Re-engineered Power Supply


I have owned a Daiwa PS-12M for some years with no problems but recently it failed twice with the same fault - the output power transistor going open circuit and the second time it caused an 80W HF PA to fail at the same time. It was therefore time to do some major changes to improve its reliability.

The initial solution was to remove the internal PCB and design a new one that would hold increased smoothing capacitance and use an L200 regulator IC driving a pair of TIP3055 transistors with current sharing resistors mounted on the rear heatsink. Unfortunately I could not make it short circuit proof and the output devices failed twice using both NPN and PNP configurations so I went back to a tried and tested design using a 723 regulator and NPN current amplifier.The original mains transformer, bridge rectifier, and indicator LED were retained. Despite the output device failures the L200 still worked perfectly.

The new circuit is shown below based on information in the 723 data sheet and previous well used designs.

The original smoothing capacitance was increased from 5 x 3300uF to 6 x 4700uF and a new current and voltage DVM (DSN-VC288 10 amp version) was installed instead of the analogue meter. RF decoupling was provided along with preset short circuit protection. The first current amplifier has a small PCB mounted finned heatsink to ensure adequate cooling. One 1N540x (300 amp peak rating) provides protection in the event of the output voltage holding up longer than the input voltage to the regulator e.g. if a battery or large electrolytic were to remain connected to the output when the PSU was switched off and the other provides reverse polarity protection to cater for back emfs from inductive loads like electric motors, solenoids and relays.

The finished power supply provides an output voltage of 3 - 15V and the current limit is set at 12A.

The output fuse will blow if: