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How To Convert DC panel meter to read AC AMPs?

Discussion in 'Electronic Basics' started by [email protected], Feb 2, 2007.

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  1. Guest

    I have beautiful DC panel meters that currently reads 0-5Volts DC by
    using an in-series resistor. I want to use this meter to instead read
    0-5 AC AMPS. This is to help me judge excessive power consumption on
    my train layout.

    I have a .15 Ohm shunt resistor in parallel with the meter, but as you
    might expect, the meter needle vibrates due to the AC. So, I put a
    diode in series with the meter to feed it DC. This works. To help
    keep the meter as sensitive as possible, I put a small electrolytic
    capacitor in parallel with the meter. This gets me closer to measuring
    Peak voltage drop across the shunt instead of RMS.

    Is this the best way to do this? I tried a full-bridge rectifier but
    found that I had even more voltage drop across the bridge. Are there
    low-voltage drop diodes I can use that will work better here?

  2. Jamie

    Jamie Guest

    Look at a precision rectifier.
  3. Ross Herbert

    Ross Herbert Guest

    Firstly, you will need to use a bridge rectifier to convert the AC to
    DC. Then you will need to use a shunt in series with the AC line as a
    means of detecting a suitable voltage range to drive the meter.

    Read Rod Elliot's article on meters and shunts.
  4. ahwii

    ahwii Guest

    Thanks for the references but building precision rectifiers seem a bit
    overkill. Are there any pre-built precision rectifiers that come on a
    single chip?

    I really didn't want to make this such a big project that I would have
    to breadboard the supporting circuitry. I was hoping a simple diode
    would do the trick. That is why I tried a half phase, single diode
    test using 1N914/1N4148 silicon switching diodes. I made up a bridge
    rectifier (made of the same diodes) but this dropped the sensitive of
    the meter quite a bit. Using a standard 50V silicon bridge rectifier
    delivered almost no voltage to the meter! I need .9mA for full meter

    Building two op amp circuits (one for each supply that I am measuring)
    and feeding the Op Amps their 5v source is more than I wanted to take
    on. The power supply is delivering 17V AC at about 5AMPs to my digital

  5. kell

    kell Guest

    You're only getting about 1 volt peak across the sense resistor at
    maximum current, so you don't have much to work with.
    Schottky diodes or germanium diodes have lower voltage drops than
    standard silicon. Using schottky or germanium bridge with a double
    diode drop might still be low enough to give you a reading, but I
    don't see why you want to use a bridge. Wouldn't the extra drop
    affect the accuracy of your meter in the lower range? You're getting
    very little voltage from that sense resistor.
    Until there's enough current through your sense resistor to exceed the
    drop of your diode or diodes, the meter will read zero. The more
    diodes you use, the more inaccurate. If it was my railroad and I
    wanted the meter accurate I'd probably use a single shottky or
    germanium diode.
    You could also look into using a current transformer instead of a
    sense resistor. Theoretically at least, you could get a linear, full-
    scale reading on your 5 volt meter. The devil is in the details and I
    haven't used current transformers, so I'll have to defer to persons
    with more expertise on how to execute that.
  6. default

    default Guest

    If you can sacrifice some more voltage in the shunt here's a simple

    Put a pair or silicon back to back diodes in series with the shunt
    resistor and connect the meter to the shunt resistor and diode pair.
    The diodes have to be rated high enough to handle 1/2 the AC current
    that you will use - and there should be a fuse or some current
    limiting (or just use a pair of 20 amp diodes)

    That will drop Point .6 volts AC and compensate for the diode drop in
    the meter's rectifier diode. You will have six tenths of a volt less
    going out to the train layout - if that's tolerable to you.

    Likewise you could use two back to back pairs and compensate for a FWB
    rectifiers and waste 1.2 volts in the compensator diodes.


    Another different and slightly more complicated approach might be to
    bias your meter rectifier on all the time with a small current
    returned to the opposite AC supply rail.

    Your AC lead is connected to the shunt and diode anode, the load is
    connected to the other end of the shunt (same place the minus of the
    meter is connected). Your basic set up.

    Add another diode and resistor connected in series from the junction
    of the meter and cathode of the meter rectifier and the opposite AC
    lead. The cathode of the compensation diode goes to the AC lead.
    Resistor value? That depends on how much current your meter needs but
    a watt or two of power dissipation should be plenty. Both diodes
    (meter rectifier and compensator diodes have to be able to handle the
    compensation current - so a 1N4148 might not be the best meter
    rectifier in that app)

    I think that idea would work, but would welcome any criticism or
    critique, if some guru would like to lambaste me. The first idea
    would work the second idea doesn't lower the voltage out to the track
    so may be more appealing.
  7. Basically you want a circuit to let you read a very low AC voltage with a DC
    meter. This is difficult to do. The original method was to use a current
    transformer, however today it is more usual to use a 'precision rectifier'
    circuit. See for an example.

    You would, of course, be better to purchase a 5 A meter.
  8. default

    default Guest

    Way back, during the dawn of time (1930) they used copper oxide
    rectifiers for meters. Very low forward drop and very low (6 V) PIV.
    They were used for meters, battery chargers, and radio detection.
    Reasonably linear for meter applications since most of the meters were
    only good for a 5% accuracy.

    I scoured the web looking for a supplier for them and didn't turn up
    any - but they still make moving coil AC meters that use them.

    I have a few moving vane meters I got surplus. They are repair stock
    for an old X-ray machine and were made by G.E.. Iron vane meters can
    be very accurate and they don't care if the power is AC or DC. Mine
    have something like 15 turns of wire on the coil and were intended as
    current meters. I suspect all the bad press iron vane meters received
    is due to the cheap battery checker and charger meters that were
    produced with 20% accuracy.

    Nice thing about old iron vane meters is you can take them apart and
    rewind the coils for different uses and change the scale markings.
  9. Ross Herbert

    Ross Herbert Guest

    should have directed this response to Jamie who was the one suggesting
    precision rectifiers.
  10. Ross Herbert

    Ross Herbert Guest

    According to this page Edal still make
    copper and selenium oxide rectifiers.
  11. ahwii

    ahwii Guest

    I just bought a few 1N5817 Schottky diodes on eBay (DigiKey way too
    expensive and my local electronics shop had none for the taking - at
    least that I found) and will give these a try instead of the silicon
    diodes I have been trying. I guess the small transformer might be the
    next step to try. Yeah, I need to keep the voltage drop across the
    shunt resistor pretty minimal as the DCC controller wants to have a
    pretty steady voltage supply and needs to be able to pull anywhere
    from 0 to 5.5AMPS.

  12. roma

    roma Guest

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