Free Electronic/electric Circuit diagram for many electronic project, electrical project and electromachanical.
Circuit Project Description
The circuit was designed to create a DC power supply that will be
rated with 13.8 V and 20 A while having a current limiter
and short circuit protection.
TerminologyField Effect Transistor (FET) – used for
amplifying weak signals by controlling the current and the shape of an electric
field where the flow of current or the conductivity of material is only through
a single type of semiconductor materialDiode Bridge – also known as
bridge rectifier which has four diodes arranged in a bridge configuration where
the output voltage has the same polarity with either polarity of the input
voltageBC547 – NPN small signal transistors designed for general purpose
switching and amplification due to its low voltage, low current and three
different gain selectionsBC557 – PNP general purpose transistors used
for amplification and switching due to its low current and low voltage2N5683
– high-current complementary Silicon power transistors designed for use in
high-power amplifier and switching circuit applications BD330 – PNP
power transistor with high current and low voltage, used in power switching and
amplification, especially in portable equipments7812 – 3-terminal 1A
positive voltage regulator with short circuit protection, thermal overload
protection, output transistor safe operating area protection, output current up
to 1A, and output voltages from 5V to 24VCircuit Explanation
By producing 13.8 Volts at around 20 Amps, the power
supply unit is suitable for ham radio transceivers that are often receiving
currents. A separate 15 mA to 20 A current limiting output has been added for
lower currents. To achieve the desired voltage of 13.8V, the power transformer
should be proficient enough to provide 17.5 V to 20 V at 25 A. Low power
dissipation is the result of low voltage supply. Capacitor C1 smoothens the
current form the diode bridge rectifier. To do this, the value of the capacitor
should not be less than 40 uF or 50 uF. This is very true to the fact that 20 uF
should be provided per ampere. It is not necessary to position a higher value
capacitor, instead a parallel combination of smaller capacitors can be used to
achieve the desired value.
The foundation of the circuit design is focused around the 7812 IC 12V regulator.
The desired value of 13.8V can be calculated using the two attached resistors
R5 and R6 with the formula: U=12(1+R5/R6). For the 7812 regulator to
function properly, a low current of 15 mA should be maintained in the circuit.
In the event that the current exceeds 15 mA, R4 will receive the voltage drop
from the IC. This will cause Q3 2N5683 to open, which manages the high
output current. As a PNP transistor, the factor for current amplification is at
least 20.
A current limiting circuit is applied on this design with a resistance of
0.03 ohms for the maximum output of 20 A and a power rating of at least 15 W.
To create such circuit, several resistors may be connected in parallel or a
resistance wire that will give a total of the preferred resistance and power
value. To compute for the resistance, the formula is used: RL=0.7/Imax. A short
circuit automatic fuse can be created from the combination of RL and Q2,
a 3A PNP BD330 which will be opened by the voltage drop across the
resistor RL upon reaching the maximum current of 20 A. this will limit the
base-emitter current of Q3. To provide an adjustable current source for smaller
currents, an adjustable current limiter is placed in parallel to the fixed
output.
LED1 will shed light with the aid of Q1 which is connected in parallel with
Q2 while LED2 lights up in every switching ON of the PSU. IC1 can only be
damaged with a full current flowing through it. This is due to the bridging of
R3 by Q2 when the fuse is active. To prevent this from occurring, resistor R4
is included to limit the current of IC1 to 15 mA. The addition of resistor R4
will avoid IC1 from heating up, thus reducing the need for cooling aid.
The construction of the circuit is very simple that it does not require any
current sensing resistor. But the same functionality exist in the form of the
Rds-on resistance of the N-channel FET. It is responsible for handling the load
cutoff from the source. The diagram 2 helps show how the FET functions. It
behaves as a resistor but after a knick, it functions as constant current
source. The knick is produced during the fast rise of the tension Uds across
the resistance Rds due to the rising of current Id. The Uds voltage of the FET1
is detected by the R3, D2, and base-emitter connection for the Q4. The Q4 will
stop the current flow through FET1 and provide shortcut for FET1 gate to mass
and if the voltage rises enough. But then again, a certain voltage gate is
necessary to enable the FET1 to open. This gate voltage will be supplied by the
voltage divider which consists of R8, R9, Z1, and P1. With this, the minimal
voltage will be around 3.6 V and the maximum gate voltage will be that of the
Z1 which will identify the maximum current flowing through the FET1. As shown
in the diagram, the current at 20 A should have around 9.6 V and for 5 A the
Uz1 should be 5.6.
The reaction time or velocity of the limiter will be determined by the
capacitor C4. A reaction time of around 100 ms will be made possible by the
addition of 100 uF while adding 1 nF will make a reaction time of 1 us. The
current output is limited in the range of 15 mA to 20 A with the operation of
P1. The value of the RL will limit the total output current, although both
outputs can be used simultaneously. If the transformer can handle higher
current requirements, the PSU can de designed for higher outputs. This time Q3
will require enough cooling.
Application
The power supply unit having a fixed value of 13.8 VDC output and
continuous running time of 90 mins is ideal for application with citizen’s
band (CB) radio, auto equipment, and communications transceivers. It has
short circuit and overload protection. It is also used for testing car audio
amplifiers and other communications equipments since it has extremely low noise
and ripple and excellent regulation.
