DC Ammeter Sensitivity Increased?

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

--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
"Rudeness is the weak man\'s imitation of strength."

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

Nope. Still a spring.

Moving iron vane - The moving iron vane meter operates on the principle of magnetic repulsion between like poles. The measured current flows through a field coil which induces a like magnetic field into a fixed and moving vane causing the moving vane to deflect a pointer in proportion to the current or voltage applied to the coil.

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I hear what you say about a spring, and it makes sense that there may be one there. But it's interesting that the reference link you gave mentioned "springs" when describing the first two meter types, but not the moving iron one. Wonder why? Probably just an oversight.

Jeff (Who is NOT going to tear that old meter on the Eico 1050 apart to find out.)

--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
"Rudeness is the weak man\'s imitation of strength."

Perhaps they felt it was redundant. It's like holding two magnets N to N and S to S - you are the spring holding them together.

Jeff Wisnia wrote in news:Mt2dnbI78os5YIfYnZ2dnUVZ snipped-for-privacy@conversent.net:

You can test for a spring without dissection. First, does the meter jump to a position and oscillate a bit before settling? If so, put your ear (or a stethoscope) to the meter panel and tap it sharply. If there's a spring in there it should ring like a bell. If you see that resonace in the meter movement, yet hear no spring, then you and Homer are probably right, the spring would be entirely magnetic.

The pointer itself might ring with the test I described, but dull sound, not the sustained ring you'd likely hear from a spring designed to maintain a pointer position.

Hit the 'Back' link at the top of the page above and you'll see the spring.

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Could it be that the meter was calibrated to compensate for the fact that when it's hooked to a battery, it only supplies current when the charger exceeds the battery's voltage, while on a lamp it would draw current from zero on up?

-- John

No.

John

Hello John,

Then maybe the meter was calibrated so it would adjust the indicated current according to the consumer price index. Now wait, with a build date of 1965 that doesn't computer either...

--
SCNR, Joerg

http://www.analogconsultants.com

Also comes to mind that if there is a magnetic pointer restraint this is unusual, and perhaps the meter would (as usual) have bal;ance weights to restore to zero, so perhaps it is calibrated for when the meter face is vertical (panel mount style)? A moving iron movement responds to RMS so waveform should not be a problem.

Peter Dettmann

Got it, thanks!

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
"Rudeness is the weak man\'s imitation of strength."

FWIW I just went down and tried an orientation test with the meter reading 5 amps and the pointer stayed in the same position whichever way I tipped the unit.

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
"Rudeness is the weak man\'s imitation of strength."

Cheap high DC current meters were almost always iron vane and never had a shunt.

Yep, that is the standard construction; i think a spring was used for setting / resetting zero position.