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Jeff Wisnia wrote:
> Ken wrote:
> > Jeff Wisnia wrote:
> >> Arcane question, and sort of long, but here goes....
> >>
> >> Last night, young son was in the process of adding foglights to his
> >> car and asked me for some wire to extend the harness. I wasn't sure
> >> what the current draw would be so I grabbed one of the foglight
> >> assemblies and connected it to the Eico 1050 battery charger/DC power
> >> supply which has been part of my garage tool clutter for nearly 50 years:
> >>
> >> http://home.comcast.net/~jwisnia18/temp/eico1050.jpg
> >>
> >> When I cranked the voltage up to 13, the bulb lit brightly and the
> >> ammeter on the Eico read around 9 amps. That seemed sort of high to me
> >> so I asked the kid, "How big a fuse is in the harness which came with
> >> those lights?" he looked at it and told me it was a 15 amp fuse. 15
> >> amps for TWO 9 amp bulbs? Obviously something was't right, so I got my
> >> Simpson 260 and saw that the current drawn by that bulb was really a
> >> bit less than 5 amps.
> >>
> >> Since SWMBO was out getting some "retail therapy" I had some free
> >> time, and taking the Eico into my workshop, I opened it up,
> >> disconnected the leads to the ammeter and fed it with my bench supply.
> >> That verified again that it was reading almost twice the DC current
> >> passing through it.
> >>
> >> By a couple of "cut and trys" I found that about 4 inches of 18 gage
> >> solid copper wire shunting the meter made it read correctly enough for
> >> "gummint work", so I soldered that wire in and closed the Eico back up.
> >>
> >> I believe the ammmeter is what I used to know as a "moving iron" type,
> >> and IIRC the restoring force was supplied by some kind of permanant
> >> magnet field, not by a mechanical spring. Am I right about that?
> >>
> >> I doubt that Eico installed a defective meter when they built the unit
> >> around 1965 (The date marked on the meter.) and I'm guessing that the
> >> meter's restoring magnet weakened over 50 years, increasing its
> >> sensitivity to nearly double.
> >>
> >> Anyone have any similar experience with those kind of meters, I'd
> >> enjoy learning more, just for the shits and grins of it.
> >>
> >> Thanks guys,
> >>
> >> Jeff
> > Isn't there normally a shunt across the meter movement? If so, is
> > it accurate? If it had changed in value that could account for the
> > increased reading.
> There was none across the outside, and the meter was crimped shut, so I
> didn't bother looking inside.
>
> But IIRC that kind of meter just used a few turns of heavy wire
> connected across it's terminals to create a magnetic field which altered
> the total dc magnetic field inside and made a piece of iron on the
> pointer shaft change its position.
>
> Jeff
Those moving iron ones didnt usually use shunts, as you say. They were
cheap, nonlinear, undamped and inaccurate, and I expect the meter's
always been that way. It was probably a bit of marketing spin.
NT
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