own LCD Panel Meters
The meters above display the volts and amps
of my battery bank.
up I bought a couple of those cheap LCD panel meters from
Dick Smith for about $12 each, though most electronic component
outlets will carry then. The meters include a datasheet with
details on changing the voltage range, and moving the decimal
place. My meters were factory set to read 200mV full scale,
but some come ready to measure 20V
Now these need 9 volts to run, but this 9
volts must be isolated from the voltage you are measuring,
ie, you can not measure the meters own power supply. So we
need a floating or isolated 9 volt power supply to power the
meters. Oatley Electronics sell a little isolated power supply
board for just this reason, http://secure.oatleyelectronics.com/files/K212notes.pdf,
and in the March 2007 edition of Silicon Chip magazine there
was a article all about it and using these meters as a volt/amp
meter. But I found a cheaper way.
The old network cards, the ones with the BNC
coax connector, have a 5 to 9 volt isolated inverter on the
If you have one of these old network cards
in the junk box, have a look for the model number on the inverter
and do a google search.
This one is a PM7202, and I found a PDF datasheet
As you can see, these inverters are tough
little buggers, and built to last. So remember next time you
throw away those old computer cards, check for any networkcards
with the inverters on board. In fact there are lot of goodies
on old computer cards and main boards, always have a close
look before dumping.
Now the inverter has a input voltage of
5 volts, so we need to add a 5 volt regulator, such as a LM7805,
and a couple of filter caps. This, with out little inverter,
will give us a isolated 9volt power supply for the meters.
Measuring our battery voltage is easy, we
just connect the meter across the battery terminals. We need
to set up our panel meter to read 20 volts full scale ( or
200 volts full scale if we are using a 24, 48 or higher voltage
battery bank ), and this is done by changing a couple of resistors
on the back of the panel meter. The instruction booklet that
came with the meter should explain how this is done.
To measure current we need to use a Shunt
Resistor. A shunt resistor is a low value resistor placed
in series with the battery bank. And I mean LOW resistance,
like 0.01 ohms. You could use a length of wire as a shunt
resistor, but I used a length of 6mm ( 1/4 inch ) threaded
rod. Using threaded rod means I can adjust the amp meter reading
by moving the connections along the rod.
The two large black wires are the main battery
cable, and they are bolted to each end of the threaded rod.
The smaller black and brown wires lead off to the panel meter
( ignore the blue wire ). I can adjust the brown wire along
the length of rod to calibrate my amp meter.
The rod shown is about 170mm long, and I found
120mm was the magic point to give a accurate reading on the
panel meter. As my panel meter reads 0.2 volts full scale,
and this shunt gives 0.001 volt per amp, then my meter can
read up to 200 amps.
Ok, here's the complete circuit, you can click
on the image to see full size. As my meters were mounted on
the battery cabinet door, I used a length of 4 core cable
between the meters and battery bank to make the installation
easier and neater. Make sure you use a two separate wires
for the shunt -ve and battery -ve terminals, as noted on the
circuit diagram, or the amp meter will not display correctly
( there will be a phantom amp measurement of several amps
if you try to use one wire as a common -ve ).
A bit of nostalgia.
old meter was once a db audio meter from the old Telecom Telephone
exchange I worked at many years ago. I saved it from the rubbish
bin, back then Telecom, been a typical government agency,
had a stupid policy of destroying anything it threw out. So
any electronics had to be smashed before it was dumped. Like
I said, stupid. Anyway I saved this meter because it was very
old and I liked the look of it. It then spent several years
in a box in my workshop, waiting for a use worthy of its age.
I eventually decided to turn it into a amp
meter. I designed and lasercut a timber box from 9mm plywood,
gives a nice aged look. Then I took the meter apart to get
access to the meter scale and mechanism. The meter scale was
scanned into PaintShop using a flat bed scanner, a new scale
drawn up to read amps, printed out and glued over the old
scale. By scanning in the old scale, I made the sure the new
scale would fit exactly into the meter case. I then adjusted
the meter movement to convert the meter into a centered meter,
the meter would originally stop at the left. With careful
adjustment of the springs I made the meter center, and modified
the zero screw on the front case to adjust the center position.
It was a fiddly job, and one slip would have stuffed the meter
for good. To finish off the meter I added a shunt resistor
made from a strip of 2m zinc plate, a couple of trimpots,
and a rotary switch to select between +-10 and +-100 amps.