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Q on useage of Fluke 8845A

Discussion in 'Electronic Design' started by Robert Baer, Apr 19, 2007.

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  1. Robert Baer

    Robert Baer Guest

    Say one has an adjustable current limiter driven by a 15V supply, the
    load is a zener in series with the 100 ohms current monitor (Fluke input
    Z in 100uA mode), and the zener voltage is also monitored by the Fluke.
    Now add in a program to switch relays where in one position the Fluke
    is connected to measure the voltage but the diode current goes thru a
    dummy 100 ohm resistor; the other postion connects the Fluke current
    monitoring (as originally mentioned) and disconnects the voltage side
    (and connects a 10Meg dummy resistor).
    The program switches the Fluke to the desired voltage or current
    scales and switches the relays.

    Now comes the problem.
    If the relays are switched first, then the current terminals (or
    voltage terminals) get voltage before the Fluke is switched to the
    appropiate mode. Ditto if the range (voltage / current) of the Fluke is
    switched first.
    I think it is safe to assume the external relays will switch slower,
    so the programming "solution" of switching both "at the same time" will
    still result in the fault conditions mentioned.

    Nobody (not even Fluke) talks about voltage across the voltage
    terminals when the meter is in current mode, also, nobody talks about
    voltage across the current terminals when the meter is in voltage mode.

    So, what is the answer?
    What are the safe limits (both cases)?
     
  2. Jim Yanik

    Jim Yanik Guest

    Use separate meters for I and E.
    OR,use a fixed 100 ohm R and use the relay to alternately measure E across
    the R and the Zener. Calculate the I.

    Use an AM503/P6302 current probe/scope in place of the Fluke for current
    measurement.It works down to DC.Then you can use a very low R wire in place
    of the 100ohms of the DMM.
    15 volts across the 100ohms of the 100ua scale will be FAR over the scale.
    Thus not good. Will the Fluke autorange? That may be another problem.
     
  3. Robert Baer

    Robert Baer Guest

    1) I cannot afford *two* Fluke 8845As at $1K each (i am Socially
    Insecure - on Uncle Sugar fixed "income").
    2) I cannot afford a 0.01% 100 ohm resistor to get a similar number of
    significantly reliable digits.
    3) AM503/P6302???? You have to be joking - i could get slide rule
    accuracy with a VOM!
    4) Who said that there would be 15V to begin with?
    a) The zener drops some voltage.
    b) If the meter *was* on !any! current scale, the voltage drop would
    be 100mV full scale.
    c) If the meter is on (any) voltage scale, the current terminal would
    appear to be either open or "high" non-linear resistance (do not have
    meter yet - still accumulating $ for it).
    d) It seems that if one programs it, then it does not autorange (if it
    does that at all). Not clear on that aspect of the meter; i have seen no
    information concerning autoranging - only autozero which can be turned off.
     
  4. Jim Yanik

    Jim Yanik Guest

    You never mentioned what resolution you were seeking for your measurement.
    (accuracy and resolution are different things)

    Are you designing some sort of precision V reference?
    temperature will have an effect on your zener.
    Current limiter driven by *15v* is what you said;since if the circuit
    opens(by the relay),even briefly,the limiter will try to raise the current
    by raising the VOLTAGE to it's max,that is where you get the 15V from.

    (I learned that the hard way,got a 500V shock from a current source dialed
    down to zero current,an old meter calibrator)
     
  5. Robert Baer

    Robert Baer Guest

    Crude description of circuit: +15V, then current limiter to zener to
    be tested, then 100 ohm for current sample; top of zener to voltmeter.
    Currents to be forced: 10uA to 560uA in log steps for a total of 8
    currents.
    Relay switches DVM: meter voltage line to either top of zener ("V"
    mode) or open (top of zener switched to dummy 10Meg = "I" mode); current
    line to either open (bottom of zener switched to dummy 100 ohm = "V"
    mode) or bottom of zener to meter current line.
    So far,it seems the best program order is (fixed width font):
    relays | V | V | I | V | V
    -------+---+---+---+---+---
    DVM | V | I | I | I | V

    So the absolute maximum voltage at the DVM current input(s) would
    happen when one half of the relay is (still) in the voltage mode, the
    other half switched over to current mode, and the DVM in the voltage mode.
    That would be 15V minus about 1.7V current limiter drop minus zener
    voltage (seems will be in the region of 8V), or about 7V worst case.
    Since the contacts are being moved by the same armature, it would
    seem that both arms would move about the same time and one would not
    remain in the NC position while the other arm was moved to the NO position.
    So i think that i have taken reasonable precautions, but $1000 out of
    my pocket is a major expense and i would like to protect that investment
    (hence the posting of the Q).

    I have not taken precise measurements or used a scope, but it seems
    these relays actuate in about 5mSec and bounce for about 1mSec.
    The relays i use are Axicom V23105-A5003-A201 with 12V coil driven
    from +15V and 270 ohm series resistor (that series combo has a diode
    snubber across it).
    I do have some NEC MR62-12UKSRY relays that i could use instead (used
    a DIP socket).
     
  6. joseph2k

    joseph2k Guest

    I think you need 3 relays, one to go across the current sense path (perhaps
    with a small series resistor to match meter loading), one to switch the
    leads to the current path for measure measurement, and one to switch the
    meter to the zener voltage measurement. Sequencing becomes obvious and
    relay operating speed is almost a non-issue this way. I did a lot of
    things like this 20 to 30 years ago.
     
  7. Robert Baer

    Robert Baer Guest

    I will draw that up and consider it.
    Thanks.
     
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