Tuesday, January 11, 2011

DC Power Supply with 13.8V and 20A


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.


  • Field 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 material
  • Diode 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 voltage
  • BC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selections
  • BC557 – PNP general purpose transistors used for amplification and switching due to its low current and low voltage
  • 2N5683 – 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 equipments
  • 7812 – 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 24V

Circuit 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.


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.


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