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A Pretty Good Wattmeter For Bench Use
This is a pretty good half wave wattmeter that display true watts.
Its output is a DVM floating on the power line, so be careful!



The wattmeter indicating a load of 15.08 watts.
The DVM is on the 200 mv scale;
10 mv = 1 watt.
You can barely tell from the photograph that the glow
of a Neon lamp is visible through the peep-hole in
 the lower right-hand corner of the meter.


Briefly,
• AC True Watts using two quadrant multiplier
• Optimized for 120 VAC (can be changed)
• 15 watt full scale (can be changed)
• Uses DVM floating on AC Neutral as display
• Requires moderately high level of analog circuit skill
• Very inexpensive

Find updates at www.projects.cappels.org

Introduction

I am doing some work with AC line powered switching circuits and needed a way to measure the true power consumed by the circuit.  This little circuit, which seems to have first surfaced in a  circuit by Carl Nelson in National Semiconductor application note #222 in 1979, and has been updated by twice in print and once in video Robert Pease at National Semiconductor.



In my implementation, the wattmeter circuitry is contained in an outlet box
and captive leads allow AC voltage input and connection to a digital voltmeter.
The load is plugged into the AC receptacle on the box, leaving an additional
outlet for the voltmeter or a second load. Notice that the banana plugs are the
"safety" types with sheathed contacts so that there is little exposed metal.

The circuit it self is a simple two quadrant multiplier and as such, it only calculates the load power based on one half of the power cycle. This is not a problem for most loads, which draw the same current wave form on both half cycles. For the rare case in which the load draws different current wave forms in the two halves of the power line cycle (assuming single phase power lines) one merely needs to read the power, reverse the plug, read the power, and then average the two readings.

I built one, a single range device that measures up to 15 watts with a resolution of 10 milliwatts, using a DVM with 100 microvolt resolution on its 200 millivolt scale. Actually, it can read 20 watts with a slight of linearity. The meter can be modified for other ranges by changing the shunt resistor. A multirange version is described in the references.

Most of what you need to know about this has already been published, so before going into my own observations, here are the references, or you can just jump to THE CIRCUIT.

REFERENCES

To start with, National Semiconductor Note #222, July 1979
Super Matched Bipolar Transistor Pair Sets New Standards for Drift and Noise
www.national.com/an/AN/AN-222.pdf

Robert Pease's prelude, how to make a shunt for the wattmeter, What's All This Shunt Stuff, Anyhow?
http://electronicdesign.com/article/articles/what-s-all-this-shunt-stuff-anyhow-2144.aspx


The Robert Pease article with some circuit improvements, What's All This Wattmeter Stuff, Anyhow?
http://electronicdesign.com/article/test-and-measurement/what-s-all-this-wattmeter-stuff-anyhow-2190.aspx

Another Robert Pease article, this one making it multi-range, RAP's Multirange Wattmeter
http://electronicdesign.com/article/test-and-measurement/rap-s-multirange-wattmeter2191.aspx


Youtube video in which Bob Pease explains the circuit and discusses improvements
http://www.youtube.com/watch?v=mkLCs0dBX8E
Same video, different URL
http://video.google.com/videoplay?docid=-8509961694948158452#

Gary Lecomte "Chemelec" in British Colombia, implementation of the wattmeter
including a printed circuit design and a different version of the shunt resistor.
http://www3.telus.net/chemelec/Projects/Watt-Meter/Watt-Meter.htm




The Circuit

WARNING: LETHAL VOLTAGES
 NOT FOR BEGINNERS



WHEN BUILDING, TESTING, AND USING THIS INSTRUMENT, THE USER CAN COME INTO 
CONTACT WITH LETHAL VOLTAGES. THIS INSTRUMENT IS ONLY TO BE BUILT 
AND USED BY PERSONS INSTRUCTED IN WORKING WITH LETHAL VOLTAGES,
AND THEN ONLY WHEN NOT UNDER THE INFLUENCE OF ALCOHOL, SEDATIVES,
ANTIHISTAMINES, OR OTHER DRUGS OF FACTORS THAT CAN AVERSELY AFFECT
ALERTNESS. ALL CONSEQUENCES OF MAKING AND USING THIS INSTRUMENT
ARE THE SOLE RESPONSIBILITY OF THE USER. YOU HAVE BEEN WARNED.




Use a polarized AC power plug for the input and be aware of the polarity of the power plugs.
The large prong on the power plug corresponds to AC Line NEUTRAL. Get this right because
doing so will mean that the circuitry, including the DVM will be floating near ground
on AC neutral, rather than on the "HOT" AC line.


The circuit on this web page is very similar to that given by Pease, the difference being that I added a 330 uf averaging capacitor across the output, as well as a 5.19k resistor across the output to reduce gain.

I found it necessary to use multi-turn potentiometers for both the zero and the span adjustments. The photograph shows only a single turn pot for the zero adjustment, but my experience was that a single turn pot did not provide sufficient setability.

I agree with one author who noted that a fuse, particularly on the input, would be a good idea.

