Power Supply / Control / Power protection for HomeBrew HF MOSFET Linear Amplifier

Hello! Been busy continuing work on the 1kW input low-band amp, using cheap switching MOSFETs. Been concentrating on the power supply’s control and protection circuitry. Here’s what I have come up with so far (tested and working!)



As always, click on the image for a larger picture. Though there’s a decent argument for using an embedded micro (e.g. ATMEGA8, etc) as a controller, there’s no good reason why it can’t be analogue, either! So the design is based around a simple, costs-less-than-a-quid, LM339 comparator.

All diodes, unless shown otherwise, are 1N4148. Transistors are not critical. I would suggest any general purpose NPN and PNP types should suffice.

The 100W, 7R resistor (This is a SERIOUS amp — and I wanted an anti-surge, current limiter and low power setting resistor all in one!), the 1kVA power transformer, etc are obviously not on the board.

TUNE mode is activated by biasing Q4 on, which removes the bias from the relay driver transistor (Q2) and ensures the contacts are open: thus the input to the mains transformer goes through the 7 Ohm, 100 Watt resistor which is mounted separately on a huge heat sink.

Knocked up on ‘VeroBoard’ (or some cheap equivalent), the design takes up a small pcb approx. 3 inches by 2 inches. The LEDs are mounted on the front panel of the amplifier case.

The linear amplifier power and control schematic should be easy enough to read. The roles of the major transistors are marked, except for the very obvious.


For decent surge limiting, the 100W, 7 Ohm resistor is actually better placed in series with the PRIMARY of the mains transformer, NOT the secondary as shown. You basically want the 7 Ohm resistor in series with the primary at switch on. Once the comparator circuit sees the caps have charged up (I set my threshold at about 40 Volts), the relay contacts short the resistor, and thus full mains is applied to the mains transformer.