building the regulator

8/6/2019 Building the Regulator

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Building the Regulator

An electrolytic capacitor

To build the regulator, you need three parts:

A 7805 5-volt voltage regulator in a TO-220 case (Radio Shack part number 276-1770)

Two electrolytic capacitors, anywhere between 100 and 1,000 microfarads (typical Radio

Shack part number 272-958)

The 7805 takes in a voltage between 7 and 30 volts and regulates it down to exactly 5 volts. The

first capacitortakes out any ripple coming from the transformer so that the 7805 is receiving a smoothinput voltage, and the second capacitor acts as a load balancer to ensure consistent output from the

7805.

The 7805 has three leads. If you look at the 7805 from the front (the side with printing on it), the three

leads are, from left to right, input voltage (7 to 30 volts), ground, and output voltage (5 volts).

To connect the regulator to the transformer, you can use this configuration:
http://electronics.howstuffworks.com/capacitor.htmhttp://electronics.howstuffworks.com/capacitor.htmhttp://electronics.howstuffworks.com/capacitor.htm


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The two capacitors are represented by parallel lines. The "+" sign indicates that electrolytic capacitors

are polarized: There is a positive and a negative terminal on an electrolytic capacitor (one of which will

be marked). You need to make sure you get the polarity right when you install the capacitor.

You can build this regulator on your breadboard. To do this, you need to understand how a

breadboard is internally wired. The following figure shows you the wiring:

On the outer edges of the breadboard are two lines of terminals running the length of the board. All of

these terminals are internally connected. Typically, you run +5 volts down one of them and ground down

the other. Down the center of the board is a channel. On either side of the channel are sets offiveinterconnected terminals. You can use your volt-ohm meter to see the interconnections. Set the meter's

dial to its ohm setting, and then stick wires at different points in the breadboard (the test leads for the

meter are likely too thick to fit in the breadboard's holes).

In the ohm setting, the meter measures resistance. Resistance will be zero if there is a connection

between two points (touch the leads together to see this), and infinite if there is no connection (hold the

leads apart to see this). You will find that points on the board really are interconnected as shown in the

diagram. Another way to see the connections is to pull back the sticker on the back of the breadboard a

bit and see the metal connectors.

Now connect the parts for yourregulator:

1. Connect the ground wire of the transformer to one of the long outer strips on the breadboard.

2. Plug the 7805 into three of the five-hole rows.3. Connect ground from the terminal strip to the middle lead of the 7805 with a wire -- simply cut

a short piece of wire, strip off both ends and plug them in.

4. Connect the positive wire from the transformer to the left lead (input) of the 7805.

5. Connect a capacitor from the left lead of the 7805 to ground, paying attention to the polarity.

6. Connect the 5-volt lead of 7805 to the other long outer terminal strip on the breadboard.

7. Connect the second capacitor between the 5-volt and ground strips.


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You have created your regulator. It might look like this when you are done (twoviews):

In both of the above figures, the lines from the transformer come in from the left. You can see the ground

line of the transformer connected directly into the ground strip running the length of the board at the

bottom. The top strip supplies +5 volts and is connected directly to the +5 pin of the 7805. The left

capacitor filters the transformer voltage, while the right capacitor filters the +5 volts produced by the 7805.

The LED connects between the +5 and ground strips, through the resistor, and lets you know when the

power supply is "on."

Plug in the transformer and measure the input and output voltage of the 7805. You should see exactly 5

volts coming out of the 7805, and whatever voltage your transformer delivers going in. If you do not, then

immediately disconnect the transformer and do the following:

Pull out the capacitors. Plug the transformer back in for a moment and see if that changed

anything.

Make sure the ground wire and positive wire from the

transformer are not reversed (if they are, it is likely the 7805

is very hot, and possibly fried).

Make sure the transformer is producing any voltage at all by

disconnecting it and checking it with your volt meter. See

the previous page to learn how to do this.

Once you see 5 volts coming out of the regul ator, you can test it

further and see that it is on by connecting an LED to it. You need to

connect an LED and a resistorin series -- something that is easy to do on your breadboard. You must

use the resistor or the LED will burn out immediately. A good value for the resistor is 330 ohms, although

anything between 200 and 500 ohms will work fine. LEDs, being diodes, have a polarity, so if your LED

does not light, try reversing the leads and see if that helps.

It might seem like we've had to go to a tremendous amount of trouble just to get the power supply wired

up and working. But you've learned a couple of things in the process. Now we can experiment with

Boolean gates!
http://electronics.howstuffworks.com/digital-electronics3.htmhttp://electronics.howstuffworks.com/digital-electronics3.htm


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building the regulator

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