Connect with us

Voltmeter with hold reading

Discussion in 'Electronic Basics' started by mg, Jun 20, 2005.

Scroll to continue with content
  1. mg

    mg Guest

    Hello All,

    I posted a few months ago about building a discharger for a DeWalt 18v
    battery. Thanks to everyone who contributed suggestions and ideas. We
    have our first one built and working.

    A bit of background - We have a DeWalt 18v recip saw on our rescue
    van. We have used it during extrications where we have a great deal of
    water (i.e. a stream). We use our corded recip saw in less hazardous
    environments. We have three DeWalt batteries. Each battery rotates
    from 1) charger to 2) saw to 3) spare every week. It's not unusual for
    this recip saw to go unused for almost a year at a time. We needed
    some way to check the batteries. So, we decided to build a discharge
    unit to determine the condition of the batteries.

    We would like to add one additional feature to our discharger. Some
    sort of voltmeter that we could hold the last reading. This would
    allow us to double check that we stop the discharge process at 15v.

    We have a DPDT relay that we use to control a timer. This allows us to
    monitor how much time it takes for each battery to drop to 15v. We
    hope that this will allow to determine when we need to replace a
    battery.

    Does anyone know of a voltmeter that would allow us to hold a reading
    via some sort of external relay or contact?

    TIA,
    Mark
    Firefighter Sky Valley / Scaly Mountain Volunteer Fire & Rescue
    --
    Email hint - Everything after the @ is spelled backwards.

    There are only 10 types of people in the world:
    those that understand binary, and
    those that don't.
     
  2. Hi Mark, I remember your earlier posts.

    Just how long do you want to hold the reading for?

    Simply having a BIG electrolytic capacitor across the
    battery - that is disconnected by a spare set of contacts
    when the discharge phase ends - may do. The electrolytic
    will start to self discharge once it is disconnected, so it
    won't hold the sample for very long and will discharge
    (slowly) into the typically 10Mohm meter impedance once you
    start measuring the voltage. But you could try this out and
    it may be all you need.

    If that isn't good enough, then you can use a simple unity
    gain very high input impedance operational amplifier circuit
    between the capacitor and meter. The capacitor can then be a
    lot smaller, eg less leaky. With suitable design (circuit
    layout as well as circuit design), that should hold a sample
    for the odd hundred + times longer, at a rough guess.


    If that isn't good enough - yhen two other options spring to
    mind:

    A cheap digital camera arranged to photograph the meter
    display at the end of the discharge phase. You can get them
    for under a tenner here, now. That will store the reading
    "indefinately".

    A free-running A>D linked to a D>A converter. Simply trigger
    the D>A at the time the reading is needed - or do without
    the D>A and just use a latch and a row of LEDs.
     
  3. Rich Grise

    Rich Grise Guest

    Wouldn't it make more sense to, rather than just try to see what the
    volts were when the timer timed out, monitor the volts to see when
    the discharger's done, and see how long it takes? You could use an
    ordinary (analog) electric clock - when your controller says, "15v!" you
    shut down and the clock stops. You don't even have to set it to "real"
    time - set it for noon, or something, start your discharger, and read
    the hours right off the clock face at your leisure.

    Good Luck!
    Rich
     
  4. John Fields

    John Fields Guest

    ---
    from your description, it doesn't sound like you need to hold a
    voltmeter reading as much as it does to determine the amount of time
    elapsed between when you started discharging the battery and when its
    voltage, under discharge, fell to 15V. An easy way to do that would
    be to start a clock when the discharge process started and then to
    stop it when the battery voltage fell to 15V. A simple comparator
    working against a voltage reference, a counter with a 1 second time
    base, and a seven-segment display frozen at the time accumulated when
    the 15V discharge point was reached would do it.
     
  5. Roy Lewallen

    Roy Lewallen Guest

    I've used an even simpler method for many years. The comparator opens a
    relay which removes power to an old-fashioned analog electric clock. The
    clock is set to 12:00 at the beginning of the test, and shows the
    elapsed time at the end.

    Roy Lewallen
     
  6. Brian

    Brian Guest

    www.fluke.com


    Hello All,

    I posted a few months ago about building a discharger for a DeWalt 18v
    battery. Thanks to everyone who contributed suggestions and ideas. We
    have our first one built and working.

