Help meter blowing

Discussion in 'General Electronics Chat' started by john 77, Oct 3, 2004.

  1. john 77

    Thread Starter New Member

    Oct 3, 2004
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    0
    I have made a display for my caravan using LCD 200mV panel meters. There are 3 dislays showing caravan bat voltage car bat voltage and amps in or out of the caravan batery.
    The A display uses a 1mOhm shunt.
    Unfortunatly I have blown 2 panel meters for the A dislay. The shunt gives a perfect dislay on my multimeter (200mV scale) but it destroys the panel meter after a short time, particuarly on inital conection of the caravan batery.
    I asume it is generating some sort term high voltages or current.
    How can I filter this out ? Would a capacitor accross the caravan batery terminals or accross the shunt help ? what type of capacitor ? what sort of value?
    I am a bit desprate as I have cut out holes for the meters and now I carn't get one to work.
    Any advice would be appreciated
    Cheers John
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    That's always frustrating when things don't go right. The circuit attached might help, especially as you give no indication of how you have things hooked up. The resistive divider should be close enough - the 100 to 1 ratio makes the meter able to read 20 volts full-scale. With the decimal point placed before the last two digits, a fully-charged battery should read "13.20". The .1 uF capacitor will provide a path around the meter for impulse noise.

    Be aware, though, that some meters have to be sourced from a voltage other than the one they are measuring. I've never explored why some are like this, but the little guide with the meter forecasts smoke and doom if the meter tries to measure the same voltage as the one it's running on.
     
  3. john 77

    Thread Starter New Member

    Oct 3, 2004
    3
    0
    Dear Ultra Geek
    Many thanks for your prompt reply. I am farly shure the problem is not with the resistive divider. The meter requires a divider to read 20v which I have done for the voltage display. This voltage display works fine but the Amp display keeps blowing. The Amp display uses the meter "as is " as a 200mV meter that reads the voltage drop across a shunt ( 1 mohm resitor). The shunt is a comercal product designed to work with a 200mV meter. Setting my multimeter to the 300mV setting gives the correct reading.
    You idea about the meter reading a voltage it is measuring is also a good one. I have hit this problem before, but the meter display is driven by a seperate battery.
    Your capacitor solution sonds very promissing altough I dont understand why a 12V DC system needs such filtering .
    Should the .1 uF capacitor be across the terminals of the meter across the shunt or between the + and negative supply of the battery or all the above. What type of capacitor would you recomend.
    Cheers John
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    That's an interesting mystery, especially as the VOM reads correctly across the shunt. I've used the same methodology for years.

    The bypass capacitor is not critical. Any ceramic or monolithic type should work just fine. You might look at the location of the meter with respect to the sensing resistor. If there is some long lead length, the leads could be picking up some moise by inductive coupling. In addition to the capacitor (located across the meter input terminals), try twisting the leads to try cancelling induced voltages. Those panel meters usually have high input resistances, and are sensitive to voltage spikes.
     
  5. john 77

    Thread Starter New Member

    Oct 3, 2004
    3
    0
    Thanks for the advice. I got some capacitors today and will give it a try. The leads wires are long (about 3.5m) I used some data cable that is reasonably twisted together but it is now installed and would take a bit of work to change. fingers crossed for the capacitor.
    John
     
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