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

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.

It's called Google and you don't play facile and excuse yourself with a poor tag line.

DNA "Try not to speak"

"Genome" wrote in news:MjBSg.2507$ snipped-for-privacy@newsfe3-win.ntli.net:

Oops! Looks like the missing molecules spilled over again out of the blue like they did on me some time back. On the subject of practising what we preach, DNA might do well to speak less. Move along people, nothing to see here. And don't get any on you! >:)

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.)

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.

-- Graham W

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...

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

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

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.

When I worked as a meter tech for EIL many years ago we calibrated iron vane meters of various types. Most of them had a moveable metal piece that could be adjusted to affect the calibration. Most of these meters were made to be opened easily for repair and adjustment and scale changing, but some (GE I think) had an aluminum band that was swaged on with a special tool to seal the meter. All iron vane meters I worked on were for AC use, but they might work on DC. I never tried. However, I think they relied on magnetic induction which is an AC phenomenon, similar to a motor.

The really cheap meters were Shurite, and they used (I think) a permanent magnet moving vane in a fixed coil. These were often DC meters used in automotive and battery charger applications. The permanent magnet might weaken with age, but I think that would make the meter read lower, rather than high. If there were no magnetism, it would read zero. All meters I worked on had springs (taut band types use the band as a spring). A weak spring could cause a high reading.

The only meters I know to be true RMS are dynamometer types, which use a moving coil as well as a fixed coil. When the coils are wired in series or parallel, they read voltage or current. When the coils are separate, the meter can be used as a wattmeter.

Of course, most such measurement is now done digitally, but it is interesting and educational to understand the older technology. Also, many of the lab grade meters were beautiful works of art, with varnished oak or bakelite cases, and carefully drawn mirror scales to reduce parallax, and to obtain accuracy to 1/4% or better.

Paul