Hands-On-PhysicsELECTRICITY

- Low Voltage Power Supply -

The voltage from a household outlet is NOT in a safe range, as you know if you've ever been shocked by it. A low voltage power supply (LVPS) can be built which produces voltages in a range safe for experimentation. The power supply you build here will be useful in later projects.

A transformer is used to reduce household voltage to a tenth of it's strength, and the LVPS circuit converts this lower voltage from AC (alternating current) into DC (direct current). The circuit also has a control section so you can change the output voltage by twisting a potentiometer (pot). Volume controls and dimmer switches also use potentiometers.

Figure C2
LVPS circuit sketch

The LVPS built on a breadboard looks quite complicated. However, if you build it in steps and test each step, it is really rather simple.

Building the circuit

Build the circuit diagrammed below. Notice that the top bus is used for the positive side of the circuit. People generally use red wires there. The bottom bus is the negative side of the circuit, usually wired in black. In the middle a third colored wire is used. This lets someone else proofread and understand your circuit more easily.

Figure C3a
LVPS circuit diagram

Build and check your circuit in stages, from left to right (or input to output). For example, first plug in the transformer and read it's output with your multimeter set to the scale for ac voltage (VAC).

Figure C3b
Transformer

As you build GET A FRIEND TO PROOF READ YOUR CIRCUIT. You may also want to have your teacher check it before plugging it in.

Start building the LVPS by putting the rectifier on the breadboard. You can use short lengths of bare wire to make arches so the clip leads from the transformer can be easily attached. (See sketch at the top of this page)

Connect the transformer to the rectifier and measure the rectifier output, using both the VAC and VDC scales.

Figure C3c
LVPS rectifier

Now add the voltage regular along with the potentiometer and resistor. This part of the circuit controls the size of output voltage. The voltage regulator heats up, so it needs a heat sink attached to it to dissipate this excess heat. A heat sink is a piece of metal, cut so it has a lot of surface area for getting rid of heat. You can use a manufactured heat sink or make your own. Binder clips makes good heat sinks.
Test the output with your multimeter. Does twisting the pot change the voltage?

Figure C3d
LVPS voltage control

Finally, add the capacitors and test the output of the LVPS once again. It's important to wire the capacitors in the correct direction, so double check.

Figure C3e
LVPS with capacitors

You can use short lengths of bare wire to make arches so the clip leads can be easily attached to the output. (See sketch at the top of this page)

See if it all works. Hook your multimeter up to the output, and set it for DC Volts. Adjusting the pot should give you a variety of readings. If the pot is turning the wrong way to adjust the voltage up, switch the pot leads in the circuit board.

If it works first try, congratulations!! You're unusually good.

If it doesn't, don't panic. Check your connections and proof read your circuit once again. Then check for any places the wires are touching where they shouldn't (a short). Then try it again. If it still doesn't work ask your teacher for help. Either you made a mistake or one of your components isn't working.

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