Is it possible to reduce an analog ammeter scale? - Page 1
Forum section: electronics / beginners

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 04:48:23 pm
content: Hi, In the next weeks I will be building a metal detector and looking for parts at home I found an analog 20A scale meter that I was going to use for a project I never finished.The metal detector has got an audio output and since audio it's a sine wave I thought that attaching it in series with an ammeter should make the arrow move. The thing is that even if I connect this ammeter to the circuit, since it's 20A scale, I think the arrow won't move at all because the audio current must be very very small. note: The audio signal does not vary in tone but just in volumeIs there any way I can use this ammeter with my project? Is it possible to reduce the scale so that it would work with an audio signal? I may buy another ammeter but first I need to know if this is even possible.For the moment I'm just thinking about the project design and stuff like that, so I would like to know if I can include this thing into the project so that I can imagine where am I going to put it xDI can attach a pdf with all the schematics of the project if it's necessary. Can anyone help me? Thanks

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 05, 2017, 04:58:23 pm
content: An Amp meter measures the voltage over a shunt resistor. Your meter either had the shunt resistor externally so it is much more sensitive now, or it has a built in shunt resistor which you can remove. Careful trying or careful opening the meter will tell you what is going on.

author: w2aew
author_rank: Super Contributor
timestamp: February 05, 2017, 04:59:33 pm
content: The scale on the face of a panel meter *rarely* matches the actual full-scale deflection current for the meter movement. Analog meter movements are usually paired up with shunts or series resistors to adapt the meter movement to the required current or voltage reading that is needed. So, it's worth making some measurements to see what the actual meter movement FSD (full scale deflection) current is, then you can add necessary external components to make it what you want.Here's a video I did on the basics of the D-Arsonval meter movements:Here's a second video which shows how to calculate a new shunt resistor value to obtain a new (higher) full scale deflection:

author: Vtile
author_rank: Super Contributor
timestamp: February 05, 2017, 05:00:46 pm
content: There have been historically a few designs of ammeter movements, every typical one is somesort of coil, which forms a moving or rotating action. If you can reduce the scale, is a question that only basicly can be answered by opening the meter and see if the meter is done from micro or milliampere coil adding shunt (current divider). I suspect it will have current divider, but...Someone from the older generation propably knows better.

author: David Hess
author_rank: Super Contributor
timestamp: February 05, 2017, 05:41:36 pm
content: There is almost certainly a high current shunt inside the meter's enclosure. Removing it will restore the meter to its native sensitivity. The current shunt might have been external (and lost) but measuring the meter resistance with an ohmmeter will reveal if the shunt is inside.

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 05:48:17 pm
content: Quote from: w2aew on February 05, 2017, 04:59:33 pmHi, thanks for the fast detailed answer. The problem with measuring the ammeter is that I don't have a variable power supply (I want one, but I don't have it yet...) and the ammeter is AC and rated 20A , so I would have to generate a 20A current to measure if it's really 20A? Is that what you say? I don't understand it very well.

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 05:52:54 pm
content: Quote from: David Hess on February 05, 2017, 05:41:36 pmI measured the meter's poles with an ohmmeter and it says 0.8 to 0.9ohm . What does that indicate? xD I'm a bit lost, I was specting a high resistance if there's a resistor in there

author: Vtile
author_rank: Super Contributor
timestamp: February 05, 2017, 05:58:30 pm
content: Quote from: azigta on February 05, 2017, 05:52:54 pmIt says there is 17 volts over the meter when 20 amps is flowing through.. If it do have internal shunt. It might have a diode bridge inside also, since it AC meter..? A picture of the face plate (the board with needle on top and the scale printed to it. If the meter is even half decent make, it usually have some inspection markings and data) or any other markings might help us to give any sort of good quess.. Until then we are shooting in the dark.Edit2: If it is mains rated meter and works, get a cheap one for your projects. These meters are pretty expensive to get so you might want to save it for trade or better use. A picture would still be good.

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 06:16:18 pm
content: Quote from: Vtile on February 05, 2017, 05:58:30 pmThis meter wasn't expensive at all, just a cheap maybe 6usd chinese meter. When I bought it I think it said it could read 250v (mains in spain it's 230v) So I guess is mains rated. It should work since it has never been used. I attachsome pictures. I could open it and take pictures of the inside if necesary.

author: David Hess
author_rank: Super Contributor
timestamp: February 05, 2017, 06:21:36 pm
content: Quote from: azigta on February 05, 2017, 05:48:17 pmSince it is an AC ammeter, it uses an iron vane movement which works with AC or DC and responds to the RMS value which oddly enough may be a good match in an audio application although I am not sure how well they work at audio frequencies. They are not very accurate.Quote from: azigta on February 05, 2017, 05:52:54 pmIron vane meters are typically wound with very low input resistance. Even without the shunt, the 20 amp ones I have are about 0.2 ohms. They are however easy to modify by rewinding the stator which is easy to access. The meter movement responds to the number of amp*turns so if you raise the number of turns by 10 times, it becomes 10 times more sensitive.You will almost certainly want a high current low voltage amplifier to drive the meter even if you rewind it.

author: Vtile
author_rank: Super Contributor
timestamp: February 05, 2017, 06:27:13 pm
content: Thx, David.I'm not sure it is iron vane movement. The faceplate markings indicates according VDE 0410/3.68 (german .. old as ..)2.5% error, D'Arsonval type (i'm not 100% of my translation), with internal diodes. (and vertical use and test voltage over 500V) .. if so that might be possible to hack it to work.

