[VAC] Re: transformer/converter question

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxxxx>]

I've heard of that slotted strip for the control panel ammeter
calibration. Its a form of shunt. One would calibrate it by connecting a
load through a standard ammeter, and adjust the bolt position so the
control panel ammeter read the same as the ammeter on the load. That
task works best if the load is near full scale on both ammeters. Things
that affect the calibration are the length and size of the wire from the
shunt to the control panel ammeter, and the sensitivity of the control
panel ammeter. Likely there were variations on both so they made the
shunt adjustable. Especially variations between trailers. There's no
fundamental reason the control panel ammeter couldn't read within a few
percent of correct. Good ammeters have been made to be portable and to
hold calibration within 3 or 5%, laboratory meters can hold calibration
to 1% or better, but tend to be large, and fragile. Automotive type
ammeters tend to be crude by comparison and probably only good to a
tolerance of 25 or 30%. But their direction is probably accurate (though
I have one on a tractor where the resting zero position is a couple amps
to the right of the scale zero). In the grand scheme of battery systems,
often knowing that the charger is putting in more than the load is more
essential than knowing what each is to the nearest 1% unless you are
keeping a log to compute the status of the battery charge. That's the
job of Jim's e-meter which is a modern solution to the old problem of
knowing the state of a battery's charge, charger, and loads. Way back in
the early electric automobile days (20s or earlier) a DC watt hour meter
was used with direction friction so it added charging watt hours slower
than discharging watt hours since it takes more watt hours to charge
than you get from the battery. The modern e-meter is more compact and
probably more accurate.

Gerald J.