Disconnected multimeter - AC Voltage : RF pick from PC power supply ?

Discussion in 'General Electronics Chat' started by bthomas, Aug 10, 2015.

  1. bthomas

    Thread Starter Member

    Aug 15, 2009
    11
    1
    My multimeter (Mastech MS8226T) measures about 200 mV without connecting the probes to anything, if placed near my Desktop PC. This can be as high as 400 mV if my meter leads are drawn out (as opposed to coiled or tangled). The voltage drops to about 10-25 mV when I move away from my PC to any other point in my flat. The multimeter without any leads connected shows about 25 mV when near my PC but settles to about 3 mV when away from my PC.

    I wonder if others have had similar issues. I presume this is RF pick up with the leads acting as an antenna. Can there be any other causes ? What is an effective resolution ? Does your multimeter also have similar behaviour ?

    regards
     
  2. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    What issues? Consternation that a high impedance meter detects energy when attached to leads and near an energy field? This is not going to affect your in-circuit readings. The resolution is psychological. Either don't measure the air near a power supply or try not to feel bad about it. Your meter is not broken. It is merely responding to reality.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Yes, it's common for unconnected leads to pick up various stray AC signals, such as from nearby electronics and the AC wiring in your building. These signals can be seen with a voltmeter or with an oscilloscope. It normally has little effect when you connect the leads to make a measurement as that effectively shorts the stray signals.

    What do you mean "an effective resolution"? :confused:
     
  4. bthomas

    Thread Starter Member

    Aug 15, 2009
    11
    1
    Bummer. My PC is on my table. A cheap and cheerful Chinese function generator (FeelTech FY3224) is beside it. I was trying to construct a bode plot of a simple first order RC filter (just for the learning experience). But the RF pickup just swamps my measurements. At times with the multimeter leads not connected I can measure upto 6 volts ! (with function generator not powered on but connected to the multimeter ). The function generator can only generate about 7-10V. So I was trying to make my measurements more accurate by eliminating all interference. The whole point is to get a interesting result with the simplest possible equipment.
     
  5. #12

    Expert

    Nov 30, 2010
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    Please show your circuit of a first order RC filter. I think we would be very curious about a capacitor that shows several volts of AC across it with the function generator turned off.
     
  6. bthomas

    Thread Starter Member

    Aug 15, 2009
    11
    1
    Sorry I meant that if I connected the function generator directly to the multimeter and at least a couple of times (depending on where and how I placed the meter and leads) I could get a couple of volts measurement. I was trying to locate the interference when I noticed such behaviour. This is erratic and not reproducible. The RC filter (aprox 1.5 kHz cut-off) is just a series resistor (1K) and capacitor (0.1 uF) connected across the function generator. I will give it another shot with me meter placed away from the PC. I am also considering making a mutlimeter lead from coax and see if that gives me better results.
     
  7. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    I understand what a long procedure it can be to, "make friends" with your multimeter. Every scale has its limitations, so there is a lot to learn. Even when you're using it correctly, strange readings result from the resistance of the wires, stray noise pickup, unstable connections, rust flakes, conformal coating...and the list goes on.

    Keep stabbing at it and we will help when we can.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    A function generator, when off, likely looks like an open circuit.
    What do you measure when the generator is powered?
     
  9. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    The instructions for my old 4-1/2 digit DVM gave a hint for reducing pickup like your are getting: Twist the meter leads together.

    Another comment on your measurements... you may find the that your meter has too little bandwidth to make bode plot measurements. Check the specifications for your meter.
     
  10. bthomas

    Thread Starter Member

    Aug 15, 2009
    11
    1
    With my multimeter connected to the function generator, with the generator in the powered off state I measure a frequency of 49.95 Hz (power line) and an AC voltage of 5.07 V .

    Now when I power on the generator and set it to a frequency of 0.001 kHz I am unable to get a stable frequency measurement. I can only get a stable frequency measurement if the frequency is at least a couple of kHz. At 10 KHz my meter reads an AC voltage of about 3.307 V.
     
  11. bthomas

    Thread Starter Member

    Aug 15, 2009
    11
    1
    Good point. Thank you. The meter (Mastech MS8226) can measure frequencies in the range 5Hz to 10MHz. However the AC voltage measurement range is given as 40-400Hz. So you are probably right and this makes it difficult for me to use it to construct a bode plot.
     
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