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 06:35:26 pm
content: Quote from: Vtile on February 05, 2017, 06:27:13 pmI tried to open it to see what was inside. When I removed the 2 screws at the back, a piece of it came out. You can see in the pictures in the one that shows the rear of the ammeter, that in the lower part there are 2 screws. I removed those 2 screws and then I found a toroid coil and some resistors and diodes.Some pics hereDoes this mean it's possible to "hack" it? xD

author: Vtile
author_rank: Super Contributor
timestamp: February 05, 2017, 06:39:19 pm
content: Quote from: azigta on February 05, 2017, 06:35:26 pmLets wait to someone that have done something similar, I suspect that toroid is a current transformer and the board is the rectifier. Unfortunately I haven't done modification on these AC meters, so it would be more convenient to have someone explain it to you who have 1st hand experience for such conversion. It looks promising though.Edit. The toroid is there (assuming it is a current transformer) for safety and take a small enought fraction from the current that is going through (for usually pretty sensitive D'arsonval movement). The output of the toroid is still AC. The D'arsonval type of movement is DC and the rectifier is there to make the AC to DC.

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 06:42:09 pm
content: Quote from: Vtile on February 05, 2017, 06:39:19 pmSure, I'll wait. I have time. I wish I could mod it, it would look cool on a metal detector xD Thanks

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 07:17:01 pm
content: Quote from: Vtile on February 05, 2017, 05:58:30 pmHi again, I measured the coil without the toroid and the circuit and it measures 158.3 ohm (let's say 160 I guess) What does this mean? I tried to measure an AAA battery in series with a 1k resistor and the arrow moves all the way to 19 (and fast).With a 10k Resistor the arrow marks 2amps. So if the battery has 1.5v (actually it has 1.19v) and the resistor is 10k, the Intensity should be 0.000118A -> 0.118mA. Maybe tomorrow I will connect it to an audio tone and see how it behaves

author: Vtile
author_rank: Super Contributor
timestamp: February 05, 2017, 07:31:00 pm
content: Quote from: azigta on February 05, 2017, 07:17:01 pmRemember that the audio tone is propably some sort of AC and your meter movement is DC. In all negative half cycles (the alternating current have positive and negative halfcycle if we simplify a bit ) the meter tries to go left and positive to right. So it will broke down or shows zero.If you have ammeter in your DMM hook it up series with the 10K resistor and the analog movement as previous test. If the analog meter still shows 2Amps then multiply the DMM reading by 10 to get the analog meters full scale deflection current (=F.S.D). I'm now going to watch Expendaples 3, so I have no time.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 05, 2017, 07:32:18 pm
content: Quote from: azigta on February 05, 2017, 07:17:01 pmMake sure to leave the rectifier pcb in place when you feed it audio, otherwise the needle will just shake a bit.So use the blue and red wires as an input.

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 07:57:21 pm
content: Quote from: PA0PBZ on February 05, 2017, 07:32:18 pmOk so I removed the toroid and attached the blue and red wires to the pcb, and the red and white wires to the coil. Now I'm going to try to close it and that's ready for an analog signal test. Right?

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 05, 2017, 08:01:43 pm
content: Quote from: azigta on February 05, 2017, 07:57:21 pmApart from the fact that the wires were already attached to the pcb (did you take them off?) that sounds right

author: azigta
author_rank: Regular Contributor
timestamp: February 05, 2017, 08:09:50 pm
content: Quote from: PA0PBZ on February 05, 2017, 08:01:43 pmWhen I did the previous tests I was doing the tests without the board, when I wanted to remove the board, I didn't see what I was desoldering (I was a bit tired) And desoldered the wrong cables xD So then I had to desolder the right cables and that's how I ended up desoldering all the cables :'D xDD Now I have soldered the cables to the bolts (quite a challenge soldering them without the plastic melting all over) and now it measures 121.5Kohms. No idea what that means, but that's what it is.In case the thing can't measure an audio tone, could something be done without having to rewind the internal coil?edit: By the way, how do I test a sound with this? I guess that connecting it to the computer's analog audio isn't working, right? Maybe with some resistor in between? How do I do this? I said I was going to do it but I was thinking connecting it directly to the audio output, which would not be in series, so I guess that's not a possibility

author: David Hess
author_rank: Super Contributor
timestamp: February 05, 2017, 08:21:24 pm
content: Ah, so it is AC only and uses an internal current transformer. That explains the low DC resistance.You should be able to apply a DC signal directly across the meter coil where the output of the diodes are.

author: Vtile
author_rank: Super Contributor
timestamp: February 05, 2017, 10:09:23 pm
content: You need to rectify the audio signal (if you do want to measure it with DC metermovement), for that you can use the board with the diodes as it were already build in.. Just fix it as it were. (Like said above)It should be ok.I also do want to give a small warning. The current trasnformer can produce really high voltage spikes in secondary side of it, if you do not have any sorts of resistor in place. So handle it with respect if you toy around with it.