Panel Meter - Unexpected Coil - Page 1
Forum section: electronics / repair

author: yngndrw
author_rank: Regular Contributor
timestamp: March 23, 2025, 02:52:28 am
content: Hello,I have an old panel meter with no labels or markings. The scale is a percentage scale (0-200%) and I was expecting it to have a normal range such as 0-10V or 0-5V but upon inspection, it is a strange setup inside. (See attached)The wire used for the coil is massive, about 2mm in diameter. There's no separate voice coil for the meter, no separate shunt resistor - Just a single massive coil with 4 or 5 turns. I'm assuming I'll have to re-wind this meter in order to be able to use it for my purposes (A load meter for a VFD, so I need a 0-10V input range) but can anybody tell me about how it's currently setup - Is it currently configured as a DC ammeter or is this a different configuration? I believe that for a regular voltmeter the configuration is a winding and a resistor in series, is there a nice way of working out the correct winding and resistance or is trial and error the easiest? I'm unfamiliar with analogue meters and couldn't find what I was looking for when I searched online.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 03:31:34 am
content: The single heavy-wire coil must be for a relatively large current, without using a separate shunt resistor.Is there another coil in there somewhere with smaller wire? That could be for a power meter, where the torque is proportional to the product of the currents through two windings.

author: yngndrw
author_rank: Regular Contributor
timestamp: March 23, 2025, 03:53:42 am
content: It's definitely just a single coil. I've disassembled it a little further and have attached a couple of pictures. The whole needle assembly comes out (The casting is horrible!) and leaves behind the massive coil.

author: Ian.M
author_rank: Super Contributor
timestamp: March 23, 2025, 04:06:09 am
content: https://electrical-engineering-portal.com/moving-iron-instruments-voltmeter-and-ammeterItsprobablya moving iron ammeter. A picture of the scale could confirm that.IMHO its not suitable for conversion to a 0-10V proportional load meter. For that you'd need a d'Arsonval meter movement with 1mA or better sensitivity for FSD.https://electrical-engineering-portal.com/darsonval-meter-movementConfirmed by followup photos.

author: yngndrw
author_rank: Regular Contributor
timestamp: March 23, 2025, 04:23:56 am
content: Thank you, I've never looked into the workings of analogue meters and didn't realise the different types available but those pages explain them well. It looks like I need to find another meter for my application, but at least I now know what I'm looking for.I don't know the history of this meter or what it was used for. The scale doesn't give much away! There are no other markings on the meter to show the maximum current, maybe it was custom-made for a specific application.

author: Ian.M
author_rank: Super Contributor
timestamp: March 23, 2025, 04:29:42 am
content: I'm very surprised its scale was that linear, not cramped near the ends.

author: yngndrw
author_rank: Regular Contributor
timestamp: March 23, 2025, 03:57:06 pm
content: Maybe it was just intended as an indication, it's hard to know without knowing the original usage.

author: CaptDon
author_rank: Super Contributor
timestamp: March 23, 2025, 05:47:25 pm
content: Why would it be cramped at the ends? Its movement is linear to current flow.

author: Gyro
author_rank: Super Contributor
timestamp: March 23, 2025, 06:03:06 pm
content: No it isn't, because the force between the fixed and moving iron elements is non-linear with distance. Roughly square law.It's a while since I saw the inside of a small moving panel iron meter, but that one looks reasonably well constructed, apart from the lousy alloy casting. It might be possible to make it a bit more linear by shaping the iron elements (although I can't think how). My guess is that the scale is 'indicative' and nowhere near as linear as the markings would lead you to believe.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 06:06:58 pm
content: If I remember correctly, moving vane meters do not sense polarity (as opposed to d’Arsonval meters with permanent magnets), and can be connected to AC without rectifying.When both types were common, moving vane meters were cheaper (and looked cheaper).

author: Gyro
author_rank: Super Contributor
timestamp: March 23, 2025, 06:19:28 pm
content: Yes, they work equally 'well' with AC and DC, ignoring frequency effects.