Looking for help in designing a low profile 12v power supply

For my pwm project I decided to include the power supply inside the box instead of having a wall wort on the outside. I know the basics on how to build one (step voltage down, rectify, regulate). My problem is finding a transformer that will fit inside what remaining space I have inside my project box.

I took apart one of those really small wall worts and was very surprised to find out it is much more complicated than what I was expecting. I won't go into the details of my discovery, but the design that was used makes use of a transformer that is the size that I was hoping to use... so I have hope.

I would like to make a power supply with the follow criteria:
120v ax input
~12v output
~0.5 Amps (my design uses .4 max)
Max Height of the transformer (or any component) ~1/2"

I searched digikey for the transformer, but was unsure if the specifications listed would meet my criteria. If anyone can clue me in on how to make a power supply that is not super complicated or bulky I would really appreciate it!

why dont you just use the guts of the wall wort?

That was the intent, but it broke in the process.

I got 'em coming out my rear, do you want one? No charge, just shipping.

You could use a diodes to construct a full-wave bridge rectifier and then use a Zener diode(s) to regulate the voltage to 12 VDC? It's not the most efficient design, but functional. This is all coming from a mechanical engineer, but the science seems fairly straight forward.

Rectifier circuits : DIODES AND RECTIFIERS
Zener diodes : DIODES AND RECTIFIERS

Nevermind - it was for 40v input

So the wall wort you took apart was probably a switching power supply. The circuitry for those is quite a bit more complex as it requires a controller to regulate the amount of current going into an inductor, as well as additional circuitry to filter out the noise that is usually present in such a design. The reason the transformers are so much smaller is two fold, one they are switching at a much higher frequency than the 50/60hz mains voltage, which allows for a much smaller core, and two, the open circuit voltage is typically quite a bit higher than the traditional transformer into a diode types.

You're best bet is to buy an off the shelf unit and modify it, unless you want to design a printed circuit board to handle the circuit.

What are you supplying power to? That will determine what sort of DC power you require. A simple DC supply is fairly easy to put together, but it might not suffice for your needs.

To me, it seems like a lot of work just to move the DC converter into the unit. I think I'd prefer to convert the 120V at the wall and have DC running into the box. Slightly less dangerous if there happens to be a short inside somewhere...

The simplest approach would be to go to a thrift store (Value Village) and rummaged through their hundreds of wall-warts which they sell for a couple of bucks.
Second option is a simple linear rectifier constructed of a low VA transformer, some diodes, capacitor, and an lm7912 ic.
Final option would be to pick up a Stepdown converter IC from Maxim with all the external components.
Both the second and third option require some circuit design and pcb layout. While this is fun, it might be more bother than it's worth at the moment.
If you're really interested in building the circuit let me know and I could point you in the right direction.

Thanks for the help everyone. I ended up buying a slightly bigger project box so that I can fit the huge transformer in it. I also bought a 1amp/12.6v transformer from radio shack. I took apart a wall sort from goodwill but the transformer was 18v and I don't like the idea if my voltage regulator wasting that much energy just to give me 12v.

Was the purchased transformer rated for 1amp overkill? Or could I of used a transformer rated at .45amp. I have tested my setup and it can just about draw .45 under worst-case conditions.

Since you've already picked up the 1 A transformer, and if it fits I'd say there is no harm in being oversized.
Regarding the 0.45A draw, are you saying that your box is actually pulling 0.45 A of current ? What are you driving?

brewhacker said:

Since you've already picked up the 1 A transformer, and if it fits I'd say there is no harm in being oversized.
Regarding the 0.45A draw, are you saying that your box is actually pulling 0.45 A of current ? What are you driving?

Click to expand...

I think I measured .41 or .43 amps. The circuit was my pwm circuit powering a 12v computer fan. I set it to 100% and held the fan still with my hand to simulate a worst case condition.

Normally I would not run the fan a full speed for a stir plate (i would assume) and the fan would always be allowed to rotate unless the stir bar was massive or something. Under operating conditions I think I was drawing .3 amps at full speed.

If you think I could get by with a .45 amp transformer then I might take back the 1amp.... It is just so big and heavy.

With you're worst case measurement, it should be fine for you to drop the size. You would be limited though if you ever wanted to add pilot lights or some other addon.
I think you've done well to oversize it.
Good luck!