All you need to know to make
regulated power supply . . .
3-terminal voltage regulators
regulator is one of the most common components to be added to a project.
It's the heart of what we call a "built-in" power supply. It
allows the project to operate from almost any type of voltage. It can be AC or
DC and any voltage (within prescribed limits). The voltage can come from
batteries, a plug pack or a transformer. The only other components that need to
be added are diodes, a few capacitors and electrolytics and the power-supply
section of a project is complete. The voltage regulator has made the
designing of a power supply a relatively simple task.
However, before we take the designing too simply, there are a number of features
and facts that must be taken into consideration.
Here are three pointers:
1. The power supply section must be "tight." In other words, the track-work
for the power supply must be short so that the capacitors across the regulator perform
their function of removing parasitic oscillations.
2. The regulator must be heatsinked so that you can hold your finger on it for
at least 30 seconds or so.
3. The input voltage must be at least 5v above the output so that the diode
bridge can have at least 1.5v across it and regulator at least 3.5v across
it (so it can perform its job of regulation).
Here are the two most popular types of voltage regulator. The only difference is
the current capability:
1. The 78L05.
This is a +5v, 100mA device that looks exactly like a
2. The 7805. This is a
+5v, 1 amp device.
There are also higher-voltage regulators such as: +6v, +8v, +9v, +10v, +12v, +15v,
+18v, +24v, as well as negative voltage regulators, adjustable voltage
(+1.2v to +37v) and high current
regulators (5 amp, 10 amp). The performance of these devices can be created by using the 7805 with
additional components. See below.
The most important fact to remember is the different pin-outs for the positive
and negative types. This prevents an incorrect type
being fitted to a PC board and damaging the rest of the circuit. Basically,
the Input and Ground are reversed on the two devices, but because all the
regulator have different pin-outs, you should refer to a diagram before
Note: The Ground terminal is also called the
3-TERMINAL VOLTAGE REGULATORS
||+1.2V to +37V
for a positive 5v regulator is shown below:
The 7805 connected to
heatsink with thermal grease.
There are four important points to remember when designing a power supply:
1. The regulator must be close to the edge of the board so it can be attached to
2. The heatsink must also be attached to the PC board so that is does
not move and allow the leads of the regulator to break.
3. The 2200u and 100n must be mounted close to the regulator.
4. The input voltage must be higher than the output to allow for the
voltage drop across the bridge and regulator. The minimum for a 5v output is 7v
AC or 9v DC.
Note: It does not matter which way around the AC or DC supply is connected to
the bridge as the bridge will automatically adjust for either polarity. This is
one of the advantages of placing a bridge on the input of a project. It allows
either-way connection to power the board.
The diagram below shows a 7805 regulator circuit built on breadboard. This
is not the same circuit as shown above as the 100n in the output has been
replaced with an electrolytic. This open-type arrangement is only suitable
for a very low output current as the 7805 is not heatsinked and it needs to have
short leads between the output and a 100n monoblock to prevent it from oscillating
A 7805 "test
circuit" mounted on breadboard
The circuit above can be fitted onto a small PC board using a
W04 1.5 amp bridge, a 2200u electrolytic, a 10u electrolytic and an indicator
LED with 220R dropper resistor. The circuit for this is shown below.
A 7805 power supply module
The 7805 regulated supply
with a LED
Here's an example of how not
to draw the diodes in
The wonderful part of electronics is its universal nature.
Imagine if every country had their own colour code for resistors! The same
applies to laying out circuit diagrams. Stick to the universally accepted way of
presenting "building blocks." The diagram above takes quite a while to
work out that the diodes are actually forming a standard bridge. By merely placing
them on an angle (as shown in the first circuit above) you can see that all the
diodes are facing the one direction. This allows you to put a single diode in
the centre of the diamond to represent the bridge.
