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repairing a remote voltage monitor circuit.

Discussion in 'Electronic Repair' started by Andy Baxter, Oct 21, 2005.

  1. Andy Baxter

    Andy Baxter Guest

    I've been helping out a local environmental centre by trying to fix a
    remote voltage monitor circuit they have with their small windmill /
    battery bank. It runs off the 24v battery bank they have, and relays
    readings of the current voltage and charge / discharge rate down to some
    meters down in the valley where they live. (The windmill is on top of a
    hill, with the battery and inverter in a shed next to it.)

    The voltage is measured directly from the battery, and the current by
    measuring the voltage drop across a short length of the cable that the
    inverter and windmill charger connect to the batteries through.

    The original circuit was designed and installed by someone else - he's
    left an instruction manual with a complete wiring diagram, but no internal
    circuit diagram for the electronics. I'm still trying to get in touch with
    him to ask if he can help, but without luck so far. According to the
    manual, the readings are relayed down to the meters in the valley using
    standard current loop driver chips. The current loop runs down some eight
    core signal cable that was installed for this purpose.

    So far, what I've done is just rewire the whole thing following the
    instructions in the manual, and replace all the fuses. (It had been partly
    disconnected after someone else had a go at fixing it a few years ago.)
    The story is that it originally stopped working after a lightning strike,
    but there's no visible sign of this.

    Now I've got to a point where everything obvious has been tested (except
    the continuity of the signal cable down to the valley, which I'll do next
    week) and it's still not working. The power light comes on, and the two
    fault lights come on for 10 seconds (a self test mode?) then go off, but
    the meters at the bottom of the hill are stuck against their stops (on the
    'no current' side).

    Does anyone have any general advice on how to approach something like
    this? E.g. I'm not sure what's the best thing to do if the fault is in
    the electronics - is it best just to replace all the chips and try to test
    the other components in situ, or take the whole thing home with me and
    build a test rig? Also, what kind of precautions should I be taking about
    working with 24V DC? So far I've been wearing rubber gloves and making
    sure everything is disconnected before I touch anything to do with the
    battery bank. I have some photos of the setup if anyone wants me to post
    them.

    cheers, andy
     
  2. Jasen Betts

    Jasen Betts Guest

    if there's only a few cheap chips in there replace them.


    if it was a lightning strike the bits that would have got hit the hardest
    are those nearest the cables, at both ends of the cable.
    so check the circuitry behind the meters and that the
    meters work...

    damaged parts aren't always visibly damaged.

    Bye.
    Jasen
     
  3. Guest

    Here's what I'd try. First, the meters can be loop powered (two wires)
    or locally powered (four wires) - look at the meter connections. If
    they're supposed to be locally powered, make sure that power supply is
    working. It also wouldn't surprise me if this setup brings +24 V from
    the top of the hill to power the meters separately from the signaling
    loop; if the +24 V doesn't make it down there, the meters won't work.

    If that checks out, then: Assuming it's 4 to 20 mA signaling, get a 24 V
    power supply or 24 V worth of (small) batteries. Connect a 1 K resistor
    and 5 K pot in series, and connect this across the 24 V supply. You can
    now draw 4 to 24 mA (at 24.0 V nominal) from the supply. (If you don't
    like going over 20 mA, use a 1.2 K resistor.) Go to where the meters are,
    disconnect one of the meters from the signal wires that go up the hill,
    and put the meter in series with the pot, resistor, and batteries. The
    meter will have some internal resistance, so you won't be able to go to
    the full 24 mA, but you should be able to vary the reading on the meter
    by adjusting the pot. If you can't make the meter respond, then there
    is a fault in the meter.

    If you can make all of the meters move with your test rig connected
    directly to the meters, try the same test, but from the top of the hill.
    Disconnect the signal cable from the electronics up there and use your
    battery-resistor-pot rig to send a varying current down the cable. If
    the meters don't move, suspect the signal cable. If they do move, you
    probably won't get exactly the same readings as you did before, due to
    the added resistance of the signal cable, but you should get some kind of
    varying reading.

