Un-Regulated Power Supply
by Martin E. Meserve
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Power Supplies | Un-Regulated Power Supply
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Power Supplies generally
consist of two parts, the AC to DC Rectifier/Filter followed by
the Regulator. The AC to DC Rectifier/Filter,
described here and shown in the diagram below, is kind of like a
pre-conditioner for the Regulator. AC Input voltage is converted
from the power mains (117 VAC) using Transformer (T1),
converted to DC with the Rectifiers Diodes (D1-D4), and then filtered
by the Electrolytic Capacitor (C2).
This web page takes you through the steps of designing the AC to
DC Rectifier/Filter part of
a power supply. It allows you to define some initial information,
like current and voltage required, and then calculates the necesary
transformer specifications, rectifier diode specs, and filter capacitor
requirements. The program basically follows the same structure as similar
to a program in Ham Calc, written by George Murphy, VE3ERP.
It is assumed that, power supplies
designed with the aid of this web page will plug into 117 VAC.
If you are designing for another primary voltage you will need to adjust
the specifications accordingly.
In many cases you may wish to convert to
voltages higher than the input voltage, but this web page is really intended
for working with power supplies that convert to lower voltages. You can still
use this web page for making high voltage power supplies but you won't
be able to use the LM317 Regulator for anything over 37 Volts.
This web page is split into several parts.....
Initial Transformer Calculations section you are asked to
supply an output voltage which is then used to calculate the optimum
transformer for the job. It is assumed that the output voltage, under
no load, will be 1.41 times the full load output voltage. This factor
is used to calculate the regulation percentage and is due
to a Full-Wave Bridge Rectifier being used.
The previous step is likely to compute
a non-standard transformer voltage rating. In the
Transformer Secondary Voltage section you are asked to supply
a more standard output voltage. The voltage specified should be greater
than, or equal to, the calculated value, from the previous section.
A new set of power supply output voltage specifications are then computed.
The output might be higher than originally intended but you can follow
this pre-conditioner with a
LM317 Voltage Regulator to obtain an more exact voltage.
Just make note of the Full Load Output Voltage and the
DC Output Voltage and transfer them to the regulator design.
Then, in the section on
Load Currents you are asked to supply your expected maximum current drain
and your expected DC ripple percentage. The data in the table can be used as
as a rough guide, when specifying the required ripple percentages. It
gives you a general idea of the ripple percentages that can
be tolerated by certain pieces of equipment. This data was extracted
from a program in Ham Calc, written by George Murphy, VE3ERP.
CW transmitter multipliers & amplifiers
Linear amplifier plate voltage
Linear amplifier bias supply
VFOs, speech amplifiers and receivers
0.01% - 0.1%
Non-critical audio devices
1% - 10%
Devices not requiring DC smoothing
10% - 100%
The value of a filter capacitor
to achieve these specifications will be calculated. Almost invariably,
this is going to be a non-standard value. Following the calculations, you
are asked to supply a more standard value and new ripple percentages will
If you don't have a single capacitor
near the calculated value, one or more capacitors can be placed in
parallel to approximate the calculated value. For example, if 10,500 uF is
the calculated value, two 5,000 uF capacitors can be connected in parallel to
make 10,000 uF. This value would then be entered for the standard value.
The Final Design
section presents you with complete design information. A fuse and bleeder
resistor are included for safety purposes and a LED/Resistor combination
is included as a visual power-on indicator.
Initial Transformer/Load Calculations
In the text box below, enter
the required power supply DC output voltage. You do this so that the
web page can calculate the necessary transformer and load specifications.
Required DC Output Voltage (Volts)
The chart on the right lists the Primary/Secondary
transformer voltages based on the specified DC Output Voltage. Then, using your
specified DC Output Voltage, the No Load DC Output Voltage is calculated
along with the Load Percentage. If you
already have a particular transformer output voltage in mind,
than skip over this section and go to the section for specifying the
Transformer Secondary Voltage.
Specifications for a Un-Regulated Power Supply with
x Volts DC Output
Primary Voltage -
x Volts AC
Secondary Voltage -
No Load DC Output -
x Volts DC
Full Load DC Output -
Load Regulation -
Standard Transformer Secondary Output Voltage
In the text box below, enter
the Transformer Secondary Voltage for the transformer that you wish to use.
The voltage entered should be greater than, or equal to, the voltage calculated
in the previous section.
Transformer Secondary AC Voltage
The chart on the right lists the power supply
DC output voltages based on the AC secondary voltage that you specified.
Further transformer specifications will be
in the Final Design section.
Power Supply Specifications using a Transformer with
x Volts AC Output
Load Current Calculations and Standard Value Capacitor
In this section you are asked to define your Maximum DC Current
requirements and the Maximum Ripple Voltage that you require.
Not that, the ripple percentage, presented in this section, is not
the same thing a the load regulation percentage mentioned earlier.
The output of the bridge rectifier is a pulsating DC voltage
and the capacitor on the output is used as a storage device. During positive
transitions of the DC voltage, some of the current goes directly to the
load and some gets stored in the capacitor. In between these positive
transitions, the capacitors provides the current to the load. This
effect smooths the DC voltage waveform. In general, the higher the
value of capacitance, the more energy the capacitor can store and thus,
the less ripple on the output DC voltage waveform.
Use the text boxes, on the right, to enter your Maximum DC Current and
Maximum Ripple Voltage requirements. The text below these boxes will
then reitterate your requirements and the value of the capacitance required
to meet these requirements. The Maximum Ripple Voltage
should be expressed in a percentage. These two values drives the
capacitor value calculation. High DC output current and
low ripple percentages mean that the required capacitance
is going to be big.
DC Output Current (Amps)
Maximum Ripple Percentage (0-100 %)
A DC Output Current of x and
a Maximum Ripple Percentage of x %
would require a Filter Capacitor, C1, of x
uF @ x WVDC.
Now, in the text box below, enter the value of the
capacitor you will actually be using. As previously mentioned, one or more
standard capacitors can be placed in parallel to approximate the calculated
value. Entering a value less than the calculated value will
cause a ripple percentage that is higher than previously
specified, at the specified current.
Standard Value Capacitor (uF)
(No commas please)
Note that the
filter capacitor will not have much effect on the load regulation.
To get better load regulation use the power supply designed here and
follow it with a
LM317 Voltage Regulator. Make sure the the output voltage,
from the power supply on this page is at least 2 volts higher than
final required output voltage.
Final Power Supply Design Data
Listed below is the final power supply design data
based on the information you supplied above. Any changes in the design
requirements will be automatically reflected in the output below.
The parts list below is for a power supply with a no-load output
of x, a Full Load Output of
x and a Ripple Percentage of
Final Parts List