A simple and having stable voltage and amperage regulation plays an important role to power up an amateur radio station which when it operates generate leakage Radio Frequency power to the ambient area within the station location. Even if, precautions have been made to reduce the RF leakage, chances are there are still possibility that an RF leakage situation might be present within the installation of an amateur radio station. Therefore, a reliable DC 13.8 Volt power supply must be used to provide a safe operation which is free from voltage fluctuation due to RF interference to the reference circuit of a power supply being used. Of course, there are available power supply in market that suits the standards to be used to power an amateur radio station equipment. But if one wants to build a reliable power supply that has good voltage, amperage and free from RF interference then this article may be used to the needs.
Here are the schematic and a photo which shows an example layout of components of the reference section of the power supply which employs LM723 voltage regulator unit.
Figure 2. Components layout of LM723 voltage regulator
The schematic employs current sensing and current limiting features of the LM723 through pin 2 and 3 as depicted in the schematic. The maximum current is sensed and limited based on the characteristics of Ohm law, V = I x R. By using 0.1 Ohm / 5 W resistor at each emitter of 2N305, the maximum allowable amperage rating of each 2N3055 transistor is limited to approximately 6 Amperes. The maximum rating of 6A is less than the maximum amperage rating of 2N3055 which is 15 Amperes. This is to ensure that each final transistor is only loaded about less then 50% of its maximum rating. This will ensure a safe operating for the final transistors.
If there is more than 6 Amperes current flows through 0.1 Ohm resistor which is connected to pin 2 and 3, the LM723 will cut-off its output at pin 10 which is used to drive Q1 2SD313. This is then will turn off the Q1 and the finals transistors. This protection will shutdown the power supply once each transistor is driven to supply more than 6 Amperes. This current limiting is based on Ohm law by which V = 6 A x 0.1 Ohm = 0.6 Volt. The current limit of the LM723 is set to work once the voltage difference between pin 2 and 3 exceeded 0.6 Volt.
The final transistor must be installed on appropriate heat sink to prevent overheating. In addition, to prevent voltage drop, ensure that a proper size of cable is used (i.e., cable with diameter greater than 2.5mm). This diameter cable size is recommended to wire the plus – minus rails, secondary winding of transformer to the bridge diode, and the collector and emitter pins of the final transistors. Furthermore, in reducing voltage drop, the wiring layout for the parts of the circuit which carry high current must be designed as short as possible.