The output voltage of a supply can be increased by "jacking up" the
voltage produced by the 7805. The way the 7805 works is this: It maintains a voltage
of 5v between output and common terminal. If the voltage on the common terminal
is increased (jacked up), the output voltage will be 5v higher. The 7805 always
maintains 5v between output and common. The
circuit below produces an output of 12v.
1 amp 12v Power Supply Circuit
Almost any voltage
between 5v and 30v can be obtained by this method. This saves stocking the
complete range of regulators.
The output voltage is determined by two resistors in VOLTAGE DIVIDER MODE. Five
volts is always present across the 120R resistor and if another resistor is
placed in series, it will have a proportional voltage across it. In the circuit
above, 7v is developed across the 180R resistor, making a total of 12v on the
To increase or decrease the voltage, only one resistor has to be changed in the
circuit above. The 120R is retained and the 180R is changed. If it is increased to 220R, the output voltage will be 14v, for a 330R, the output voltage will be 18v. The resistor in the common line
can be a potentiometer. This will produce an adjustable output
voltage. The dropper resistor for the LED will also have to be
increased so that the LED is not over-driven on the higher voltages.
A meter can be placed on the output to monitor the voltage and current
taken by the load.
There is only one problem with an adjustable supply. The regulator
must be heatsinked so it is capable of dissipating the heat for the worst condition. In
addition, the input voltage must be sufficient to cater for the maximum
ADJUSTABLE OUTPUT VOLTAGE
The output voltage can be adjusted (varied) from 5v to 24v via a potentiometer
connected to the common line of the regulator. The input voltage and
heatsinking of the regulator must be sufficient for the output voltage and
current. The output may not
deliver more than 100mA @ 5v due to the heat produced by the regulator if the
input voltage is say 24v - 36v. See above discussion.
5v to 24v Power Supply Circuit
The circuit below is dangerous! Do not use it. The output of a 3-terminal regulator will jump to full output voltage when
the selector switch is changed from one voltage to another. If the input voltage
is 36v (for 0-24v power supply), the output will rise to 36v or more as the
selector switch is changed from one position to another. This is because one set
break before the next contacts are connected and thus the common terminal will be FLOATING
during this short interval of time. Any project being powered by the supply may
be instantly damaged!
The layout on this board could be improved
The photo above is an example of how not
to layout a PC board.
The pot has been placed in the centre of the board and this makes it difficult
to get to the connections, if you wish to add an external pot. In addition, the pot chosen for
the project needs a small screwdriver and is difficult to adjust. Always use a
pot with a small shaft, so you can adjust it with your fingers, and it can
have a screwdriver slot as well. Axial electrolytics are more
expensive than radial types and take up a lot more board-space. Axial
electros went out with the Ark. If you want your PC board to look modern, use
only electros that stand-up.
If any power supply is supplied with a voltage more than 4v above the output voltage, this
is called an
OVER VOLTAGE and can cause
most serious will be overheating of the regulator. Most circuits do not
require a steady current and thus it is impossible to give an absolute size
heatsink for a particular application. The simplest way of determining the
size is to be able to hold your hand on it almost
constantly. If you have to let go, the regulator is getting too hot.
The heat it dissipates is simply a matter of working out the voltage across the regulator multiplied
by the current flowing. In most cases the current flow will fluctuate and the
value of heat dissipation
will be very complex and
beyond the scope of this discussion.
However, the only thing we can say is the need to keep the input voltage
as low as possible (providing it is above the minimum stated above). Every
volt above this minimum is wasted as heat in the regulator and if 1 amp is
flowing, the waste will be approx 1 watt.
Sometimes you can connect a 12v 1 amp plug pack to the power supply, drive
a particular load, and the regulator will get fairly warm. If you then
connect another 12v 1 amp plug pack, the regulator will get extremely hot!
