Small DC Power Supply


Introduction: Small DC Power Supply

About: Electrical Engineering Student

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!!!

Step 2: Rectifying the Stepped Down Voltage

So now that the voltage has been stepped down from the wall, we want to make make it DC. The first step to this is to rectify voltage signal.

My preferred method of doing this is the use of a bridge rectifier. There are a couple ways to do this however, if you have rectifier diodes hanging around from something else that you have taken apart, you can build your own easily. Or, you can get one of the prepackaged ones that are available from Fairchild or other component companies.

Pretty much the only thing to worry about is to make sure that whatever you are using will be operating within its range. Check any available data sheets to make sure that you aren't going to try and pull too much current through the rectifier. If datasheets are unavailable, i.e. you used scavenged diodes to build your own rectifier, I usually just go ahead and build it anyways, and see how much everything heats up.

Step 3: Filtering

So now that we've rectified the voltage, we have changed the voltage signal from a sine wave into what is more or less the absolute value of itself. Now what remains is to smooth it out. So we are just going to insert a capacitor in parallel between ground and voltage.

This is a really easy step, especially since electrolytic capacitors are common, and you probably have something broken lying around that you can pull one out of. Here, a larger value is better, but you don't really need to go overboard. I just put in a large value and then later replaced it with smaller and smaller values until the ripple got bad enough for me to worry about it.

As with all things, make sure that you are using a component that will work within its safety limit. Here, you have to make sure that the capacitors rated voltage isn't exceeded. It would be a good idea to measure this just to be sure.

Also: Make sure to put the capacitor in the right way. The side with the stripe on it is the side that you must put to the most negative voltage. I've only ever seen it once, but when an electrolytic capacitor is put in backwards, it can explode.

Step 4: Regulation

This step is necessary to take the smoothed out voltage, smooth it out a little further and give you your final, desired output voltage.

Again, you could do this step in a couple of different ways. First you could whip up a zener regulator, if you happen to have zener diode around that fits the output voltage you wanted.

Personally, I prefer another way. More of a 'plug and chug' approach, this method just uses a prepackaged voltage regulator readily available from any number of different companies. Pretty much all you have to do is make sure that it will handle the current you are going to pull from it, and that you are supplying it with a voltage inside of its input range. One of the ones I built needed to have the voltage dropped down a bit further so I figured out the size of the resistor needed to put the input voltage inside the proper range. If you need to do this, just keep in mind the power dissipation.

Also, some regulators need a small capacitor in parallel with the output in order to stabilize it. The datasheet will mention if it requires one or not.

Step 5: Other Safety Considerations and Finishing Up

So, now you've got a small DC power supply. It can be mounted permanently on a board, or used temporarily if you leave it on your breadboard.

Just watch, what you hook up to it, so that you don't fry any of the components, and be really careful with that transformer, as it is plugged into the wall.



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    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 ;)

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

    2 replies

    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.

    Thanks, but actually I have typed it by mistake.

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

    1 reply

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

    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..

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

    1 reply

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

    would a 1000uf capacitor work?

    I want to aske this expremenit have data?

    '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.

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

    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.

    1 reply

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

    3 replies

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

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

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

    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 ( ), 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. (,_RT-ANGLE_.html ). You might also find them at an electrical distributors, if they'll sell retail.

    Yeah, I'm a wet blanket :-)