Re: what size shunt do I need?
- From: Larry <noone@xxxxxxxx>
- Date: Sun, 13 Aug 2006 23:15:29 -0400
ondawada <no@xxxxxxxxx> wrote in news:pigvd25tllb7pb09gpt090k41nuq8chphd@
4ax.com:
Cat: 321T
RGE: 50 MVDC
SCL: 0-50 ?ADC (one of the letters is rubbed out, MADC?)
I have a 70A alternator and 220AH house battery capacity. I'd like to
do this myself if I can. I have a small shunt from a 12v battery
charger but I don't think that will work. If the correct answer is
"you need a 50mv, 70A shunt", where do I get one? Lot's of people
selling 100A, 500A etc for solar pannels. Thanks for your time.
If you have a meter calibrated for full scale at 50A to use with a 50A,
50mv shunt, that means at 50A through the shunt it drops 50mv, which is
what the meter movement is looking for. If you use a 100A shunt, it will
produce 25mv at 50A, making the meter read half of what is actually
flowing through the shunt. You simply multiply the reading by 2 to get
the correct current through the shunt. The meter would read 50A when
100A were passing through the shunt.
Your other problem is calibration....my previous life's specialty. If
the meter were calibrated to 50mv full scale and the shunt calibrated for
50mv at 100A, they will read within a few percent of correct. However,
in a cheap meter, that just costs too much to achieve so the manufacturer
takes whatever meter reading he can get from the cheap movement and files
down the shunt, which is always too low in value by design, until the
meter reads correctly, ignoring whatever voltage this happens at, just as
long as it will read the correct current within the tolerance limits
specified. Look at the shunt and you can see the file marks where
someone on the assembly line matched that particular shunt to that
particular meter movement. A cal lab, like the one I worked in for the
Navy, can do this calibration, and does, even on matched 50mv units. So,
using just any old so-called 100A, 50mv shunt with that particular meter
won't guarantee it will read correctly, unless you can know some standard
current and test it like the calibration techs do....caveat emptor
Sure wished they'd let me work on meters and movements....instead of
complex microwave and RF test equipments. Meters are EASY!...(c;
--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.
.
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