While the transformer testing
process is outside the scope of this
article, the conditions for OC and
SC testing bare directly. OC
testing looks at full voltage, low
input current, and no-load current
(open secondary) conditions.
SC testing looks at low
voltage, high input current, and
no-load voltage (shorted secondary)
conditions. The Kill-A-Watt has
the potential to provide all of the
needed for both OC and SC
testing, except for one problem. It is designed to operate
at a full AC voltage (nominally 120V). If this voltage drops
below about 60 VAC (say via a variac autotransformer
Figure 1), the simple internal DC supply for the
microcontroller begins to fail, and the power meter
displays erroneous results.
OC tests work fine as full voltage is used, but for SC
testing only enough voltage is used to drive the
transformer to the rated input current. This may require
only 10%-20% of full voltage, and the stock power meter
will not work with such a low input voltage.
This article describes a simple modification that safely
powers the meter for accurate operation in this low
Open the Meter
The lead photo on the previous page shows the Kill-A-Watt EZ model 4460 front. Notice it is not plugged in!
Nothing inside is isolated. Three screws on the back will
separate the covers and provide access to the two internal
circuit boards. Figure 2 shows the inside of the top with
the AC board separated.
We will make no changes to this printed circuit board
(PCB). The controller board is to the right and does not
need to be removed from the case. The microcontroller
(not visible from the top side) requires a standard 5 VDC
power supply. Derived in part from a dropping resistor on
the AC PCB, it fails when the input AC is too low. Though
the internal supply uses a 78L05 voltage regulator already,
we will add another regulated, switchable, and battery
powered supply for the low AC region of operation.
The schematic of Figure 3 shows the simple
LM78L05-based supply which is powered through a
pushbutton from a 12V "N" size cell. The output diode
(D2) provides isolation and the ground diode (D1)
compensates for the output diode voltage drop. The entire
circuit fits on a small perf-board so that the only
"permanent" modification is drilling a hole in the top of the
back cover for the pushbutton actuator.
Make the Mods
In Figure 4, there are two wires visible (red and black)
connected to the Vcc (+5V) and digital ground for the
controller. Insure that your K-A-W PCB matches the layout
July 2015 23
■ FIGURE 1. A typical
Photo courtesy of
■ FIGURE 2. The
internal PCBs: low
voltage PCB on the top
right;AC section on the
■ FIGURE 3. Schematic and parts for modification circuit.
■ FIGURE 4. Power connections.