The second plug pack may actually produce a higher voltage. The
12v rating is only an arbitrary value and the actual no-load output may be
15v to 18v. The manufacturer allows the voltage to drop to 12v on full
load. This is called "regulation" and applies to
"transformer regulation." This is one of the hidden problems you
may have to take into account when determining the size of the
heatsink. That's why the only way to check the project is to feel the
The 7805 has an internal thermal shutdown when the temperature reaches a
high value. It is obviously important that the regulator does not reach
this condition, however if the circuit shuts down, the regulator will cool
and turn on again. In this case the regulator will not be damaged.
The regular also has an internal short-circuit detector. If the output is
shorted, the regulator will shut down.
If you require more than 1 amp, the 7805 can be
combined with other components to provide an output of up to 3 amps, with
the circuit below. The current is
switched through the TIP2955, so the 7805 can be run without a heatsink since it
only regulates the voltage. Note the 3-amp diodes in the power supply.
For currents greater than 3 amps, additional TIP 2955 transistors can be
"piggy-backed" on top of the TIP in the circuit. If the gain of each
transistor is approximately the same, the transistors will current-share the
load and get equally hot.
A 3amp diode (1N 5404) compared with
a 1 amp diode (1N
A negative supply
can be produced with a 7905 voltage regulator. Three things that
have to be remembered are:
1. Check the connections of the 7905 before fitting it.
2. Reverse the 2200u and 10u electrolytics and LED.
3. Reverse the bridge so that the negative goes to the "in" pin
of the 7905.
regulated supply circuit
This just about
covers the intricacies of the power supply. Almost every project
needs a power supply and provided you adhere to the rules above, it
will be a simple task to add a supply to a corner of your project
and produce a reliable design that will not need any further attention.
I can remember, "in the old days," the power supply was always giving
trouble. TV's had electrolytics that dried out, diodes became
open-circuit, voltage-doubling electros failed and switch-mode power
supplies blew up for no apparent reason.
It was the Japanese, with their quest for improvement, that created
transistors with improved reliability and higher voltage ratings.
Also, IC's (in the form of voltage regulators) simplified the power
supply to a single device.
That's how things have changed.
A voltage regulator has the capability of smoothing a 3v ripple to less than
1mv. This is an improvement of 3,000:1. At the same time it is capable of
delivering 1 amp.
A link to a Dual
Five more Power
Supplies with some helpful notes from Bill Bowden.
A variable 0.7v to
24v Power Supply with adjustable current 50mA to 2 amp using discrete
components - ideal as a bench supply.
A very clever
circuit to convert a meter from 0-24v to 0-2amp.
circuit. The advantage of an LM317 regulator is the supply will go down
to 1.2v. If you add two diodes (in series with the output line), the voltage
will go down to 0v.
The University links that were tried, did not go through or were .pdf files
and were very messy to search. However this should be sufficient to cover the
ONE FINAL TIP
If it is at all possible, keep your power supply to 1amp rating. This is the
cheapest and easiest to implement. Once you go over 1.5 amp, the power
transformer becomes expensive and bulky and impossible to get as a "plug
pack." (A plug pack is a transformer tightly- housed in a plastic box
with two or three pins mounted in the plastic so that the whole assembly can
be plugged into a power point. Some of these are rather large and bulky and
only allow one power-point to be used, when a dual power-point is available.
However the plug pack is the safest and best way to get 12v to 15v @
The latest release is a switch-mode plug pack. The model I purchased was 12v @
3amp and weighed only 1/10th the weight of a 12v 1.5amp plug pack. The cost
was double the cost of a 12v 1.5 amp plug pack but when you work it out, the
cost is about the same on a performance-basis and occupies only one outlet. The pack runs
totally cool as it is more than 85% efficient. This is the way the plug pack
is going. There is also a 5 amp version. This beats the transformer
The next stage in power supply design will be switch-mode.
ONE MORE . . .
There is one more amazing trick you can perform with a 3-terminal regulator. It
can be wired to produce a CONSTANT CURRENT output. See the next page.