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SIMPLE POWER METER WITH DUMMY LOAD
By Rahul Srivastava, VU3WJM

The design is based on a kit sold by Elecraft. The RF voltage developed across 50ohm resistor combination is rectified and measured using a multimeter. Cheap digital ones from China are highly suitable.

Dummy load consists of series parallel combination of 32 nos 100 ohm 1W carbon resistors. (Don’t substitute a higher wattage wire wound one). This gives us a non inductive 50 ohm load. A germanium diode from the centre tap of combination gives us potential for power measurement. Original design was capable of measuring 20W max..

With many of us using MOSFET based linear at 30 plus voltages slightly more power handling was required. This design can handle 35-40W for short durations. DC potential developed at TP1 can measured on a multimeter and corresponding power could be derived from look up chart given below.

Alternatively power can be calculated from the formula:

Power = ((Volt at TP1 X 1.414) + 0.2) sq / 50

0.2V is taken as general value for the diode drop it will vary from device to device. For silicon diodes like 1N4148 it will be 0.7V.

Schematics:

Watt Meter - Schematics

Watt Meter - Look up Table
Look up Table

The circuit is simple enough to be taken up by anyone. It can be assembled over a piece of scrap copper clad PCB material. Care should be taken to keep component leads to minimum. For purists I am also including a PCB layout.

The accuracy of meter is within 10% and it should work beyond 2 meters. No Bird this one but good enough.

PCB

Watt Meter - PCB
PCB Design

Watt Meter - Component Layout
Component Layout

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