what size shunt do I need?

Ahoy, what size shunt (can I make one?) do I need for this gauge?
Specs:
D.C. Ammeter - range to 75 amps
Triplett Corp.
Hendricks Electric Inc.
Sticker on the back says:
EIL
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;

 

I initially read your name a No Logic which is appropriate as there is
no logic to your reply!

I have a 60A alternator which happily fully charges my 330AH domestic
set under my normal cruising pattern.
Charging batteries is dependent upon three parameters, the voltage,
the current available,and the time,availabl,e saying that a 70A
alternator will not charge fully is nonsense without knowing the other
two parameters!

Richard

Nb "Pound Eater" Parkend G+S

 

Where do we FIND these guys??!! A 10A battery charger, hell even a 2A
battery charger will charge a 660AH bank of beasts! It just takes
longer....

The slower you charge them...the BETTER they charge as the chemical
reaction to charge them is a slow process, itself.

As long as his load is less than 70A, which I'm sure it's much less, those
little 220AH batteries will charge just fine.

NOONE CAN CHARGE A LEAD ACID BATTERY IN 20 MINUTES NO MATTER HOW BIG THE
ALTERNATOR IS! The chemistry is just too slow for this old wives tale.

Geez.....

 

So is my meter calibrated for 50A or 75A? I'm still confused about the
specs on the back of the gauge. SCL- what does that mean? The meter
reads 0-75 amps on the face but the SCL is 0-50 something or others (I
can't quite read the lable; I think this thing's from WWII.)

Ok, Mouser Electronics has base mounted shunts for $20.
100A/50mv,.. a resistance of 0.0005 ohms or,.. a 50A/50mv unit, R of
0.001 Still not sure which of these is right for me.
There doesn't seem any way to open the gauge but is there any chance
there could be a shunt inside it already? The +/- lugs have 12 gauge
wires snipped off. (someone has marked, "good" on the back).

Sure wish I'd stayed in school,.. Thank you very much.

 

You have all the information you NEED right there in your POST...
Your meter needs a 50mv DC Shunt, to read correctly and the meter
is a 0-75Amp DC Scaled 50 ma DC Meter.
If you would like to check my information, call Paul Hendricks
at Hendricks Electric, located at Fishermans Terminal in Seattle,
Washington USA.

Tell him "Bruce in alaska" sent you..... he'll know who you mean.....

 

From WW2? Neither. It's not calibrated at all....

At the rated current, its meter terminals have the rated voltage, in this
case 50 mv across them. .0005 ohms is VERY hard to measure, in possible
with two terminals at that current level. The only way we measure it is
with a calibrated, steady current and calibrated millivoltmeter.

If there is a shunt inside the meter case, the terminals would have to be
HUGE to handle 75A through the internal shunt. Checking that is easy.
Put an ohmmeter across the terminals on the meter case. If it pins or
moves at all from the ohmmeter's tiny current level, it has no internal
shunt and you'll see the resistance of the meter movement, much higher
than something near zero ohms.

The wire size may sound like it has an internal shunt, but the ohmmeter
test above will prove one way or the other. If it came from military
applications, they'd put wires on it heavy enough to stand the torpedo
hit so it would still work. These are the same military bureaucrats
buying special IC chips to sense atomic attacks, with the mentality that
the airplane they're in will still function if we protect the memory of
the computer from atomic EMP pulses...a silly notion.

 

The 100A shunt can be filed down between the meter contacts to increase its
resistance so it will output 50mv at 75 amps, even calibrated for this
particular old meter movement. All you need is a 100A calibrated current
source and a rat tailed file...(c; Been there, cal'd that. I calibrated
one for a circuit breaker tester a long time ago. It was 50,000A at 50mv!
It was used as the sensing shunt for a huge circuit breaker tester that
failed calibration. I fixed it. You have just GOTTA see a 50,000A circuit
breaker trip in milliseconds when that huge tester pulses it! It sounds
like a bomb going off!....(c; The bus bars to the breaker jump right off
the floor! Must be magnetic pull against the Earth's field...(c;

Some things a cal lab tech gets to do are lots more fun than others....(c;

 

Ah, would that Hendricks were still there... but alas, it is closed.

bob
s/v Eolian
Seattle

 

Ah shit.... and another Good Guy hits Retirement .... Best Wood/Freeman
Autopilot mechanic in the business.....

Bruce in alaska