3. Uses surface-mount
components to reduce size.
Circuit Design
The first item to select was
the LCD voltmeter. Digi-Key listed
three models of Martel’s low-cost
V-Series voltmeters for under
$30. Let’s look at the specs: +-
200 mv full scale, with auto-zero
and auto-polarity. User selectable
decimal points. Three samples
per second. Power requirements
are 7-12V at a paltry 1 mA. Three
sizes of numerals are available:
0.25”, 0.50,” and 0.60”. I chose
the 0.50” size so I could read it
across the bench.
Unfortunately, these meters
have a downside. They require an
isolated power supply, like a 9V
battery. You are not allowed to
connect the minus (or plus) of
the power supply to either input,
i.e., a common ground, due to
the V-Series’ internal circuitry.
There are tricks you can use —
like plus and minus supplies —
but that’s usually not practical. One nifty solution
would be to employ an isolated DC-DC converter,
which I’ll discuss later on.
Now … what about battery life? A quick search of
the Internet for the specs on a 9V Duracell Coppertop
showed a life of 250 hrs with a 2 mA load and 7V
cutoff. The Martel unit only uses 1 mA which means
the service life would be approximately 500 hrs.
Therefore, if it was used for four hrs/day, we could
expect a lifetime of 125 days. Sounds good.
Then, I had a brainstorm. When I’m checking out a
circuit, I seldom need to read the voltage or current
continuously; a brief look now and then is usually
enough. This would also extend the battery life. So, I
added the auto-turnoff circuit shown in Figure 3. In
operation, each time the button is pushed, it charges
the 10 mF capacitor, turning on the FET and LCD.
When the button is released, the one meg resistor
slowly discharges the cap until it falls below the Gate
Threshold Voltage (VGS(th)) of the FET and turns off
the LCD.
Since the FET was being used as a switch, I
wondered how much current — if any — might still be
flowing when it’s turned off. Would it drain the battery?
A check of the spec sheet showed a maximum Zero
Gate Voltage Drain Current (IDSS) of just 1 µA — not a
problem.
■ FIGURE 2. The new self-contained in-line Wall Wart Power Monitor displays
readings either continuously or at the touch of a button.
I double-checked the auto-turnoff circuit on a
solderless breadboard to make sure my calculations
and assumptions were valid.
This brings us to the DC-DC converter that I
mentioned earlier. The idea was to use a small amount
of power (1 mA) from the wall wart to power the LCD.
The biggest problem was that the input voltage could
vary from 3. 3 VDC to 24 VDC, which is a major stretch
for most DC-DC converters.
So, I poured over the catalogs looking for an
isolated converter with just the right mix of specs. To
make a long story short, the design became way too
complicated. It looked like I would need to add a diode
bridge (to handle positive and negative pins), pre-regulator, small heatsink, DC-DC converter, and raise
the minimum input voltage to 5V. Too messy. The KISS
principle won the day. Battery power was the best
choice!
Bells and Whistles
Just when I thought the design was pinned down, I
realized there were times when I wanted to leave the
monitor on for a half hour or so, not just 15 seconds.
For example, when I’m checking out a GPS module.
So, I added a new switch for “continuous” operation,
designated as SW4 in Figure 3. I chose a center-off
February 2011 43
