**Three
Terminal Regulator Circuits**

Three terminal
regulator circuits have been around for many years. They are
one of the most popular choices for power supply regulators circuit in
use today. Easy to use, highly reliable and low in cost are all factors
which have popularized their use. Over the years manufacturers
have created a wide variety of regulators. They range from fixed
voltage and current models to precision adjustable dual output types. The
focus of this article is to highlight the use of the more generic brand
of three terminal regulators; the kind that you are most likely to find
in your local electronics parts store. These regulators tend
to be either fixed output voltage or of the adjustable type. Clearly
many other types exist and if you are planing a project of any size it
would be good to check out the links provided at the end of this article
to explore these types further. For our purposes we will use
some simple design rules to create a reliable power supply that will provide
years of service.

Before we get started I would like
to say that three terminal regulators are not without their shortcomings.
Most significant is that most three terminal regulators are limited
to one maximum current limit value. Another problem often encountered
is that many are designed for a specific fixed output voltage. These
short-comings aside we can design and assemble a reliable, inexpensive
power supply that will meet with most hobbists needs.

**First
Step:**

**Determine
the Voltage and Current Requirements for your Power Supply.**

Essential to any good design is establishing a clear goal early in the project. Nearly all of the components that we will need will depend heavily on what our expectations are for our power supply. I have created three charts for you to use in determining various characteristics for your power supply design.

( Note: Three are provided for those of you that may want a positive, negative and fixed output power supply.)

Power Supply #1 Requirements | ||

Minimum voltage | Maximum voltage | Range Adjustment |

Minimum Current | Maximum Current | Range Adjustment |

Power Supply #2 Requirements | ||

Minimum voltage | Maximum voltage | Range Adjustment |

Minimum Current | Maximum Current | Range Adjustment |

Power Supply #3 Requirements | ||

Minimum voltage | Maximum voltage | Range Adjustment |

Minimum Current | Maximum Current | Range Adjustment |

**Second
Step:**

**Review
the Requirements for your Power Supply.**

In the first step we
formulated a wish list of our requirements so we could develop some ideas
about what we want. Our second goal is to determine what is required
to meet that design criterial. In this phase we will need to
determine if there are any "off the shelf" regulators that meet
our design requirements.

If you indicated fixed voltages and only
maximum currents we can formulate some pretty straight forword solutions
for you design. Shown below is a table of readily available fixed
voltage/current regulators.

If you have indicated a variable output supply
design, follow this link to the variable output design section.

Positve Fixed Output Voltage Regulators |
||

Output Voltage |
Maximum Output Current |
Regulator Type |

+5V |
0.1A or 100mA |
LM78L05 LM340LA-5.0 |

+12V |
0.1A or 100mA |
LM78L12 LM340LA-12 |

+15V |
0.1A or 100mA |
LM78L15 LM340LA-15 |

+5V |
0.5A or 500mA |
LM78M05T |

+12V |
0.5A or 500mA |
LM78M12T |

+15V |
0.5A or 500mA |
LM7815T |

+5V |
1A or 1000mA |
LM7805T |

+12V |
1A or 1000mA |
LM7812T |

+15V |
1A or 1000mA |
LM7815T |

+5V |
1.5A or 1500mA |
LM340T-5.0 |

+12V |
1.5A or 1500mA |
LM340T-12 |

+15V |
1.5A or 1500mA |
LM340T-15 |

+5V |
3A or 3000mA |
LM323K |

+12V |
3A or 3000mA |
Output Voltage |

+15V |
3A or 3000mA |
Output Voltage |

In
the case of negative supplies use the chart below.

Negative Fixed Output Voltage Regulators |
||

Output Voltage |
Maximum Output Current |
Regulator Type |

-5V |
0.1A or 100mA |
LM79L05 |

-12V |
0.1A or 100mA |
LM79L12 |

-15V |
0.1A or 100mA |
LM79L15 |

-5V |
0.5A or 500mA |
LM79M05T LM320M-5.0 |

-12V |
0.5A or 500mA |
LM79M12T LM320M-12 |

-15V |
0.5A or 500mA |
LM7915T LM320M-15 |

-5V |
1A or 1000mA |
LM7905T |

-12V |
1A or 1000mA |
LM7912T |

-15V |
1A or 1000mA |
LM7915T |

-5V |
1.5A or 1500mA |
LM320T-5.0 |

-12V |
1.5A or 1500mA |
LM320T-12 |

-15V |
1.5A or 1500mA |
LM320T-15 |

-5V |
3A or 3000mA |
LM345K |

-12V |
3A or 3000mA |
Output Voltage |

-15V |
3A or 3000mA |
Output Voltage |

Now that we have selected a fixed output voltage we can determine the rough schematic. For multiple output fixed power supplies you have several options and these will be depend on the type(s) of transformers that you have available.

In the circuit shown above we the design is farily straight forward. With A few simple calculations we can find the values for the fuse, transformer, and power supply filters. Please note that a negative supply will be very similar to one above and you may find it by clicking here.

**Third
Step:**

**Preform
some simple calculations to determine what parts we should look for.**

The
easiest way to determine the size of the, filter capacitors, transformer
and fuse is to work from the output back to the input. For the regulators
mentioned we need to know what is often called the "input /output
differential dropout voltage". This is the minimum voltage that
is required "across" the regulator to maintain proper operation.
Component manufactures list this characteristic under a variety of
headings in their data sheets, however it is important to consider is that
all regulators require some voltage drop across them in order work properly. Consider
this their "take" in the process. Typically we see
about a 2-3 volt drop from input to output. Some newer regulators
require as little as 0.5V, however conservative practice has shown that
if you plan things too close, you will probably suffer later. My
suggestion is to consider about a 3-5V differential. So how
does this relate to our project? If you are considering a 12V supply
then the input supply voltage must be a minimum of 3-5 volts higher than
12V ( minimum of 3-5 lower if you are considering a negative supply). Based
on this example theinput voltage must be 16 volts. (Don't worry I
will summarize all of this in a little later).

