Introduction: Create a DC Power Supply
How to make a DC power supply using the AC wall voltage!
Step 1: Build a Basic Full-wave Bridge Rectifier
Your circuit should contain:
1. A transformer with a turn ratio of about 6. Connect your transformer to a power strip with an on and off switch for safety.
The voltage from the wall is generally about 120 volts in the United States. This voltage it too large for our ¼ watt resistor to handle, so we will use a transformer to decrease the amplitude of the AC voltage in. We pick a transformer that has an appropriate turn ratio. For transformers, the turn ratio Nprimary /Nsecondary = Vprimary/Vsecondary. Vprimary = 120V and we are shooting for Vsecondary to be about 20V, so our transformer should have a turn ratio of about 6.
2. Four 1N4007 diodes or a bridge rectifier.
Our circuit diagram shows 4 diodes. However, these can also be replaced by a bridge rectifier like the one we used in our circuit in the lab.
How to hook up your bridge rectifier: Connect the legs with the squiggles to the outputs of the transformer, the minus side to ground, and the plus side to the rest of the circuit.
3. A 1/4 Watt 1kΩ resistor.
Important: When building your circuit, make sure that you have the polarities of the diodes correct. ALWAYS TURN YOUR POWER STRIP OFF BEFORE MODIFYING YOUR CIRCUIT.
Your graph of the output voltage should appear to be the absolute value of a sine wave with an amplitude of about 20 volts. This circuit eliminates the negative portions of the voltage. However, the voltage retains a lot of variation. We can add other components to make the voltage less variable.
Step 2: Add a Low-pass Filter
Add a capacitor to your circuit to reduce some variation in the output voltage.
1. Take the previous circuit and add a 100µF capacitor in parallel with Rload.
The capacitor creates a low-pass filter which filters out the lower voltages.
Important: Make sure you get the polarity of your capacitor correct before you turn on your power strip. The negative side should be connected to ground.
Lower voltages are filtered out in the output voltage. However, this circuit retains variation in V_out.
Fun Fact! The ripple in the voltage can be calculated if you have the average voltage.
∆V = ∆t*I / C = (∆t*Vavg) / (R*C)
You can further improve this DC power supply by adding a zener diode or a 7805 Terminal Regulator.
Step 3: Add a Zener Diode
1. Add a zener diode and a resistor to your circuit to create a 5V power source and stabilize the voltage even more.
Add a 5.1 V zener diode (we used a 1N4733A) in series with a resistor, R_zener (we used a 68 Ω and 680 Ω resistor in series). These two components should be added in parallel with the 100µF capacitor.
You should pick an RZener that accounts for the worst-case scenario, which is when the current splits evenly between the load and the zener diode. For a load current of 10 mA, the worse-case scenario is 20 mA through the zener diode.
RZener = V*I = 15V*0.02A = 750Ω at most.
RZener should be a 1/4 W resistor of not more that 750 Ω. We used 68 Ω and 680 Ω resistors in series to obtain an equivalent resistance of 748 Ω (remember that resistances simply add in series).
Important: Make sure that you have the polarity of the zener diode correct before turning on your power strip. The current flows opposite the direction that the zener diode points.
The resultant voltage out should have an average of about 5V and a small ripple.
2. An even better alternative to the circuit with the zener diode is using a 7805 Terminal Regulator in place of RZener and the zener diode.
Remove your RZener and zener diode. Hook up your 7805 terminal regulator in parallel with your capacitor.
How to hook up your 7805:
From left to right, the 3 terminals are your INPUT, COMMON, and OUTPUT.
INPUT should be hooked to the capacitor. COMMON should hook to the ground. And OUTPUT should be your V_out that is measured by your oscilloscope.
You should observe a flat signal of about 5V with no ripples and only some noise.
Congratulations! You've created your 5V DC power supply from the AC wall voltage!