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A Pretty Good Wattmeter For Bench
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!
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.
• AC True Watts using two quadrant multiplier
• Optimized for 120 VAC (can be
• 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
I am doing some work with AC line powered switching
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
In my implementation, the wattmeter circuitry is contained in an outlet
and captive leads allow AC voltage input and connection to a digital
The load is plugged into the AC receptacle on the box, leaving an
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
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
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
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.
To start with, National Semiconductor Note #222, July
Super Matched Bipolar Transistor
Pair Sets New Standards for Drift and Noise
Robert Pease's prelude, how to make a shunt for the
wattmeter, What's All This Shunt Stuff, Anyhow?
The Robert Pease article with
improvements, What's All This Wattmeter Stuff, Anyhow?
Another Robert Pease article, this one making it
Youtube video in which Bob Pease explains the circuit
Same video, different URL
Lecomte "Chemelec" in British Colombia,
implementation of the wattmeter
including a printed circuit design and a different
version of the shunt
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
so will mean that the circuitry, including the DVM will be floating
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.
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
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
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.
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.
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
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
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.
I hope that whenever
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.
Contents ©2011 Richard Cappels All Rights
Reserved. Find updates
First posted in June,
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