    If all of that checks out, you're left with the electronics at the top
    of the hill. You might check the input wires from the battery bank and
    the shunt to make sure the right signals are getting to the box. If
    that's OK, then connect your 1 K resistor across the electronics box in
    place of the wires to one of the meters, and measure the voltage across
    it to see if the box is trying to drive the meter. If you can, put a
    load on the batteries to see if the voltage across the 1 K resistor
    changes. If you can't get a reasonable output from the electronics,
    I would then vote for taking the electronics box home where you can test
    it at your leisure.
    I wouldn't worry about getting shocked as much as I would about shorting
    the output of the battery bank. It's possible to shock or burn yourself
    on 24 V but you have to work at it a bit. It's pretty easy to melt a
    wrench by dropping it across the terminals of a battery. If there are
    no fuses in the system, now would be an excellent time to add some.

    Matt Roberds
     
  4. Andy Baxter

    Andy Baxter Guest

    mroberds said:
    They are loop powered, according to the manual I have.
    Thanks, I'll try this.
    I've tested all these cables for continuity, and they're OK.

    Thanks for the detailed reply.

    andy.
     
  5. Andy Baxter

    Andy Baxter Guest

    Jamie said:
    Why? It was working perfectly well before it broke, and the signal wire is
    still in place. (If it's damaged we can use a different set of wires, or
    install a new one which they have anyway). so as far as I can see there's
    no need to rebuild the whole thing, which is adding extra complications
    and costing them a lot in parts at the same time.
     
  6. Jamie

    Jamie Guest

    put a wireless system in.
     
  7. Franc Zabkar

    Franc Zabkar Guest

    I'd leave the rig at the bottom of the hill. At the top of the hill
    I'd put a 100 ohm resistor between the two ends of the signal cable.
    That would enable me to detect shorts in the cable, and to watch the
    meter while I'm adjusting the loop current.

    -- Franc Zabkar

    Please remove one 'i' from my address when replying by email.
     
  8. Franc Zabkar

    Franc Zabkar Guest

    It may help us visualise your setup if you described the various chips
    at both ends, particularly the interface ICs. One current loop
    implementation that gave problems for me involved a 7406 open
    collector driver at one end and an optocoupler at the other.
    I'd be interested in knowing what circuitry is between the meter
    movement and the interface. Does the current loop interface chip
    merely translate 4mA and 20mA currents to 0 and 1 logic levels? Does
    the meter operate in PWM mode, ie does it average the current pulses
    to get an analogue reading? Does it possess any "smarts"?

    What do you see on the meter if you drive it directly from the diode
    range of a DMM? My DMMs produce 1mA on this range. You could use a
    second series connected DMM to measure the current. Or you could
    replace the meter with your DMM and measure the loop current directly.
    -- Franc Zabkar

    Please remove one 'i' from my address when replying by email.
     
  9. Jasen Betts

    Jasen Betts Guest

    put a wireless system in.
    Next time a lightning bolt strikes that hilltop,a wireless
    system will survive a whole lot better than the wired system.
    (especially if the aerial is mounted low) fibre-optic is another
    option.

    it's a hill top, they get struck quite regularly.

    It all depends on how often you get electric storms in that area, and how
    often you want to be repairing that device.

    Bye.
    Jasen
     
  10. Andy Baxter

    Andy Baxter Guest

    Franc Zabkar said:
    There are some pictures at www.niftybits.ukfsn.org/electronics/

    p1010087.jpg is a photo of the main circuit board from the top. The main
    part of the circuit is two AD694 current loop drivers, each fed by a
    IMA118 instrumentation amplifier - one for the voltage measurement and one
    for the current shunt.

    I think I can probably work it out - I just got spooked by the thought of
    having to reconstruct the circuit diagram from the circuit itself, and
    I've never done any diagnostic / repair work before, so I'm not sure
    what's the best way to get started on something like this.

    It looks like the bloke who built it should be sending me a circuit
    diagram, so it shouldn't be as hard as I thought.
    It's just a standard 4-20 mA current loop. Not PWM, looking at the
    datasheet for the driver. The meter circuitry just has a couple of
    resistors and a diode on each meter. The meters are driven directly from
    the current loop, not by any active circuitry.
    Don't know, but I'll try some test like this next time I go up.

