1. technology and computing
2. electronic components

Simple Regulated Supply (SRS)
Visit www.mitchelectronics.co.uk for more electronic kits and PCB
fabrication services!
Simple Regulated Supply
MitchElectronics 2015
Page 1
The importance of power supplies!
All electronic circuits require a source of electricity to work. Some examples of power
sources that are commonly used would be batteries while the main supply would be an
other example. But the problem with most power sources is that they are usually
“incompatible” with most devices because most if not all power sources output an
unregulated voltage that may even be constantly changing. For example, as a battery is used
the voltage across the battery slowly decreases. This is bad for most circuits because fixed
voltages are typically needed (such as the 7400 series which needs 5V). So how do we take
a supply of electricity that is changing and output a constant voltage? We can use special
components to create a power converter!
The supply being built in this kit must NOT be connected to an input voltage greater than
24V and the current drawn must NOT exceed 100mA. While this means that the supply
cannot be used with the mains or power current hungry devices it does however make this
kit perfectly safe to experiment with and is perfect for those who wish to practice with or
design power supplies.
The Design – AC rectifier
Below is the schematic of the simple regulated supply. The supply consists of several stages;
AC rectifier, Smoothing + Bleeder, Regulator, Decoupler and Indicator.
The four diodes (D1, D2, D3 and D4), form a very special circuit arrangement called a
rectifier. Its function is to take a supply voltage of any polarity and output the same polarity.
If P1 is positive and P2 is negative then D1 and D4 conduct as they would both be forward
biased. This would result in the cathode of D1 and D3 being a positive voltage and the anode
of D2 and D4 negative. This would result in the cathode of D1 and D3 being a positive
voltage and the anode of D2 and D4 negative. Notice how no matter the input polarity
changes the output polarity of the rectifier does not change.
Simple Regulated Supply
MitchElectronics 2015
Page 2
Rectifiers are also drawn as shown in the schematic below. Notice how that the output DC,
while having a constant polarity is dramatically changing between 0V and the maximum
output voltage. This bumping effect needs to be removed before it can be regulated and this
is where smoothing and bleeding comes in!
The Design – Smoothing and bleeder
Now that the power supply has a voltage with a constant polarity the input voltage needs to
be smoothed. This is because the later stages of the supply require an input voltage that is
above the regulation value (the output voltage). Even though the peek of the input voltage
can be larger than the final output voltage there may also be times when the input voltage is
smaller that the output (as shown in the image above).
The smoother is just a capacitor connected across the output of the rectifier. When the
output from the rectifier increases the capacitor charges and thus energy is stored in the
capacitor. When the output falls the capacitor will discharge and thus put energy back into
the circuit. This discharging will actually raise the voltage and thus stop the voltage from
dropping too quickly. If a large enough capacitor is used and the input supply is consistent
(like the mains), then the voltage will never drop too low (shown below).
Simple Regulated Supply
MitchElectronics 2015
Page 3
The resistor is included for safety reasons and is known as a bleeder resistor. If this supply
was connected to the mains then the smoothing capacitor could easily store up to 300V
across it which would be fatal to any unfortunate soul! When the supply is turned off the
capacitor needs to discharge so that the device does not A) output any voltage and B) make
the device safe to open and service.
To discharge the capacitor a simple resistor (100k is sufficient for our design), is included
across the capacitor. When the supply is disconnected or turned off the resistor will
discharge the capacitor and make the supply safe to service or handle. In our design an
other capacitor is included on the input for removing power spikes. This capacitor is typically
small (100nF), and placed near the regulator.
The Design – Regulation
This is perhaps the most important stage as it takes the somewhat smoothed supply and
converts it to a continuous steady output. The regulator used in this supply is known as a
linear regulator and has three pins; The input, ground and output.
The input takes in a voltage that must be larger than the output voltage. The ground is
ground and common to both the input and output. The output is the regulated supply. The
specific device being used in our supply is the most iconic regulator found (7805), and
outputs a continuous 5V.
Simple Regulated Supply
MitchElectronics 2015
Page 4
T092 Package
Typical 7805 regulator in the typical T0220 package. Note that our 7805 is in a T092 package
The Design – Decoupling and indication
So the supply has been rectified, smoothed and regulated so that should be it right? Well
not just yet because a good power supply designer also considers what the power supply
will be connected to.
The circuit that uses the supply may have high frequency components such as processors or
oscillators and these devices can create noise as well as suddenly draw large amounts of
power. To prevent noise on the power supply lines decoupling capacitors (100nF), are placed
in parallel with the output power lines. To cope with sudden power demands a large
reservoir capacitor is also place in parallel with the output supply as shown below.
The LED and resistor form a simple indicator that lights up when the supply is active. This
can inform the user that the supply is in fact working as well as warning that the input is live.
So that's it! You just learned how to create a basic power supply! So go and build the kit!
Simple Regulated Supply
MitchElectronics 2015
Page 5
The PCB layout
Below shows the component arrangement of the PCB.
Component
Value
D1, D2, D3, D4
Diode (Black)
C1
100uF Electrolytic Capacitor
C2, C3
100nF Ceramic Capacitor
C4
10uF Electrolytic Capacitor
LED
Blue LED
R1
100k Resistor
R2
1k Resistor
U1
LM7805 (T092 Package)
----------------------------------------------------READ THIS---------------------------------------------------• DO NOT CONNECT TO THE MAINS! THIS IS FOR PRACTICE AND SMALL PROJECTS
ONLY
• CAN ONLY SUPPLY UP TO 100mA
• Supply voltage should not exceed 24V
• This can be powered with a battery larger than 5V! And the best part is that it does
not matter what orientation you put the battery in, the rectifier will fix that for you
----------------------------------------------------------------------------------------------------------------------
Simple Regulated Supply
MitchElectronics 2015
Page 6