oscilloscope and function generator

Discussion in 'General Electronics Chat' started by PG1995, Apr 17, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Hi

    I wanted to know how I can check the AC waveform on the O-scope. First thing I would need to do is set the O-scope's mode to AC waveform. Do I need to use a series resistor to limit the current, or does the O-scope have its own built-in automatic system to limit the current? If it has some automatic mechanism then that would mean I can directly connect the O-scope with the main. Any other suggestion or advice? Please guide me.
    Thanks.

    Regards
    PG
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If the average DC voltage from the function generator is near 0VDC then the O-scope can be set for AC or DC.
    The input resistance of an O-scope is very high (1M to 10M ohms) so it draws almost no current from the signal.

    The mains voltage might blow up the input circuits of an O-scope. Use a step-down transformer to view the mains waveform.
     
  3. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thanks, Audioguru.

    Here the mains voltage is 220V. Can't I use 1000 ohm resistor in series? Then only 0.22 A current will pass through the resistor, and I can connect O-scope in parallel to the resistor to read the mains voltage. What is your thinking on this? Please let me know. Thanks.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If you connect a 1k resistor in series with the input of the O-scope then there is nothing to prevent the input circuit of the O-scope from blowing up due to over-voltage.
    The 1k resistor does not limit the voltage, it limits the current into a short circuit. But the input circuit of an O-scope is not a short circuit, it is a delicate amplifier.

    Use a step-down transformer.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Because you're a newbie at this, we should ask: Why do you want to connect a scope to the mains?

    It is not terribly difficult to use an oscilloscope to monitor voltages like AC line voltages (and higher), but it can take some special equipment that a newbie probably won't have. There are also safety considerations that a newbie probably won't understand. Thus, we discourage beginners from doing such things.

    If you really have to do this, the best approach, like Audioguru said, is to do it through a step-down transformer to bring the voltage down to a relatively safe level -- say, in the 1 to 10 volts range.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Make and post a video of your O-scope blowing up when you connect its input to the mains!
     
  7. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Besides, 0.22A through a 1k resistor is almost 50W.

    The best way to get the right step down transformer is to take apart a small stereo, alarm clock, or anything which requires low voltage DC, and probe the output with your scope. NEVER connect the earth terminal of the probe to neutral (you may create a small explosion *especially* if live and neutral are reversed); connect it to the other end of the transformer's secondary.
     
  8. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thank you all of you. I understand your advice and suggestions. The follow-on questions are simply out of curiosity. Audioguru, your Uncle Scrooge is very nostalgic for me. Some years ago, I and my sister used to watch this cartoon everyday!

    Please have a look on this image:

    [​IMG]

    If the mains voltage is 220V, the potential difference across R1 would be 55V. Here the O-scope is simply a voltmeter with very high resistance.

    This is the picture of the O-scope I have access to:
    http://img8.imageshack.us/img8/5408/oscillo.jpg

    I think this scope has internal resistance of 1MΩ. But you will see 25PF. What is this "25PF"?

    Does this O-scope have any setting to increase the sweep time so that I can see the waveform being drawn?

    Please help me with the above queries. Thanks alot.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I don't know if the input of your 'scope can survive the peak voltage of 240VAC which is plus and minus 340V, plus some more due to voltage spikes.

    The input circuit of your 'scope and its wiring are 25pF.

    Every 'scope has a sweep time adjustment switch and potentiometer.
     
  10. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    If V is 240V. Then current through the circuit (R1 and R2) will be 240/4000 = 0.06A. The voltage across R1 will be 0.06x1000 = 60V. So, won't the input voltage for the O-scope 60V because it is making parallel connection with the R1?

    I don't get that 25ρF thing. The unit ρF is used for capacitors. Would you please explain it a bit?

    Thank you for all this guidance.
     
  11. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    240 volt mains is 240 volts RMS. The scope measures peak to peak voltage. 1v RMS is 2.82v P-P. 240v RMS is a lot P-P.

    The scope has a "Time/Div" selector switch and a "Variable-Cal" control which determines the sweep rate. With the VAR knob full CW to the Cal position, the Time/Div switch will tell you how fast the sweep moves across the screen for 1 major division. If your mains are 50Hz then 1 cycle will occur in 20ms. If you set Time/Div to 20ms/div then you will see 1 cycle in 1 division. If you set it to 2ms/div, you will see 1 cycle in 10 divisions. Generally, in the Cal position, the sweep is only accurate to 3% or so.

    1 megohm/25 pf means that the scope input "looks" like a 1 megohm impedance with a 25 pf capacitor across it.
     
    PG1995 likes this.
  12. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    You're still neglecting the fact that those resistors will very quickly go up in smoke unless they are high power 10W resistors and even then they will get very hot. Especially note that in some failure modes resistors can short potentially giving the full 230V across the scope's inputs. Also, note that the EARTH terminal on the scope is actually connected to the earth usually by means of plumbing or a direct wire into the ground. The earth terminal is often tied to neutral at some point near the substation so you will effectively be shorting live to neutral with a 1k resistor. If you have a decent electrical system set up at home or an office you will hopefully trip the RCD/GFCI and/or breaker, but not always!

    Use a transformer, it is MUCH cheaper than a new scope and/or fire damage!
     
    Last edited: Apr 18, 2011
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your 4k ohms worth of resistors will dissipate 14.4W! Why not 40k or 400k to reduce the heat?

    The 240VAC mains has peak voltages of plus and minus 340V plus more for spikes. Most electronic circuits blow up with such a high voltage.
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Since one side of your scope is earth ground, using your schematic, it is possible to let a lot of the magic smoke out! USE A TRANSFORMER UNTIL YOU FULLY UNDERSTAND PROPER USAGE OF YOUR TEST EQUIPMENT!!!
     
  15. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    If the OP does so, they will then have to take into account the scope's input resistance as it will no longer be negligible.
     
  16. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    IF you have 2- 10X probes which will divide the incoming voltage by a factor of 10, AND IF your dual trace scope can do differential measurement (ch A minus ch B) then you might be ok.
     
  17. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    You might be okay as the 1 Mohm resistor will dissipate only 50.3mW and if the scope is not using a small SMD resistor it could be handled... but, then again, what semiconductor is connected to the 230V? That may well break down and render the scope's input useless... be very careful.
     
  18. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    That's a $300 Pintek PS-200 scope. It comes with a pair of 1X/10X probes. Max scope input is 400v AC + DC. It can do A + B and invert B, so it'll do A - B.
     
  19. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    It is probably limited to 20V/div though and the offset can usually move by two display heights but no more. 10X would be safer... but I still would be nervous with connecting such an expensive bit of kit to the mains. Transformer = so much cheaper than a new scope.
     
  20. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thank you very much, everyone. I appreciate your replies and they were helpful. I will ask some follow-on questions soon.

    Best wishes
    PG
     
Loading...