    A bit of background - We have a DeWalt 18v recip saw on our rescue
    van. We have used it during extrications where we have a great deal of
    water (i.e. a stream). We use our corded recip saw in less hazardous
    environments. We have three DeWalt batteries. Each battery rotates
    from 1) charger to 2) saw to 3) spare every week. It's not unusual for
    this recip saw to go unused for almost a year at a time. We needed
    some way to check the batteries. So, we decided to build a discharge
    unit to determine the condition of the batteries.

    We would like to add one additional feature to our discharger. Some
    sort of voltmeter that we could hold the last reading. This would
    allow us to double check that we stop the discharge process at 15v.

    We have a DPDT relay that we use to control a timer. This allows us to
    monitor how much time it takes for each battery to drop to 15v. We
    hope that this will allow to determine when we need to replace a
    battery.

    Does anyone know of a voltmeter that would allow us to hold a reading
    via some sort of external relay or contact?

    TIA,
    Mark
    Firefighter Sky Valley / Scaly Mountain Volunteer Fire & Rescue
    --
    Email hint - Everything after the @ is spelled backwards.

    There are only 10 types of people in the world:
    those that understand binary, and
    those that don't.
     
  7. mg

    mg Guest

    Just to clarify a few points.

    We have the elapsed timer working well.

    We discharge the battery very, very slowly. The elapsed times are
    running between 5.5 and 6 hours. Because of the long elapse time, we
    can't just have someone hang around or remember to return to the
    station at the correct time to check the voltage.

    What we want to do is record or hold the ending voltage somehow. As
    this is our first attempt at a discharger, we want some way to double
    check that we don't drop below 15v. We are pretty green at putting
    this together. And let's just say we aren't 100% confident that we
    have everything right.

    Our first thought would be to use some sort of voltmeter that had an
    external method of triggering a "hold". We are open to suggestions.

    TIA,
    Mark
    --
    Email hint - Everything after the @ is spelled backwards.

    There are only 10 types of people in the world:
    those that understand binary, and
    those that don't.
     
  8. ehsjr

    ehsjr Guest

    Mark,

    It makes more sense to test the discharge circuit than to
    monitor it, in this case. Connect it to a variable power
    supply set to say 17 volts, and start the discharger.
    Slowly reduce the voltage from the supply, while monitoring
    with the DMM, until the discharger turns off. Do that
    a few times to make sure you get the same reading, and
    you will know what the discharger cut off point is.
    You can repeat that test whenever you want.

    If you don't have one, you can build a variable supply for
    less than 10 dollars worth of parts - a 24 volt dc wall wart
    from Allelectronics costs $4.50, and you need an LM317 and
    the circuit on page 9 of
    http://cache.national.com/ds/LM/LM117.pdf

    That will give you a regulated variable supply capable of
    600 mA. I don't know what the discharger load draws,
    but if it is taking 5.5 to 6 hours to discharge the
    packs, 600 mA is more than enough.

    I don't remember your discharger circuit - was it the TL431
    driving a transistor that operated a relay?

    Ed
     
  9. So, why not try my idea of using a capacitor to store the
    value - together with a relay contact that will isolate the
    capacitor at the end of the dischage phase?

    From the above, I guess that you want to hold the value for
    a few hours, so an electrolytic isn't going to manage - so
    use a polypropylene instead.

    Try this as a quick test: Get say a 10uF polypropylene
    capacitor, shove it across a battery to "sample" the battery
    voltage - disconnect it and leave it for whatever time you
    think you need. Then stick it across a multimeter and take
    the immediate* reading and compare it to what the battery
    voltage was when its voltage was "sampled". You may find
    that the readings are near enough for what you need.

    *The voltage on the capacitor will start falling quite
    quickly - once the multimeter is connected - so you do have
    to take an instant reading.
     
  10. I don't know if that will work with the capacitor.However, I have a nicd
    battery charger/discharger for my digital camera.The digital camera uses
    only partially the charge of the nicd battery, so they need to be discharged
    first.mg however wants to find if the batteries are ok, AFAIK nicd batteries
    last up to 1000 charge-discharge cycles, and I think IMHO is a good idea to
    run the saw from time to time so that condensed humidity will evaporate.The
    nicd batteries according to my experience can keep their charge for months,
    as I have a pocket light with nicds.
     
  11. mg

    mg Guest

    Ed,

    A very good point about testing the discharger. I'll mention this to
    some of the crew tonight (we do training every Tuesday evening).

