3 wire or 2 wire volt meter

Discussion in 'General Electronics Chat' started by kiwimagic, Jan 11, 2018.

  1. kiwimagic

    Thread Starter New Member

    Jan 11, 2018
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    Hi All
    I'm building a control board for my model train layout. I would like to add a voltage meter for each of my tracks. (4)
    As the tracks each have a reversing switch which reverses the polarity to the track.
    Would a 2 wire volt meter be able to work with reversed polarity or do I need to use a 3 wire volt meter to cope with the reversed polarity.
    Or could I just add the 2 wire volt meter to the rotary power controller .

    cheers
    kiwimagic.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    What type of voltmeter?
     
  3. kiwimagic

    Thread Starter New Member

    Jan 11, 2018
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  4. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    The third wire to for power to the meter. It has nothing to do with measuring the voltage. You will need to power it if the measured voltage os more than 30V.

    The specs mention nothing about measuring negative voltage.

    Why do up think you need a voltmeter? Maybe you should explain your need first.
     
  5. ElectricSpidey

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    Dec 2, 2017
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    Neither one will work with reversed polarity.

    I would use the 2 wire version with a diode bridge, and simply use math to figure out the voltages.

    IE: voltage displayed + 1.4 = actual voltage

    The 2 wire version won’t light till about 5 volts, where the 3 wire will light with any measured voltage as they have two power leads and a voltage test lead.

    But, your best bet is to connect them before the reversing switch.

    The 2 wire version is the most convenient, but as I said they don’t light until a certain voltage is applied, but you probably don’t run your trains at that low a voltage anyway.

    Also be sure to get yourself some really small hardware to mount those things.
     
    Last edited: Jan 11, 2018
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  6. tranzz4md

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    Apr 10, 2015
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    Get a meter with reversible polarity, or a non-polarized one. There used to be analog bi-polar meters with a 0V center position, and the needle moved right for (+) and left for (-). They're probably hard to find anymore. Galvanometers were like that, but are typically very low voltage.

    We'll assume that your train runs on rather low voltage D.C. and reverses polarity to reverse the traction motors, and you've used a volt meter to test this before. There are some that use different schemes.
     
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  7. ScottWang

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    Aug 23, 2012
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    You can buy the lower voltage version as 3V~30V with two wires.
     
  8. spinnaker

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    From what I understand the new model locos have micros now. There is no longer need to reverse track polarity or do the old fashioned track blocking to stop a train on a certain part of the track while others moved along. It might be better to just buy a new loco. I would bet they cell retro fit kits for the older models.
     
  9. crutschow

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  10. Alec_t

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    Sep 17, 2013
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    You could use a reversing switch with extra poles devoted to reversing the meter input.
     
  11. Tonyr1084

    Well-Known Member

    Sep 24, 2015
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    Would be helpful to know your full intents and desires (aka plans) for what you're asking. A drawing or two would also benefit.

    So far you've been told to use a single meter before the polarity reversing switch - which is a good idea when coupled with LED's to indicate polarity. But I'm wondering why you need to know the voltage. If you're going for the "COOL" factor then I'd opt for using an older D'Arsenval movement type meter with zero in the center position (also already suggested).

    My thoughts on a model railroad, something I'd like to do in the next 10 years, would be to set up bi-colored LED's at each track segment, sort of like signal lights. Red could represent a reverse polarity, so that an engineer (imaginary) in the engine could know that he needs to stop. In other words, if the track hits a positive AND a negative polarity, canceling each other out - the train would stop. What I mean is assume the "North" track rail (for instance) is positive and the south track is negative. The train would move forward. But if it encountered a set of rails who's polarity would be reverse of that then two positives (or two negatives) would provide no electro motive force, and the train would stop. Just make sure to hook it up so that you don't short your power supply. Isolation transformers connected to an auto-transformer would do a sweet job at isolation to avoid short circuiting any given power supply. Use of LED's would give a ready indication whether a track segment is ready to accept an on-coming locomotive or not.

    If you want digital then I'd look for one who's voltage display range is from zero to twice your power output (or greater). Unless you're setting up a console to mimic the look of a modern real locomotive cab then I'd go with the auto transformer, some rectifiers and some very large capacitors. Build (or buy) one of those units that you can throw the throttle to full open and the train will slowly and realistically build up speed. The capacitors would act to slowly slow down the engine realistically even if you throw the throttle to full emergency braking.

    Personally, I like the idea of LED's located at certain track segments indicating polarity. But I like the older versions. I've never heard of a locomotive with a microchip. But why not ? ? ? ! They're putting chips in coffee mugs now-a-days.

    [edit]
    Would be nice if you could post a schematic of what you're thinking. People here are pretty darn bright. Caps, MOSFETS and microcontrollers - the guys here (not discriminating against female; by "guys" I mean the folks here) are pretty good at all that stuff.
     
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  12. kiwimagic

    Thread Starter New Member

    Jan 11, 2018
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    Yes the new thing is DCC but I prefer the older Dc versions. Also I have about 15 Locos so the cost of converting to DCC would be quite high.
     
  13. Tonyr1084

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    Sep 24, 2015
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    I bought some N scale because a local shop was going out of business and the stuff was REALLY cheap. They were just about cleaned out for HO scale. And what I have is still in a box. Haven't done anything with it. Really don't have a space for it right now. I'd have to clean up my electronics lab in order to build a set. Or just set it up on the kitchen floor. But then the cats wouldn't be able to resist batting the cars around like a "Godzilla".
     
  14. spinnaker

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    I have a whole bunch of old train stuff. Sadly I face the same problem. I do have a spare room but it is so tiny. I don't think it would make much of a setup for HO. Maybe N guage. Frankly I don't have as much of an interest anymore.
     
  15. kiwimagic

    Thread Starter New Member

    Jan 11, 2018
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    For N scale you do not need much room at all. You can build a layout on the back of an old door and hang it off a wall.
     
  16. Tonyr1084

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    Sep 24, 2015
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    N gauge, just because it's small doesn't mean it won't take up much room. Unless all I want is a train to go in circles. I'd want at least a simulated mile of track. Prefer a couple miles, not more than five. N probably takes up two thirds the room N gauge would. For five miles of track I'd need a large room with either scale set. Since my basement is already finished and has dedicated spaces, setting up a train set just isn't in the cards right now. I'd have to give up my electronics room - or - my wife's shoe storage room (over 166 pair at last count a couple years ago). Nope! Not exaggerating.

    HEY! Maybe I can build an O gauge railroad to deliver her shoes. Now THERE's an engineering "FEAT".
     
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