Types | Dual supplies |
Transformer | Rectifier |
Smoothing | Regulator
Next Page: Transducers
Also See: AC and DC | Diodes |
Types of Power Supply
There are many types of power supply. Most are designed to convert high voltage AC mains
electricity to a suitable low voltage supply for electronics circuits and other devices.
A power supply can by broken down into a series of blocks, each of which performs a
For example a 5V regulated supply:
Each of the blocks is described in more detail below:
Power supplies made from these blocks are described below with a circuit diagram and
a graph of their output:
- Transformer - steps down high voltage AC mains to low voltage AC.
- Rectifier - converts AC to DC, but the DC output is varying.
- Smoothing - smooths the DC from varying greatly to a small ripple.
- Regulator - eliminates ripple by setting DC output to a fixed voltage.
Some electronic circuits require a power supply with positive and negative outputs
as well as zero volts (0V). This is called a 'dual supply' because it is like two
ordinary supplies connected together as shown in the diagram.
Dual supplies have three outputs, for example a ±9V supply has +9V, 0V and
The low voltage AC output is suitable for lamps, heaters and special AC motors.
It is not suitable for electronic circuits unless they include a rectifier and a
Further information: Transformer
Transformer + Rectifier
The varying DC output is suitable for lamps, heaters and standard motors.
It is not suitable for electronic circuits unless they include a smoothing capacitor.
Further information: Transformer | Rectifier
Transformer + Rectifier + Smoothing
The smooth DC output has a small ripple.
It is suitable for most electronic circuits.
Further information: Transformer |
Rectifier | Smoothing
Transformer + Rectifier + Smoothing + Regulator
The regulated DC output is very smooth with no ripple.
It is suitable for all electronic circuits.
Further information: Transformer |
Rectifier | Smoothing |
Transformers convert AC electricity from one voltage to another with little loss of power.
Transformers work only with AC and this is one of the reasons why mains electricity is AC.
Step-up transformers increase voltage, step-down transformers reduce voltage.
Most power supplies use a step-down transformer to reduce the dangerously high mains
voltage (230V in UK) to a safer low voltage.
The input coil is called the primary and the output coil is called the secondary.
There is no electrical connection between the two coils, instead they are linked by an
alternating magnetic field created in the soft-iron core of the transformer.
The two lines in the middle of the circuit symbol represent the core.
Transformers waste very little power so the power out is (almost) equal to the power in.
Note that as voltage is stepped down current is stepped up.
The ratio of the number of turns on each coil, called the turns ratio,
determines the ratio of the voltages.
A step-down transformer has a large number of turns on its primary (input) coil
which is connected to the high voltage mains supply, and a small number of turns
on its secondary (output) coil to give a low output voltage.
| turns ratio = ||Vp
|| = ||Np
||power out = power in |
||Vs × Is = Vp × Ip|
|Vp = primary (input) voltage
Np = number of turns on primary coil
Ip = primary (input) current
||Vs = secondary (output) voltage
Ns = number of turns on secondary coil
Is = secondary (output) current
There are several ways of connecting diodes to make a rectifier to convert AC to DC.
The bridge rectifier is the most important and
it produces full-wave varying DC. A full-wave rectifier can also be made from
just two diodes if a centre-tap transformer is used, but this method is rarely used
now that diodes are cheaper. A single diode
can be used as a rectifier but it only uses the positive (+) parts of the AC wave to
produce half-wave varying DC.
A bridge rectifier can be made using four individual diodes, but it is also available in
special packages containing the four diodes required. It is called a full-wave rectifier
because it uses all the AC wave (both positive and negative sections). 1.4V is used up in
the bridge rectifier because each diode uses 0.7V when conducting and there are always two
diodes conducting, as shown in the diagram below. Bridge rectifiers are rated by the maximum
current they can pass and the maximum reverse voltage they can withstand (this must be at
least three times the supply RMS voltage so the rectifier can
withstand the peak voltages). Please see the Diodes
page for more details, including pictures of bridge rectifiers.
Alternate pairs of diodes conduct, changing over
the connections so the alternating directions of
AC are converted to the one direction of DC.
|Output: full-wave varying DC|
(using all the AC wave)
Single diode rectifier
A single diode can be used as a rectifier but this produces half-wave varying DC
which has gaps when the AC is negative. It is hard to smooth this sufficiently well to
supply electronic circuits unless they require a very small current so the smoothing
capacitor does not significantly discharge during the gaps.
Please see the Diodes page for some examples
of rectifier diodes.
|Single diode rectifier
||Output: half-wave varying DC|
(using only half the AC wave)
Smoothing is performed by a large value
electrolytic capacitor connected across the
DC supply to act as a reservoir, supplying current to the output when the varying DC
voltage from the rectifier is falling. The diagram shows the unsmoothed varying DC
(dotted line) and the smoothed DC (solid line). The capacitor charges quickly near
the peak of the varying DC, and then discharges as it supplies current to the output.
Note that smoothing significantly increases the average DC voltage to almost the peak value
(1.4 × RMS value). For example 6V RMS AC is rectified
to full wave DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this
increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.
Smoothing is not perfect due to the capacitor voltage falling a little as it discharges,
giving a small ripple voltage. For many circuits a ripple which is 10% of the supply
voltage is satisfactory and the equation below gives the required value for the smoothing
capacitor. A larger capacitor will give less ripple. The capacitor value must be doubled
when smoothing half-wave DC.
C = smoothing capacitance in farads (F)
| Smoothing capacitor for 10% ripple, C =
||5 × Io |
|Vs × f|
Io = output current from the supply in amps (A)
Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC
f = frequency of the AC supply in hertz (Hz), 50Hz in the UK
Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable output
voltages. They are also rated by the maximum current they can pass. Negative voltage
regulators are available, mainly for use in dual supplies. Most regulators include some
automatic protection from excessive current ('overload protection') and overheating
Many of the fixed voltage regulator ICs have 3 leads and look like power transistors,
such as the 7805 +5V 1A regulator shown on the right. They include a hole for attaching
a heatsink if necessary.
Please see the Electronics in Meccano
website for more information about voltage regulator ICs.
a = anode, k = cathode
Zener diode regulator
For low current power supplies a simple voltage regulator can be made with a resistor
and a zener diode connected in reverse as shown in the diagram.
Zener diodes are rated by their breakdown voltage Vz and maximum
power Pz (typically 400mW or 1.3W).
The resistor limits the current (like an LED resistor). The current through the
resistor is constant, so when there is no output current all the current flows
through the zener diode and its power rating Pz must be large enough to withstand this.
Please see the Diodes page for more information
about zener diodes.
Choosing a zener diode and resistor:
output voltage required is 5V, output current required is 60mA.
- The zener voltage Vz is the output voltage required
- The input voltage Vs must be a few volts greater than Vz
(this is to allow for small fluctuations in Vs due to ripple)
- The maximum current Imax is the output current required plus 10%
- The zener power Pz is determined by the maximum current:
Pz > Vz × Imax
- The resistor resistance: R = (Vs - Vz) / Imax
- The resistor power rating: P > (Vs - Vz) × Imax
- Vz = 4.7V (nearest value available)
- Vs = 8V (it must be a few volts greater than Vz)
- Imax = 66mA (output current plus 10%)
- Pz > 4.7V × 66mA = 310mW, choose Pz = 400mW
- R = (8V - 4.7V) / 66mA = 0.05k
choose R = 47
- Resistor power rating P > (8V - 4.7V) × 66mA = 218mW, choose P = 0.5W
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© John Hewes 2007, The Electronics Club,