Convert old Analog Ammeter to voltmeter?

Discussion in 'General Electronics Chat' started by Wharf1999, Apr 16, 2015.

  1. Wharf1999

    Thread Starter New Member

    Apr 15, 2015
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    I have an cool large industrial AC ammeter that reads 0 to 1200 amps. I believe it was used in an old electrical transmission panel in an MCC, and was attached to a selector switch. I am speculating the selector switch could be used to measure the draw for different circuits - major electrical feeds to buildings or industrial plants, or maybe three large pieces of equipment
    I'd like to use the meter for a display in a restaurant. How can I make it actually show a reading, so the patrons could use the selector swtich and register a change to the meter needle.
    I think the best way would be to use a 9 volt battery or similar low volatge power source, to keep it safe and simple. So how can I get the meter needle to show a reading or move with such low voltage?
     
  2. Reloadron

    Active Member

    Jan 15, 2015
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    Look very closely at the meter face. Old analog meters like this generally were driven by a current transformer or current shunt external to the meter. Frequently you will find some small print on te meter face telling you what the meter movement actually is scaled for and reads. I would not suggest putting any voltage across the meter until you know what you have. You don't want to toast what might be a delicate meter movement. Look below the case on the face of the meter itself.

    Ron
     
  3. pwdixon

    Member

    Oct 11, 2012
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    You need to take the meter apart and remove the shunt first (that's a shorting element across the terminals). Then the meter action will respond to much lower currents, much more in line with something you can get from a battery. Then you can add appropriate series resistance/potentiometer components to be able to dial-up a needle position, that will be a trial and error basis and depends on the coil itself.
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    The meter should state what the full scale deflection current is and what the meter resistance is. When used as an ammeter, shunts are used in parallel with the meter to adjust the amount of current passing through the meter. Same principle is used for a voltmeter.
     
    Last edited: Apr 16, 2015
  5. Wharf1999

    Thread Starter New Member

    Apr 15, 2015
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    There are serial numbers and I think scaling numbers on the face of the meter as you mention.
    I'll take a look and post them tonight.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    A 1200A meter would not have an internal shunt for the current, it would be a large external shunt.
     
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  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Likely there is no internal shunt in that meter. However, since it is an AC meter, there is likely a rectifier inside.

    Start with a 9V battery. Put various resistors in series with the meter and battery. Start with 20KΩ (~ 500uA) and see if you get a deflection. If not, try 10K, 4.7K, 1K until you see the needle move. Finally, find a resistance that will give full-scale deflection on the meter. The trick is to approach from the high-resistance side...
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Well I have to take that back. I pulled a couple meter movements out of my parts bin and one gave full scale deflection current and one didn't; neither gave coil resistance.

    When you're trying to figure out current for full scale deflection, be aware that it could be as low as uA; so start there and work up.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    You might also just take a DMM and try to measure the meter resistance directly. Unlikely that a DMM will overdrive the meter, particularly if you start on the higher range settings first.
     
  10. profbuxton

    Member

    Feb 21, 2014
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    Is the ammeter scale linear? If its not linear it may be a moving iron meter. If it is linear then it will most likey be driven via a current transformer and these "usually" put out about 5 amps or 1 amp at full scale to the meter.
     
  11. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I was just digging through my bin of "panel devices" and found 4 vintage yokogawa panel meters with a full scale deflection at 95microAmps. Even with external ~35k of shunt resistance attached, my dmm drove it to half scale. With out that shunt resistance i suspect i could have damaged the movement.
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
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    This worked for the two models of Simpson meter movements I have. Both measured to be about 85 ohms and had 20% deflection on the 200 ohm setting; but I started from the highest range and worked down. One meter was labeled 1mA full scale, so I can infer that both have the same specs.
     
  13. Wharf1999

    Thread Starter New Member

    Apr 15, 2015
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    I think Profbuxton is correct. I talked to an EE in my company, and he was certan that the panel meter is likely driven by a CT, and the only way to actuate is to put 1 to 5 amps through it. It could be done at lower voltage,. he suggested using a transfromer feed by 120v, bring it down to about 6 volts. then use a coupl resistors to show different loads on the meter. he's send me a sketch. I looked at few transformers & for about $75 I can do it. Now I must decide if a display piece is worth it.
     
