Moving iron AC ammeters OK for DC as well? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 29, 2015, 06:58:29 am
content: I have a few NOS moving iron ammeters, the sort with a compressed low end of the scale. The ones you always see in the bottom of boxes at rallies when all the decent moving coil ones have sold.I have always used these as seemingly designed, for measuring AC current, but I needed a DC ammeter for up to 20 Amps, so tried one. It seems to work fine on DC, although dampening is not great. Up to the 10 amp DC current limit of my multimeter the two track fine. Is there any reason *NOT* to use one on DC? I even noticed the terminals are marked + and - on some of them ....Thanks.

author: Simon
author_rank: Unknown
timestamp: August 29, 2015, 07:10:51 am
content: surly there is some sort of rectifier in them ?

author: IanB
author_rank: Super Contributor
timestamp: August 29, 2015, 07:43:05 am
content: I am sure they were used to indicate the charging current in old style car battery chargers, which must have been DC. I don't imagine there is any reason not to use them for DC.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 29, 2015, 08:01:34 am
content: They work on both AC & DC--in fact,they read True RMS,albeit on a weird non-linear scale.Back in 1959,we were taught about them,but told they were "pretty much obsolete"!;Dhttp://electrical-engineering-portal.com/moving-iron-instruments-voltmeter-and-ammeter

author: German_EE
author_rank: Super Contributor
timestamp: August 29, 2015, 08:42:54 am
content: Simon, no rectifier required! The bad news is that there is a fixed coil and a moving piece of soft iron attached to the pointer, this gives a non-linear scale due to the inverse square law. It's been a number of years since I've seen one of these.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 29, 2015, 09:10:29 am
content: Excellent, I may get a few more, no one seems to want them at the rallies! Thanks for the information

author: SeanB
author_rank: Super Contributor
timestamp: August 29, 2015, 10:53:45 am
content: The nice thing about them is they work fine on AC or DC, and even will give a good RMS reading for high crest factor current ( think of a transformer with a high current rectifier and an oversize capacitor which only charges over a few degrees at top of the cycle) as well. Damping can best be described as poor to none, but they do work well.Just note that many are orientation sensitive, and will read different if used face down or mounted in a panel. The units with solid cores ( no laminates) are better damped as there is lower core loss, but in any case they only drop 100mV or so at 10A current.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2015, 04:33:38 pm
content: The types with solid core are not that good in RMS reading, as they do not correctly handle higher frequency components.Using with DC may cause a slightly larger error, as the hysterese of the material is not smoothed out from AC - but some more or less permanent magnetisation can build up. So after a large positive DC current they may read a little high on small positive currents and low on small negative ones. As reading is usually not that accurate anyway, this is no real reason not to use them on DC.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 29, 2015, 07:17:39 pm
content: Yes, since using it to set a Hall effect trip run off a choke wound on a ferrite rod for the magnetism above a certain current, it no longer settles exactly at zero, as you say, it's probably residual magnetism. Still very adequate for the job in hand though! Thanks for the replies.