Now this is the tricky part, depending on
the size of our input filter capacitor we may need to consider a higher
voltage.Shown
in the right is a view of what rectified, filtered DC looks like. Notice
the dark rippled black line at the top, this is how moderately filtered
DC looks. The gray arcs represent the unfiltered pulsating DC that
comes from our diode rectifier circuit. Indicated on the left of
the figure is a bracket that shows what is know as "ripple voltage".
For moderate values of the input filter capacitor some ripple will
be present. It turns out that from a practical point of view
this is a good thing. An input filter that is to small will lead
to excessive ripple that will cause the input voltage to be too low and
the regulator will have a hard time preventing a least some the ripple
from getting to the output. Too large a filter and we will could
find that we place a large strain on our rectifier diodes. Look at
the figure on bottom right, notice the brackets labeled "Capacitor
Charge Time", this is the time when our input filter capacitor is
recharged. If our capcitor is too large we will find that the diodes
have very little time to recharge the capacitor. This short time
period requires the diodes to provide very high currents to the capacitor.
This can lead to diode failure, radio frequency noise, and other
problems resulting in poor power supply performance. In order
to solve this problem we will use a rule of thumb estimate that the ripple
voltage will be no less than 3-5 volts.

To compute the value of capacitance required
for our circuit we will use the following formula.

Using the above formula we
can find the approximate value for our filter capacitor. To use the
formula we must have a value for the load current, capacitor recharge interval,
and the ripple voltage. In all cases we will use approximations.
From our previous example we chose a load current of 1 ampere, for
the capacitor recharge time we will use 0.0083 seconds or 8.3 ms, and finally
for our ripple voltage we will use 4 volts. Our calculation will
yield C = 0.002075 Farad or 2075 micro-Farad (uF). Since this is
an approximation any value around 2000 uF will do.

Some may argue that these values may be too
loose or too tight, however our goal here to keep our design rules simple
and to the point. Let's take a minute or two to summarize what we
have so far.

**1.
Select a output voltage and current that meets our needs.**

**2.
Assume that the Regulator input voltage will need to be at least 3-5V greater
than the output voltage.**

**3.
Assume that the ripple voltage will be 3-5.**

**Now
let's see an example:**

**12
Volts output.**

**+ 4
Volts (input output differential - dropout voltage)**

**+
4 Volts (ripple voltage)**

**Total
20 Volts (peak rectified voltage)**

Now
what does this mean? Let's take another look at that figure on the
rectified DC voltage. Notice that the rectified peak voltage is equivalent
to our

computed total voltage value. The minimum ripple voltage is equivalent
to our regulator input voltage. With this information we one step
closer to determining the transformer voltage that we need. We must
determine the voltage drop of our rectifier circuit and do a simple conversion
from the peak voltage to the equivalent AC voltage to determine the transformer
secondary voltage. For diode bridge assemblies like the one shown
in the schematic we can assume 1.4 Volts drop across the diode assembly.
What this means for us is that our transformer voltage will need
to be 1.4 Volts higher than the peak value. In our previous example that
was 20 Volts. Therefore our peak AC voltage will be 21.4 Volts. The conversion
to an AC equivalent voltage is best explained by understanding that DC
is constant and AC is a constantly changing voltage. For our purposes understand
that during some portions little or no energy is available. Engineers have
determined that the peak AC value is 1.414 times higher than what is known
as the RMS value (Root Mean Square). In short if we compare 100 Volts RMS
to 100 Volts of pure DC the energy content is the same, even though the
AC voltage is constantly changing. What this means to us is that
we need to find what the equivalent AC RMS voltage is for the peak value
that we determined. In the example 21.4 Volts will "be divided
by" 1.414. This gives us an AC RMS voltage of 15 Volts AC. Our
transformer then needs to have a secondary voltage of 15 VRMS. Transformer
selection can be a difficult process, since most suppliers carry a limited
selection. For our purposes if you have to chose between a 12.6V
tranformer and a 18V tranformer, pick the later. If your input voltage
is too low the regulator will "drop-out". This drop-out
will appear as hum on the output and in some cases irractic circuit behavior.
I would however caution against simply purchasing transformer that
are much higher that our calculated transformer voltage. If our transformer
input voltage is too high we can place our regulator in dnager of over-dissaption
(exceeding the device power rating) and over-voltage (exceeding the maximum
input voltage rating). In the next section I will discuss input voltage
limits, power dissapation, and heat sink selection.

**Fourth
Step:**

**Check
for overvoltage and over disappation.**

We
are now closing in on the design. Three terminal regulators are generally
designed for low voltage applications. In oder to insure safe
and proper operation we must check that we are operating within the normal
design limits of the regulator that we have selected. First as a
general rule most three terminal regulators are limited to an input voltage
of not greater than 40V. Several other techniques can be used to
extend this limit and several manufacturers offer high voltage versions
of their three regulators that may meet your needs.

To check for overvoltage simply look at the
calculated peak voltage from the rectifier. In our example this was
20V. With this we must also insure that we do not over dissapate
the regulator. Mounting the regulator to the metal part of
our box will help, but generally this is not enough. First and foremost
we must know a little something about the case style of our regulator.

*Disclaimer:*