Homebrew RF Milliammeter

The need for a QRP output meter was felt when the homebrew SWR meters / RF ammeters in the shack proved useless for peaking the output of a homebrew 7MHz 5W solid state CW transmitter. For their pointers barely moved.

Hence the decision to build a RF milliammeter on the same lines as my homebrew RF ammeter.

RF Milliammeter

All parts were sourced from the junk box. BNC sockets were used. The primary of the toroidal transformer is a piece of the inner conductor of RG-58/U coax. The secondary is wound with a length of solid hookup wire. A rubber grommet ensures positioning of the toroid.

Inside the RF Milliammeter

The small 1kΩ, 200μA FSD meter makes the unit quite compact. The enclosure of a defective 230V - 110V autotransformer came in handy, with the meter taking the place of the 110V socket.

RF Milliammeter - Schematic

It was roughly calibrated to read around 250 RF milliamperes maximum. The CW transmitter was then easily adjusted.

Related post: Homebrew RF Ammeter
_______________________________

Power Supply Dummy Load

A 13.8V 25A DC Linear Regulated Power Supply was in need of testing after its defective main rectifier was replaced. It had protective circuits for under-voltage, over-voltage, short-circuit and over-current.

The filaments of a couple of 12V 55/60W automobile headlight bulbs were connected in parallel and used as a dummy load, but their very low cold resistance caused instant tripping.

A length of galvanised curtain wire (~ 1.5m long), connected in series, saved the day. The power supply did not trip and the bulbs emitted a feeble glow.

Power Supply Dummy Load - Schematic

The connecting wire was then slid along the curtain wire. The illumination progressively increased and so did the load current. When the curtain wire went out of circuit, the bulbs were fully illuminated and drawing 23A from the power supply.
_______________________________