    Whoa.... The circuit on page 9 went way over my head. I'm always
    willing to learn something. Can someone translate the circuit into
    simple terms?

    We used a DPDT relay that looped back to keep itself energized. We
    have 660 ohm resistor and an adjustable potentiometer in series with
    relay. We used the adjustable potentiometer to set the relay to drop
    out at 15v. I don't understand enough electronics to use a TL431. From
    the research I did, a TL431 seemed like something we could use. I just
    didn't understand it enough. Something like a DPDT relay and resistors
    - I can understand. A TL431 seemed more exact than resistors. Again,
    if someone can explain using a TL431 in simple terms, I'm all ears.

    Just so I don't forget
    THANKS to everyone for the suggestions.

    I hope we don't have to use our recip saw on a vehicle extrication.
    But, thanks to the generosity of folks here, we'll have full use of
    the saw and batteries.

    Mark
    --
    Email hint - Everything after the @ is spelled backwards.

    There are only 10 types of people in the world:
    those that understand binary, and
    those that don't.
     
  12. John Fields

    John Fields Guest

     
  13. Any particular reason why it shouldn't be worth trying?

    The 10uF polypropylene capacitor will, of course, start to
    discharge once removed from the supply - but the internal
    leakage resistance will typically be of the order of greater
    than 1,000,000,000,000 ohms (from a spec.sheet) - so it will
    take a while (over a year to fall 2%, if that was the only
    leakage). There will be surface leakage as well, but that
    shouldn't be too great - it isn't as if it is mounted on a
    pcb and will need guard rings and the rest. Its voltage
    isn't being constantly monitored, so won't be loaded by that
    circuitry. So, it should hold the voltage for quite a few
    hours, without difficulty.

    Of course, once the meter is put across it, it will start to
    discharge into the meter input impedance. However, even if
    you take 2 seconds to read the meter, the time constant is
    such that the value will still be within 2% (and the meter
    is probably only accurate to that..). If you take 5 seconds
    to read the meter, the value will still be within 5%.

    If anyone has a 10uF polypropylene capacitor, a 15 volt
    supply and a test meter, perhaps they would give this a try
    and report back?
     
  14. ehsjr

    ehsjr Guest

    I'll try a diagram:
    D1
    +---------|<----------+
    | ----------- |
    | | LM317 | |
    Vin-----+----|Vin Vout|----+--------+--------+---Vout
    | Adj | | | |
    ----------- --- / |
    | / \ D2 \ R1 --- C1
    | ----- / ---
    | | | |
    +-----+----------+--------+ |
    | | | Gnd
    | / | +
    +---->\ --- C2
    / R2 ---
    \ |
    | |
    Gnd Gnd


    Ok, first D1 and D2 are diodes that are used to protect
    the LM317. The symbol ---|<--- shows an "arrow" ( < )
    pointing to a vertical line. The actual component is
    a little cylindrical thing with one wire sticking out
    from each end, and with a white band "painted" on one end.
    The vertical line in the symbol represents the banded end
    of the actual diode. In your case, you could probably
    omit D1 and D2.

    Next, C1 and C2 are capacitors that tend to stabilize
    circuit operation and minimize ripple voltage. They
    can also be removed for your purpose. That reduces
    the circuit to just three components, and I'll draw it below:

    -----------
    | LM317 |
    Vin----------|Vin Vout|----+------------Vout
    | Adj | |
    ----------- /
    | \ R1
    | /
    | |
    +-----+----------+
    | |
    | /
    +---->\
    / R2
    \
    |
    Gnd

    The LM317 has 3 leads, called Vin, Adj and Vout. If
    you look at page 2 on the site, you will see which
    physical lead corresponds to which place on the
    diagram. (I'll draw it below, anyway.) You are
    already familiar with resistors and potientiometers -
    R1 (240 ohms) and R2 (5K pot), respectively, in the
    diagram.

    Vin refers to + input voltage. It is the place where
    you connect the positive wire from the 24 volt wall
    wart. (Allelectronics part #DCTX2460) The other
    wire from the wall wart connects to Gnd.
    Vout refers to the output voltage (positive). You
    connect your discharger to Vout and Gnd.

