How to build a Power Supply
Background
A power supply is a useful tool to have on any experimenter's bench and is pretty easy to build. This project describes a simple 12V fixed bipolar supply.
Safety
As always, please follow proper safety precautions when building this project. This is especially true of working with the 120VAC power line. If you are not familiar with the necessary safety precautions, then contact someone who does know before proceding with this project.
Parts
- Select a suitable instrument case for the project. RadioShack sells several cases that would work well for this project.
- Select or build a transformer for this project. If you can locate a transformer, with a 120VAC input and two 12 to 15VAC outputs, it would be ideal. Make sure that the current rating of the output is large enough to power whatever you had in mind for this project. If you cannot locate a single transformer with 2 output windings of 12 to 15VAC, then purchase 2 transformers with 120VAC primary and 12 to 15 VAC secondary.
- Select a 3 prong 120VAC power cord.
- Select a rubber grommet with a large enough inner diameter to allow the power cord to pass through.
- Select solder terminals. A six terminal unit should be fine.
- 3 single banana sockets: 1 red, 1 black, 1 green.
- 4 standoffs to mount the circuit board
- 2 bridge rectifiers
- AGC panel mount fuse holder
- 22 AWG stranded wire
- 4 470uF electrolytic capacitors (The actual uF rating is not critical.)
- 2A 120VAC AGC fuses
- 3A 120VAC switch
- 7812 regulator (positive 12V regulator)
- 7912 regulator (negative 12V regulator)
Tools
- solder iron
- screwdrivers
- wire cutters
Instructions
- Take a few minutes to study the layout of your case and the parts that you need to insert into it. Sketch out the position where the various parts will be located.
- Measure the outer diameter of your grommet and drill a whole large enough to insert it into in the back of the instrument case.
- Drill 3 holes about 1/4" diameter spaced .375" apart on the front of your instrument enclosure. The purpose of this spacing is to allow you to use standard double and triple banana plugs.
- Drill 2 small holes in the bottom of the case spaced appropriately to mount the solder terminal strip.
- Drill 4 holes spaced appropriately to mount the circuit board.
- Drill a hole large enough to insert the fuse holder through the back of the instrument case.
- Drill 2 holes for each of the 2 transformers.
- Insert the power cord through the grommet in the back panel.
- Mount the 2 transformers using appropriate screws.
- Drill a hole on the front panel large enough to mount the switch.
- Mount the 3 single banana sockets to the front panel. Mount the green terminal in the center and the red and black to either side. Use a multimeter to ensure that you have no connection from each terminal to the chassis (metal of the case). Turn the multimeter to continuity (aka beep mode) and listen for a beep as you touch the red probe to the terminal and the black to the case. If you hear a beep, then you have a short. Correct this before proceeding. This is simple when using plastic single banana sockets but shorting can be a problem when you have metal sockets.
- Carefully strip and remove the rubber coating from about 3" from the end of the power cord.
- Solder a spade solder terminal to the green wire from the power cord.
- Connect the green wire spade terminal to one of the screws used to mount the solder terminal strip.
- Solder the black wire from the power cord to one end of the fuse holder. Connect the other end of the fuse holder to one end of the power switch. Connect the other end of the power switch to one end of the terminal strip.
- Solder the white wire from the power cord to the other end of the terminal strip.
- Solder one of the 2 120VAC primary wires from transformer 1 to the terminal of the terminal strip that has the black wire. Solder the other primary wire from transformer 1 to the terminal of the terminal strip that has the white wire. (Which wire connects to the black and which connects to the white is not critical at this point.) Repeat this step for the second transformer.
- Label the rectifiers #1 and #2 so the following terminal decriptions are easy to follow.
- Solder each of the 2 bridge rectifiers to the circuit board. Connect a small wire from the left AC input terminal of rectifier #1 to the left AC input terminal of rectifier #2. Connect a small wire from the right AC input terminal of rectifier #1 to the right AC input terminal of rectifier #2. This is known as a parallel connection.
Solder a wire from the left AC terminal to the terminal of the terminal strip to which you soldered the black wire. Solder the wire from the right AC input terminal to the terminal of the terminal strip to the terminal to which you soldered the white wire.
- Solder a wire from the positive (+) terminal of rectifier #1 to the negative terminal of #2. Solder a green wire to this terminal and a spade terminal to the other side of the wire. Connect this to the same end of the terminal strip to which you connected the green wire from the power cord. This is the ground for the circuit. It will also act as the center for the bipolar supply. warning - it is important that the transformers are not already grounded!!! Please read the documentation that came with the transformer or test it to ensure that it is not grounded
- Connect a wire from the green single banana socket on the front of the panel to the positive terminal of rectifier #1 (which is connected to the neg terminal of rectifier #2).
- Connect a wire from the black single banana socket to the negative terminal of rectifier #1.
- Connect a wire from the red single banana socket to the positive terminal of rectifier #2.
- Label the capacitors 1 through 4.
- Connect the negative terminal of capacitor #1 to the negative terminal of rectifier #1.
- Connect the positive terminal of capacitor #1 to the positive termial of rectifier #1 (which is the terminal that is connected to the negative terminal of rectifier #2).
- Connect the positive terminal of capacitor #2 to the positive terminal of rectifier #2.
- Connect the negative terminal of capacitor #2 to the negative terminal of rectifier #2. The capacitors help to minimize the size of the ripple.
- Label 7812 regulator as regulator #1 and the 7912 as regulator #2. Solder both to the circuit board.
- Connect the common pin of regulator #1 to the common pin of regulator #2. Warning - Refer to documentation to verify which pin is the commmon for each regulator; each can be different.
- Connect the ground of these 2 regulators to the common of the 2 rectifiers.
- Connect the input of regulator #1 to the positive terminal of capacitor #1.
- Connect the output of regulator #1 to the black single banana socket on the front panel.
- Connect the common of the regulators to the green single banana socket.
- Connect the input of regulator #2 to the negative terminal of capacitor #2.
- Connect the output of regulator #2 to the red single banana socket.
- Connect the positive lead of Capacitor 3 to the red single banana socket.
- Connect the negative lead of Capacitor 3 to the green banana
- Mount the circuit board using the standoffs. The purpose of the standoffs is to ensure that you have sufficient space between the back of the circuit board and the bottom of the case. It is important that no metal from the circuit board touches the case or a short has occured.
- Insert the fuse into the fuse holder.
- Turn the power switch off.
- Plug the power cord into a fuse-protected power strip. This step is not strictly necessary but a good precaution in case you mis-wired some of the connections.
- Turn on the power on the power strip.
- Turn on the power on the power supply.
- Use your multimeter to test the voltage outputs.
I hope you find this project useful and fun. Feel free to e-mail me suggestions and comments. Although this project describes building a supply, keep in mind that an old computer is a good source of a power supply. I have purchased old XT/AT computers specifically for the power supply.