Assembly

The circuitry was hand-wired onto a phenolic circuit board. Some of the copper pads on the other side of the board were removed to assure at least 5 mm of clearance between AC line voltage and other metal on the board (including unconnected pads).  Construction of the copper shunt is described on this web page.





Notice that in this photograph, the zero set pot is only a single turn pot. It was later replaced with a multiturn trimmer.
Also notice that the matched pair of 2N3904 transistors is only covered with green fingernail varnish and a nylon wire
tie at the time of the photograph. See the photograph below. Also note that the shunt resistors shown is not the 0.1 ohm shunt
eventually used.


This photograph was taken after wrapping copper foil around the matched pair of 2N3904 transistors.
Wrapping the foil around the transistors helped reduce the drift. I placed the nylon wire tie lower on
the board with the intention of reducing the transistor's leads' exposure to circulating air.

The matched pair of 2N3904 transistors was made according to instructions by Robert Pease, documented at this link on my website:
http://www.cappels.org/dproj/xstrmatch/Transistor_matching.html

The circuit showed considerable offset drift until I had wrapped the matched pair in copper foil and placed the whole assembly in the outlet box, with a piece of foam rubber between the circuit board and the lid, serving the dual purpose of providing thermal insulation and keeping the board stable in the box.


Calibration                                                                     

Carl Nelson's originally published circuit used fixed value components for the span (gain) and only had an adjustments to zero the circuit. Robert Pease added a span adjustment and provide a means of calibrating the circuit on the workbench using DC power supplies. I endorse this method.

My bench supply was not able to provide the full scale RMS input voltage, so I chose to calibrate while connected to the AC line. I used a ground fault circuit interrupter, an Isolated Adjustable Transformer, a Switchable Resistive Load and a True RMS Voltmeter adapter.

Plug the isolate variable transformer into the AC mains through the ground fault circuit interrupter (with the input power switch off), then plug in the wattmeter, and plug the wattmeter output into a voltmeter set to its 200 millivolt scale. Connect the true RMS voltmeter to the output of the wattmeter, and switch on the power to the isolated transformer. Adjust the variac for 120 VAC and let the thing sit for five or ten minutes to make sure everything has stabilized.

After the circuit has stabilized, adjust the zero adjust pot to zero the output. Again, let the circuit set for a few minutes to see if it wants to drift to a new quiescent operating point. Once satisfied that the circuit has settled, zero the output again if needed, then go onto the span calibration.

Connect a resistive load. The load should be approximately 960 ohms to provide a 15 watt full scale load at 120 VAC. Note the reading on the true RMS voltmeter and calculate the actual power into the load (P = (IxI)/R, but if you didn't know that already, stop and consider whether this project is at an appropriate level for you.) Adjust the span pot until the reading on the DVM corresponds to the calculated power. Remember that 100 micro volts = 10 milliwatts, so 15.000 watts, for example, would be displayed as 150.0 millivolts.

The resistive load that I used is switchable, using 960 ohm arrays of resistors in series and parallel combinations to provide a 15 watt resistive load at both 120 VAC and 240 VAC. This allowed me to check to see that power reading dropped to 1/4 of that in the 120 volt position when I switched to the 240 volt position, thus assuring that linearity is adequate.

At this point, you might want to remove the load, let the circuit settle a little bit, then readjust the zero setting, allow further stabilization, and check the full scale calibration.  When all that is done, you are finished with the calibration.

When calibrating the instrument, keep the circuit free from drafts. I wrapped the circuit board in a piece of cloth, exposing only the necessary adjustment each time an adjustment was made.


While calibrating the instrument, be careful to not come in contact with the circuitry. Even though I was using an isolation transformer, adjusting this circuit is very dangerous. There is still 120 VAC across the circuit, and I would not bet my life on all of the safety features working properly. Your most powerful safety feature is between your ears -THINK BEFORE EACH STEP and ALWAYS KEEP ONE HAND IN YOUR BACK POCKET.

   
Operation

I hope that whenever you work with circuits and appliances powered from the AC line, you are careful about electric shock. With this power meter, you need to be doubly cautious because the output signal is floating on the power line. If the wall plug is inserted correctly, everything will float on AC neutral. Otherwise, it will float on HOT conductor and this is pretty dangerous and might even affect accuracy because of the circuit's capacitance to ground.


1. Set a battery powered digital voltmeter (DVM) to its 200 millivolt scale then connect it to the wattmeter.

2. Plug the wattmeter into the AC power source and allow it a few minutes to assure that the circuit has stabilized.

3. Plug the load or device under test into the electrical outlet on the output of the wattmeter and note the power displayed on the DVM.

If there is an appreciable offset when after the wattmeter stabilizes after applying power but before connecting the load, note the offset and subtract it later. This should have little or no effect on the power measurement once the offset has been subtracted.


Very sad note: While preparing this web page on 24 June, 2011, I learned of the passing of Robert Pease in an automobile accident on after leaving a memorial for Jim Williams who had died just over a week prior. This makes June, 2011 a very dark month for the analog community.  I think the article in the link below summarizes the great losses.
http://realityandreason.blogspot.com/2011/06/jim-williams-and-bob-pease.html


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Contents ©2011 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in June, 2011

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