    You adjust the output voltage (Vout) by turning
    the pot (R2)
    ________
    | O |
    |________|
    /_______ /|
    | | |
    | | |
    | |/
    -------
    | | |
    | | |
    A V V
    d o i
    j u n
    t





    Here's the TL431 circuit - 3 resistors, a pot, a relay
    a diode, and a momentary pushbutton switch:

    Relay
    Contact2
    /
    +---------------------o o---------------+
    | |
    | Relay |
    | Contact1 |
    | / D1 /
    +----o o---+-----|<--------+ \ Load resistor
    | | | /
    | __ | | \
    +----o o---+---RelayCoil---+ |
    | PB | | |
    | R1 R4 |
    + | | | |
    Battery | ___|__/ |
    | \ / / \ |
    | R2 /<----------->/ \ TL431 |
    | \ ----- |
    | | | |
    | R3 | |
    | | | |
    +-----------+---------------+-------------+

    D1 is a 1N4001 diode
    PB is a pusbutton switch (eg MPB-1 from Allelectronics)
    R1 is 2.2K
    R2 is a 50 K pot
    R3 is 2.2K
    R4 is 1K Ohms
    Relay - # RLY-622 from Allelectronics

    Load resistor - this depends on the rate at
    which you want to discharge the battery.
    It must be at least 20 ohms, and at least 5 watts.
    Part # 50-25 from Allelectronics will discharge
    your battery at a rate of about 330 mA.

    Ed
     
  15. Yes, of course, but I mean it's not a *standard* industrial methode.Of
    course, anyone can try whatever he likes, particularly in low voltage
    circuits, which are a nice field of experimentation.Mains voltage is a
    different story.AFAIK, nicd batteries need discharging in a discharger only
    if they are partially used in the appliance before charging them, due to
    memory effect.
     
  16. ehsjr

    ehsjr Guest

    Make that DCTX-2460
    ^
     
  17. John Savage

    John Savage Guest

    I risk showing my ignorance here, but amongst the bewidering myriad
    functions on a digital camera's menu, isn't there one where you can set
    a time delay of 5.5 hours? If so, just photograph your voltmeter!
     
  18. Guest

    How about just measuring the battery voltage when you get around to it
    after the test?

    Once the relay opens the battery should stop discharging.

    Of course it'll slowly discharge but if it is any good at all a few
    hours won't make any meaningful difference in the voltage you measure.

    Say that you load the battery for 5.5 hours (no comparator circuit,
    just a timer) and come back to measure its voltage 8 or some other
    number of hours later. If the voltage is still above 15 you are sure
    that it at least met the 15V after 5.5 hours of load criterion.

    If the voltage is below 15, you know it either failed the test at 5.5
    hours or was close to failure and self-discharged to an unacceptable
    level thereafter. Either way it is no good.

    Dave
     
  19. ehsjr

    ehsjr Guest

    Suppose he measures the voltage of the battery, about 8
    hours after starting. What do each of the following
    possible readings tell him?
    1) 15.5V
    2) 15.4V
    2) 15.3V
    4) 15.2V
    5) 15.1V
    6) 15.0V
    7) 14.9V
    8) 14.8V
    9) 14.7V
    10) 14.6V
    11) 14.5V

    I think he wants to eliminate uncertainty.

    Ed
     
  20. Rich Grise

    Rich Grise Guest

    You need to let go of this mindset.

    Throw the timer away. (or put it back into stock ;-) ) Turn off the
    discharger based on voltage, not time. _That_ will ensure that you
    don't drop below 15V, _and_ it will ensure that you _do_ _reach_ 15V.

    Use the circuit that ehsjr suggested (or any threshold comparator-
    type circuit) to turn off the discharger _when the battery voltage
    decreases to 15 volts_. That way, you're assured that the discharge
    stops when the battery is at 15 volts. It doesn't matter how long it
    takes, and in fact, shouldn't be expected to take any particular amount
    of time, because it will vary depending on the condition of the
    batteries when they're mounted.

    Now, if you think it's important, you could put an ordinary line-
    operated, motorized electric clock on another set of relay contacts.
    Set it for, say, 12:00, turn on the discharger, and go home. When
    you come back the next morning, the time when the clock stopped
    will be "how long it took for this battery to discharge to 15 V."
    (or that plus 12 hours ;-P ) (don't set the clock to the actual
    time of day, unless you want to write it down and do arithmetic in
    hours & minutes.)

    And, of course, you can check the battery voltage at your leisure -
    that should hold up better than a capacitor. ;-)

    Cheers!
    Rich
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-