Interference-suppressed radio power supply unit

There are often problems with mains operation of two-way radios, because the power supply is not sufficiently immune to electromagnetic emissions. The effects of an insufficient immunity of the power supply are usually that it comes in transmission mode, in SSB or AM devices also by the modulation, to quite large changes in the output voltage. Unfortunately, the voltage may increase, which even may damage the radio. Frequently, however, a strong drop in the voltage during transmission or modulation is noticed. Incorrectly, it is then often assumed that the power supply is too weak to operate the radio. Here, a power supply circuit constructed with single transistors is shown, in which various measures have been taken to avoid direct influence from the RF emission of the radio. At the same time, the penetration of interference signals via the power network is largely prevented. Especially in the SW area this often results in a cleaner reception.

At first glance, the circuit seems maybe a bit complex circuit. But it contains various parts only needed for suppressing possible RF interferences. The capacitors shorts possibly occurring RF, wich additional is blocked by the coil. For the pure function of the circuit those parts would not be necessary.

The power supply is designed for the usual output voltage of 13.8 volts. However, it can be brought to deviating values with the trimming potentiometer within certain limits. Depending on the desired current output, a stronger type may have to be used instead of the 2SC789. This must always be mounted on a heat sink. Of course, then the transformer and the rectifier accordingly must be calculated for higher current values. In the case of significantly larger values, the smoothing electrolytic capacitor must also be matched accordingly, e.g. 3300 or 4700μF instead of the 2200μF indicated in the schematic.

The circuit is suitable for standalone accessory devices or, as shown here, as part of a mains supplied two-way radio.