Wall Transformer for Project Power Supply




Wondering what to do with those old wall transformers? Use them for power supplies to run electronics projects!

You will need some of these:
power supply (go with a junker)
Alligator clips with plastic boots
wire strippers or cutters
needlenose pliers
some tape
about 10 minutes

You will be interested to know what the output values are. This one listed 9 volts DC as the output with 600 miliamps.

The label will also tell what the polarity is. This one has a negative on the outside, positive on the inside.

It also indicated that it came from a telephone. I think it was from the dump. Nobody will cry if this is gone. You should check with any potential owner before you perform this surgery on a wall transformer.

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Step 1: Cut Off the Output Plug

Make sure it has been unplugged for a while. They hold some charge.
Cut the wires.
Cut one wire shorter than the other. It is nice if the negative is shorter, but not essential. Mostly, you want to make it so that the two tips cannot easily touch each other. If they contact, you will probably blow the fuse, which you won't want to replace, and will probably cause you to toss the transformer.

Step 2: Strip the Wires

Strip both wires about a half inch.

Step 3: Put the Boots on

Put the boots on the wires. The narrow end is away from the wire end. If you have two colors, that's great. Red, orange or yellow for positive, black or green for negative
If you forget to to put the boots on, you can get them on, but it is a pain.

Step 4: Attach the Wire

Twist the stranded wire to make it stronger and easier to handle. Put the wire through the hole in the gator clip, send it out between the two tabs in the handle.

Bend one of the tabs over the exposed wire. Make sure you have a good mechanical connection. If the wire is loose, it will fall out.

You may want to bend over the other tab, but if you decide to use the clip again, you may be able to salvage the clip and use the other tab. Just make sure you have a good connection.

Step 5: Slip the Boot Over the Clip

Jam the nose of the needlenose pliers into the mouth of the clip. It is a lot easier to do this if you have the mouth open.
Slide the rubber boot over the gator clip.

Do the same to the other clip.

Step 6: Test the Leads

Plug it in
Put the multimeter on the appriate setting for the value you think it is.
Put the clips on the meter.
Check the meter's reading.
If you pay attention to the negative symbol, you can see what the polarity of the wires is.
Label the leads with masking tape, especially if you don't have different colored boots.

Step 7: Glory in Your Completion

This thing is done.
You can use it to power projects. If your project is fussy about what voltage you need, you may need to build a control circuit.

If you are just powering something that needs to go, like a motor or something like that, then you should be able to just hook the gator clips to the project and get going.

Since this is a wall transformer, it will continue to draw power even if it is not being used. Unplug it to when not in use to conserve energy.

Have some fun!



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29 Discussions


2 years ago

QUESTION: I was wondering if this would be safe to charge a old ni-cd drill battery. My drill charger went dead. The output on the drill transformer is 12 volts, 500ma. The output on the closest matching ac transformer I have is a 13 volt, 800ma. Would this be safe?


11 years ago on Introduction

I would also like to know how close your measurements are to the ones printed on the transformer. The actual Output voltage seems to differ a lot from the printed on output value. Does anybody know why these values don't match? I have a 9v (printed on) transformer that is actually outputting 12v.

2 replies
W. RustyLsdtacoma

Reply 3 years ago

If you're measuring 12v. dc on a 9v power supply it's generally an unloaded power that you're measuring. If you put a 9v load on it, it should measure 9v. Hope this helps.


Reply 10 years ago on Introduction

The performance for these wall-warts probably isn't linear (i.e. the voltage isn't the same for all current loads). Generally, the voltage goes down as more current is drawn. If you are measuring with a multimeter, multimeters are designed to have an extremely small current flow (ideally it should be zero) to measure voltage. 9v is probably what the transformer provides with a typical current load.

They all have the volts listed on the body of the transformer- may have to get strong glasses to read it but it should be there.


8 years ago on Introduction

unless the ac adapter is regulated you probably shouldnt use that because those ac adapters are perfectly matched at the device it goes with because the device pulls the exact voltage and current on the ac adapter. using it to power any device or project that doesnt pull the 'exact' current and voltage the ac adapter will put out a much higher current and voltage than it should and that could destroy your project or device or whatever you have hooked to it. it does this because the ac adapter isnt regulated its unregulated and unregulated power supplies are not reliable, maintained, unstable, controlled and they put out very 'unclean dirty power'. use a 'regulated' power supply because the power is controlled, maintained, filtered and is pure, clean lab quality power.

1 reply

Not true, the adaptor should be rated higher in amperage than your circuit draws. The rating on the adaptor is the MAXINUM it can supply. Your circuit will only draw the current it needs (unless you've done something wrong).The voltage should be the same as that of the circuit. While some things need clean regulated power,for "cheap & dirty", this will work just fine.


12 years ago

Save yourself from a lot of heartache. Use both red an black boots and put the red boot on the wire with the white line (or textured edge). This will alway be the positive clip.

1 reply

Reply 5 years ago on Introduction

I know this is an old topic, but I wanted to say, for the benefit of beginners, that that is NOT always true. In my 25 years as an electronics technician, I have found that the polarity of the white stripe (or textured edge) various from manufacturer to manufacturer. ALWAYS measure polarity before assuming anything. Better to check and be right, than to not check and be wrong.


5 years ago on Step 7

One suggestion: you can buy the boots in different colors, so: cut off the original connector, separate and strip the wires, and test the polarity.
Slip a red boot on the positive side and a black one on the negative; the rest of the instructions as posted in the article.


10 years ago on Introduction

Well I know I am new to the whole concept of tearing things apart and re-working them but I thought most transformer lines on the wires had some kind of marking on the positive lead IE: white stripe, lettering or small rib's ???? Am I wrong?

1 reply

Reply 9 years ago on Introduction

 no you're right, but sometimes the white line means negative and sometimes it means positive and we use the multimeter to determine which is which and then we can just remember which one has the line. the line is usually on the negative. what I would do is measure the polarity before putting the clips on because then you could but on a red mask and a black mask.


Reply 9 years ago on Step 6

Your meter will have two probes, black and red. The black should be in the COM port. on mine, the red is in a port labeled V/Ohm/mA. When you put the two probes on a battery or DC power transformer, you will get either a positive number or a negative number. If the number is positive and the probes are in the correct holes, then the one touching the red is positive and the one touching the black is negative. Try it out with a battery, which will have markings for polarity until you get the hang of it. Enjoy!


12 years ago

I have a very similar setup but use one of those transformers with a 6-9-12v switch on the back (found it thrown out). Also be careful about the amperage, if you use a 1A (Amp) unit you will blow out the regulators in many small electrical devices try go for 200-400mA unit and have a 1A unit just in case. Yeah I agree with a previous comment use different coloured insulated plastic sleeves for the crocodile clips. Also "never" throw a power brick out, they always become useful for either a project or fixing things.

1 reply

Reply 11 years ago on Introduction

I was under the impression that the Amperage rating was the maximum possible the transformer can provide. Since amperage is the draw of current, having a rating in excess shouldn't damage anything, because you're only using what the circuit draws. Going over in voltage though, that's a bad idea. Anyone feel free to correct me if I'm wrong though.


Reply 12 years ago

Way cool - you ought to reformat it as an Instructable. Two questions - is 12V enough to drive the 12V regulator? I though they liked Vout + 1.2V or so. If so, is it doing anything other than cleaning up the 12V input? Second q: Got any pointers to good output protection circuits that could be added, such as short circuit protection or crowbar circuits?


Reply 12 years ago

The 12 volt regulator is redundant since the wall wart would already have handled that part of the circuit. You're right though, If the regulators going to be used, you should start out with around 13.5 VDC input.