an optoisolator.... How do you use those in power supplies?

[alexhalford]

Yes, electrolytic capacitors do explode if you put them into a circuit in the wrong polarity (if they are vented, they will pop the top open, if not they may shoot the entire casing off). The fluid inside heats and expands. You're clearly have a far more mature mind than me, you have seen a cap. blow up once... I do it for fun.

[the mechanical engineer]

AWESOME!!!! I WANT A CAPACITATOR NOW!!!!!

[sysadmn]

Um, fuse and switch? Even a simple, tabletop supply for experimenters really ought to have both. Put a slow-blow fuse and switch on the AC side of the circuit ( http://www.allaboutcircuits.com/vol_6/chpt_4/2.html ), or better yet, buy or scavenge a IEC power socket with built in fuse. They are dirt cheap mail order, but the wait can be a bummer. ( http://www.allelectronics.com/cgi-bin/item/ACS-42/915/IEC_POWER_INPUT_RECEPTACLE,_RT-ANGLE_.html ). You might also find them at an electrical distributors, if they'll sell retail.

Yeah, I'm a wet blanket :-)

[Digital_Anarchy]

Shouldn't the plug have a fuse in. In the UK all plugs have a fuse, either 3A, 5A or 13A. Is this not the case on the far side of the pond?

[juggalo_assassin183]

not usualy sometimes light strings have a built in fuse but thats about the only thing ive seen one in

[Project_Nightmare]

Now I know, before all I knew was that the symbol showed a polerized cap. (and I wasn't sure which way it went, like if the schematic could put it backwards) and I didn't want to buy one only to destroy it. I understand now!

[Project_Nightmare]

Thx, i didn't see a pos. or neg. sign on the cap. so I wasn't sure which way it went.

[ElectricJ]

when the schematic symbol for a capacitor has a curved and a straight line, the straight line is the positive side

[Project_Nightmare]

Im confused with the positioning of the capacitor, do you match the positive side of the cap. with the positive part of the diode bridge or the other way around?

[ElectricJ]

the positive end of the filtering capacitor gets attached to the positive side of the bridges DC output, if that helps I believe that its shown in the schematic in the last step

[Gnaw]

I agree a great instructable for a beginner. Cost effective as well, can’t wait to try it out!

[tstens]

I'm one of those beginners you're talking about, robonut. I've been desoldering all kinds of scavenged stuff and watching my junk box grow, but I've been trying to find a first project that's actually useful. I think I just found it. I'd greatly appreciate any more noob projects. Thanks, ElectricJ!

[robonut625]

Pretty good instructable ElectricJ... will probably be useful to some beginners. I always build these with scavenged parts... I don't think I've ever purchased a single part for a power supply.