Moving iron AC ammeters OK for DC as well? - UK Vintage Radio Repair and Restoration Discussion Forum

Chris Wilson · 29th Aug 2015, 7:55 am

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 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. Is there any reason *NOT* to use one on DC? I even noticed the terminals are marked + and - on it....

Thanks.

G6Tanuki · 29th Aug 2015, 8:21 am

Yes they're fine on DC; the standard ammeter on things like car battery-chargers has always been a moving-iron type, where they're measuring the DC output (well, it's unsmoothed output from a full-wave rectifier...).

broadgage · 29th Aug 2015, 10:46 am

Yes they are fine on AC or DC or indeed on rectified AC.
A moving iron meter measures true RMS on AC or rectified AC and is in this respect superior to the cheaper digital meters. (these measure the peak voltage or current, and then divide by a fixed scaling factor to indicate the RMS value. This is only accurate with a sine wave)

Only slightly off topic, but moving iron meters on simple battery chargers often give a very misleading reading. The meter will correctly read the RMS current, but the degree of charge put into the battery is determined by the AVERAGE current not the RMS figure. 10 amps RMS might be 6 amps average.
Given a battery charger output of 10 amps RMS, one might expect a perfect 100AH battery to charge in 10 hours, or a real world imperfect battery to charge in about 11 hours. It will actually take about 20 hours.

Andrew B · 29th Aug 2015, 3:09 pm

Moving iron meters work well in the centre part of the scale, outside that they should be used to indicate SOME current is flowing.

29th Aug 2015, 7:55 am	#1
Chris Wilson Heptode Join Date: Nov 2011 Location: Whitchurch, Shropshire, UK. Posts: 612	Moving iron AC ammeters OK for DC as well? 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 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. Is there any reason NOT to use one on DC? I even noticed the terminals are marked + and - on it.... Thanks. __________________ Best Regards, CW. 2E0ILY

29th Aug 2015, 8:21 am	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Moving iron AC ammeters OK for DC as well? Yes they're fine on DC; the standard ammeter on things like car battery-chargers has always been a moving-iron type, where they're measuring the DC output (well, it's unsmoothed output from a full-wave rectifier...).

29th Aug 2015, 10:46 am	#3
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,129	Re: Moving iron AC ammeters OK for DC as well? Yes they are fine on AC or DC or indeed on rectified AC. A moving iron meter measures true RMS on AC or rectified AC and is in this respect superior to the cheaper digital meters. (these measure the peak voltage or current, and then divide by a fixed scaling factor to indicate the RMS value. This is only accurate with a sine wave) Only slightly off topic, but moving iron meters on simple battery chargers often give a very misleading reading. The meter will correctly read the RMS current, but the degree of charge put into the battery is determined by the AVERAGE current not the RMS figure. 10 amps RMS might be 6 amps average. Given a battery charger output of 10 amps RMS, one might expect a perfect 100AH battery to charge in 10 hours, or a real world imperfect battery to charge in about 11 hours. It will actually take about 20 hours.

29th Aug 2015, 3:09 pm	#4
Andrew B Heptode Join Date: May 2015 Location: Bradford, West Yorkshire, UK. Posts: 512	Re: Moving iron AC ammeters OK for DC as well? Moving iron meters work well in the centre part of the scale, outside that they should be used to indicate SOME current is flowing.