# Create a DC Power Supply

How to make a DC power supply using the AC wall voltage!
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## 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.

PS1181 year ago
Wow! I remember using that exact breadboard back in highschool!

Memorieeeeeees.... pressed between the pages of my mind...
pfred21 year ago
Usually I see an output filter capacitor used with fixed 3 terminal voltage regulator ICs. If you include one of those it may increase the stability of your current. I also usually use a much larger input filter capacitor too. I mean just look at the size of the capacitor I used in this power supply:

http://www.instructables.com/files/orig/F25/WU6P/GSUSNEZL/F25WU6PGSUSNEZL.jpg

OK perhaps that is a bit of an extreme example. Even in this power supply I used larger capacitors though. I also run off both sides of the bridge network too to have a dual positive negative supply:

http://www.instructables.com/id/Dual-POS-NEG-Power-Supply/

On the negative side of the bridge positive gets connected to the ground because ground is higher potential.
toofat2serve1 year ago
You don't necessarily need the charger or hub. The amount of current applied to the load (whatever you are charging) is determined by the load itself. An iPhone, for instance, has a charging circuit inside that limits current to 1A. An iPad limits its charging current to 2A. The Amp rating for a given voltage on the PSU is the maximum that the PSU can supply, not a constant rating.

Remember that Current is proportional to Voltage and inversely proportional to Reisistance, so

I = E / R

Since your E is a constant (regulated) 5 volts, the Resistance, or your load, will be the one that affects the amount of current.