>Bob: Have seen several of your articles, and have never failed to learn
>quite a bit from each one. . . .
Thank you. I'm pleased to be helpful.
> . . . . . I have a T-6G, 28v system, generator with OLD
>voltage reg. The ammeter is a generator-load meter, showing only positive
>amps as the load increases, but not negative amps as the load increases on
>the battery. The scale of the meter is wrong, in my opinion. Rarely draw
>more than 30 amps, yet the meter goes to 150 amps. At normal draw of 5-10
>amps, the needle barely moves off of zero.
Obviously, a poor selection of instrumentation.
>I recently bought a combination voltmeter/ammeter.......the type that reads
>amps until you push and hold a button in order to read volts. Max scale
>is 30 amps. Both the installed meter and the new meter have an external
>shunt. The existing shunt will carry up to 150 amps, with "X" millivolts
>providing full-scale needle deflection. ( I forget what "X" is)
>The new volt/ammeter came with a shunt that is rated at only 30 amps, with
>the same "X" millivolts being full-scale (in this case 30 amps) deflection.
This is good stuff . . and why I'm forwarding this letter to some list
servers I subscribe to. A standard within the instrumentation world is
to design remote shunt ammeters such that 50 millivolts on the
instrument's
terminals will cause it to read full scale. That scale could be 1 amp or
1,000 amps . . . it matters not.
The remote shunt is simply a precision power resistor designed to drop
50 millivolts across its terminals as it passes design current levels.
Hence, take about any remote shunt style ammeter and it will read FULL
SCALE at whatever current the SHUNT is calibrated, irrespective of what
the instrument's scaleplate sez.
>My question is: How can I properly install an ammeter with a scale that
>will let me read the meter, say 0-30 or 0-50 amps? Do I have to change the
>Shunt?
Yes . . . install your new shunt in place of the old one, wire in the
new instrumenet across the shunt (use 5 amp in-line fuses in each
lead coming from the shunt . . . the fuses mount as close to the shunt
a practical).
>If so, does the rating of the shunt have to approximate the rating
>of the generator?
Sure. The original situation you cited illustrated how impractical it
was to monitor the output of a 30 amp machine with a 150 amp instrument.
For alternator load meters, I don't calibrate them in AMPS, just percent
of load. Then, I'll keep a selection of shunts around for the popular
alternator sizes. The pilot's real interest is in how much of the
alternator's
capacity is being taxed. Hence, a meter that reads in percentage of some
shunt value fills the bill. The the builder wants to but in a bigger
alternator
later, he just changes the shunt, the SAME instrument stays in place
on the
panel. Nifty huh????
>Does the shunt have to match the ammeter?
Yes . . . the SCALE PLATE has to have the same full scale reading as the
ampere rating on the shunt.
>Thank you in advance for any advice you might have. And if time doesn't
>permit you to answer these questions, I fully understand.
You're most welcome and than you for the question. We'll make time.
The institutionalized aviation community has held their "black art"
very close to the chest. While ordinary citizens are building their
own byte-thrashing computers from mail-order parts, citizens who own
airplanes are not expected to know or even want to know how they work.
It's time we stopped that and started sharing the knowledge. That's
what these list-servers and our business is all about.
>Peter C. Hunt
>
>"HUNTER"
>Capt. USN, ret. Ex Fighter Pilot, A&P, T-6 Owner/Operator.
>
>
Regards,
Bob . . .
AeroElectric Connection
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(o o)
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