Wattmeter Clamp Modification

       The idea for this mod started as a brain fart that made sense and might actually work so I set out to try it out.

       I sometimes have a need to measure watts but inserting the wattmeters used by RC modelers are kind of invasive since it has to be inserted inline to measure current.

       I have a cheap-ish AC/DC current transducer that is used to interface with oscilloscopes for viewing current waveforms. The output is then used instead of the wattmeter shunt so it is logged and displayed by the RC wattmeter.

The cheap wattmeter taken apart to be modified.

The current measurement is done by three 3mOhm shunt resistors in parallel on the negative side.

This is the original schematic.

The pullup appears to be used for nulling the input DC offset so it displays zero with no load. Or so was the plan. In reality, the cheap device fluctuates from 0.00A to 0.02A without any load connected.

The modified schematic.

The original input range is 1mV for 1A current. The transducer has 10mV per amp and 100mV per amp ranges. The former is used and a voltage divider to match the voltages.

The pull up resistor is increased in value to increase the overall input impedance of the circuit to avoid loading the current transducer output. The zero null trimmer is used to adjust zero display at zero load. The gain trimmer is used to calibrate the display for correct current indication.

The parts on the PCB are changed to suit the modification.

I tried fiddling with trimmers to get the required values.

Testing the mod to see if it works.

A DC clampmeter is used to approximate the measured current. The "conductor" is a 20 turn coil to multiply the 3Amp max current output of my bench PSU so I can test up to 60A simulated load current.

Tweaked the zero null and gain pots to achieve zero display at no current and the same reading at CC mode on the PSU.

At low currents, it appears to be fine. This is not meant to be a very precise instrument.

I plan to use it for estimating loads and power consumption and recording peak current, power and energy.

For more accurate measurements, I intend to do it the old fashioned way. Use several meters and calculate the readings.

Looked around my scrap boards for slimmer multiturn trimmers that will fit in the space and hot glued them into place.

Retesting it after assembling. Looking good...

But then the clampmeter moved. So the displays are a little off. But that is to be expected as DC clampmeters are generally not that accurate because of a lot of variables so I did not bother to readjust it.

I was later able to make it more accurate by carefully tweaking the null and gain trimmers several times as the adjustments are inter-dependent.

Testing it on the home RE system. The line measured is the input to the DC distribution busbar for lighting and small DC loads.

The BNC-binding post adapter is only temporary as can be seen below.

Here it is with the more permanent BNC connector solution.

Mounting depth was a problem so it was mounted externally.

A view from another angle.

Here it is displaying the power from the small PV array going to the grid tie inverter.

       It works as expected but stability and accuracy is not the best. I can live with that since the error is acceptable for my intended use. I will use different methods and better equipment if higher accuracy and precision is needed but that would be slower and/or bulkier so this would be perfect for those quick checks or power estimation etc.

Page created and copyright R.Quan © 26 Nov 2015.