  14. Reloadron

    Active Member

    Jan 15, 2015
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    The facility I worked in prior to retiring had one older in building sub station with two transformers and a few 0 to 1200 Amp old analog Weston manufacture meters. We brought in 5 KV 3 Phase Delta and dropped it to 480 VAC. The current meters were driven by two 1200:5 current transformers and the meters had the internal burden resistors for the CTs. There were also a pair of 0 to 500 VAC meters to monitor the line voltage switchable for each phase. That sub station went in during the late 1950s and the transformers were dated 1955. I always saw humor in the fact the transformers were made when I was 5 years old. :)

    So anyway, as I mentioned above, my guess is the original poster has similar. If the meter were opened up there would likely be a low resistance burden resistor for a CT and as Mike mentioned some rectification. Many of those old analog switchboard and panel meters were really some classic stuff. As a display it would be nice to drive the meter to around 90% of full scale and back to 0 at a slow rate over and over again. Once the OP returns maybe we will know exactly what he has. :)

    Ron
     
  15. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Which brings us back to what pwdixon already posted, even at 1A or 5A full scale current there still will be a shunt inside the meter to divide that current further into sub-mA range most likely.
    So if you can mod it you should be easily able to drive it basically in any way you like.
     
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  16. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,

    Yes there could be an external shunt and an internal shunt, or an external shunt and internal calibration resistor, etc.

    I had an old 100ma meter i converted into a volt meter and used it for years on the car to monitor a few things like the voltage drop when starting and the charging voltage and the standing voltage of the battery.

    All i had to do was open the meter to get inside, and cut one end of the shunt wire which was a small coil of wire.
    Next, i connected a resistor in series with the meter and varied it until i got the full scale voltage deflection that would match up with the numbers already on the face as well as i possibly could.
    For this meter the numbers on the face went 0,20,40,60,80,100 because it was a 0 to 100ma meter to start with.
    So dividing those mentally by 10 gives me: 0,2,4,6,8,10, and that is quick to do mentally when the meter is in use.
    But unfortunately that would have read as 0v,2v,4v,6v,8v,10v, and 10 volts is not high enough for an automobile battery monitor. I wanted more like 15 volts.
    So what i ended up doing is choosing the series resistor such that the full scale reading was twice that, so the voltages would be:
    0v,4v,8v,12v,16v,20v
    and that's a bit high for a car battery but still easy to figure out on the fly when reading the meter.
    So it turned out that a 2.2k resistor gave me 20v full scale.
    Note however that at 12v a 2.2k resistor draws about 5.5ma, and that may be too much for some applications. So one of the things you have to do is make sure that the meter does not draw too much current for a given application once you have converted it.
    Since then i went to a meter with a higher basic impedance, so now i use a movement with something like 15k in series and that draws much less current from the battery.
     
  17. Wharf1999

    Thread Starter New Member

    Apr 15, 2015
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    Ths is an AC meter. I don't think there is any shunt inside or out.
     
  18. Reloadron

    Active Member

    Jan 15, 2015
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    Why? Is it an AC meter movement or is it actually a DC meter movement scaled to read AC? Maybe the actual meter movement is an average responding RMS indicating meter simply scaled to read RMS current? You won't know till you open it but during my career I saw my share of meters as you describe using an internal burden resistor as a shunt driven by a CT. I am pretty sure this meter does not directly measure 1200 AMPS AC directly. Why can't an AC current meter use a shunt?

    Ron
     
  19. Wharf1999

    Thread Starter New Member

    Apr 15, 2015
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    You called my Bluff - I heard from my EE that the meter doesn't have a shunt. He said it reads the current directly from the CT input in the same fashion as a 4-20 signal in a low volage instrument. The meter was used to test the individual phases in a 3-phase MCC.
     
  20. Reloadron

    Active Member

    Jan 15, 2015
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    My guess is you have the same thing I had in post #14. :) They were driven by CTs and actually in that old sub station did a pretty good job of providing a rough indication of what the load was on those old transformers. Eventually as demand grew we put in a new sub and by then as I was ready to retire it was among the last projects I had anything to do with. The new sub was pretty cool, fully automated and all Siemens equipment. Any of several of us could literally log on to the new sub and access a wealth of information. Anyway, I would follow through with your original post and make the meter into a working decoration of sorts. Classic stuff or junk is all in the eye of the beholder. :)

    Ron
     
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