    Thanks for your help - I'll post again to say how I get on with it.
     
  11. Jamie

    Jamie Guest

    with the work they put into the manual, you would have thought they
    could had made a real board or at least a hand sketched one.
    oh well.
    in any case, i would suggest looking at the op-amps and input devices.
    and even the regulator.
     
  12. w_tom

    w_tom Guest

    By looking at ICs only, I believe this is a voltage and a
    current or a two channel voltage monitor. I suspect that two
    4-20 cables carry analog data down the hill.

    Two inputs would be the INA118 Op amps. Two 4-20 ma wires
    would be driven by pin 11 of the AD694.

    Step one: what does AD694 pin one connect to? If to pin 2
    or 7, then 0 to 10 volts on pin 3 cause the 4-20 ma output.
    IOW make changes to the circuit board voltage input (to inputs
    of INA118) and observe corresponding voltage changes on pin 3
    of AD694. Now you know the input is good (or bad).

    Step two: see what comes out of AD694. Temporarily replace
    the cable going down hill with a 100 ohm resistor in series
    with a digital multimeter in 200 ma range. As those inputs to
    INA118 change, then current measured by multimeter will change
    from 4 to 20 ma.

    Step three: Assuming the above tests have worked, then
    perform the same step two test, this time by replacing the 100
    ohm resistor (even sold in Radio Shack) with the cable going
    downhill. IOW restore the connections except place the
    multimeter in 200 ma mode in series with one wire of that
    downhill cable. Again, changes to the input should measure
    same 4-20 ma numbers on meter.

    Step four: report back everything no matter whether problem
    is solved or not. This is a two way street. When done, all
    should have learned something.

    Step five: warning - the most common way to cause meter
    damage is to leave it in current mode when done. As a rule,
    always switch meter back to voltage measurement after taking
    current. Just a good habit to get into.
     
  13. Andy Baxter

    Andy Baxter Guest

    w_tom said:
    Thanks - I'll let you know how I get on.
     
  14. Franc Zabkar

    Franc Zabkar Guest

    I found this datasheet:
    http://www.analog.com/UploadedFiles/Data_Sheets/390142776AD694_b.pdf
    It looks simple enough to me now. Until I saw the photo I was thinking
    that the flashing LEDs may have been driven by a microcontroller as
    part of a POST. The 4mA-20mA current loop threw me as well, until I
    saw that the 4mA offset could be switched off at pin 9 of the AD694. A
    0-20mA current loop makes more sense than 4-20mA when driving an
    analogue meter.

    I suspect the LEDS may be connected to the ALARM pins (10) of the
    AD694s. The alarm goes low when the output voltage (11) has run out of
    headroom, ie when Vout = Vsupply - 2V. This could occur if the IC is
    driving a loop with an excessively high resistance. I'm guessing that
    the 555 timer may be wired as a one-shot, and that it applies a
    momentary test voltage to each of the AD564 inputs via the relay. Or
    maybe it just momentarily disconnects the current loops. This could
    explain the flash of LEDs at power-on.

    It seems to me that the easiest test would be to substitute your DMM
    (in ammeter mode) for the load at various points in the circuit,
    firstly at the output of the PCB, then at the end of the cable.

    Just one other thought. Does the meter display 0-30V, say, or does it
    use an expanded scale, say 20V-30V? It seems to me that anything below
    about 20V (for a 24V battery) would be uninteresting. I reckon it
    would be much better to see the detail at the top end of the voltage
    range.

    -- Franc Zabkar

    Please remove one 'i' from my address when replying by email.
     
  15. Andy Baxter

    Andy Baxter Guest

    Franc Zabkar said:
    From the manual, it's a 4-20 mA loop, but I don't know how this works at
    the meter end.
    That makes sense. There's also another timing element though - the system
    was designed to operate a solenoid to turn the inverter on and off
    remotely. This worked by breaking the circuit of the voltage measuring
    current loop, and triggering a deliberate fault condition. I think he did
    it that way to save on wires. That has some kind of timing associated
    with it, according to the manual, so the 555 might be for that.
    It's using an expanded scale - something like 20-30V but I'm not sure.
     
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