1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Can I measure 150V discharge current with basic scope probe.

Discussion in 'High Voltage' started by bobledoux, Jan 11, 2012.

  1. bobledoux

    bobledoux Member

    Joined:
    Jul 16, 2006
    Messages:
    259
    Likes:
    3
    Location:
    Near Salem, OR, USA
    I need to view the current draw curve of a capacitor charged to 150 volts as it discharges into an inductor.

    My scope is a Tek TDS1012. I'd like to view the voltage drop across a resistor in series between the capacitor and inductor. The scope ground would go to the resistor input and test lead to the resistor output.

    I'm concerned because the ground becomes 150 volts above the scope ground. The scope is rated for inputs up to 300 volts.

    Is there a procedure for doing this without cooking the scope?
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

    Joined:
    Apr 17, 2007
    Messages:
    7,363
    Likes:
    973
    Location:
    Loveland, CO USA
    Can you place your current sense resistor in the ground leg of the capacitor?
    The ground clip on your probe is at ground.
     
  3. squishy36

    squishy36 New Member

    Joined:
    Jul 22, 2011
    Messages:
    164
    Likes:
    9
    My method of choice for such measurements is to use a properly-rated differential amplifier; this is the right tool for the job, but many people balk at the cost -- for a hobbyist, the differential amp can cost more than their scope. Here's one example.

    The other option is to use the poor man's differential amplifier: use two scope probes, one on each side of the current shunt resistor, then subtract the two signals; the difference waveform is proportional to the current waveform. The disadvantage, of course, is that it uses up two channels of the scope.

    You can also float the scope, but I don't recommend this technique. I've used it successfully, but I felt the risks outweigh the benefits -- some day, an accident will occur and you only get one fatal accident in your life. That differential amp is a lot cheaper than an emergency room visit or a funeral.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. bobledoux

    bobledoux Member

    Joined:
    Jul 16, 2006
    Messages:
    259
    Likes:
    3
    Location:
    Near Salem, OR, USA

    Thanks for the comments. I'm a hobby person who rarely needs a differential amplifier.
     
  6. cowboybob

    cowboybob Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2011
    Messages:
    3,050
    Likes:
    479
    Location:
    James Island, SC
    Essentially this is a tank circuit, which is going to resonate, but to be sure of some things, can you provide a schematic of the circuit with power source(s) and grounds?

    I think I know of a simple way for you to get the reading you want, but it depends on circuit design issues we can't know of without the schematic.
     
  7. bobledoux

    bobledoux Member

    Joined:
    Jul 16, 2006
    Messages:
    259
    Likes:
    3
    Location:
    Near Salem, OR, USA
    I've attached the schematic. The load is a solenoid with a DC resistance of 4.7 ohms.

    I've added a resistor to the ground lead for current measurement using voltage drop, as suggested above.
     
  8. cowboybob

    cowboybob Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2011
    Messages:
    3,050
    Likes:
    479
    Location:
    James Island, SC
    OK. So not a resonant circuit. just a controlled relay.

    Looking at the schematic, C1 looks like it's serving as a filter cap. When Q1 fires, the 150 VDC will (should) supply the power to energize the relay coil, i.e., the cap won't discharge much of a discernible current. Of course, when the 150 VDC is removed (at power down, or whatever), then C1 will discharge eventually, but since we don't know what sort of bleeder resistors may be in the power supply circuitry, the time constant isn't known.

    That said, if the 0.5 ohm resistor is connected to an actual ground (not floating) then your ground test lead hookup looks fine.

    I might add, though, that it's unlikely that you'll see much of a voltage presence across R1 except at the initial application of power as the cap charges up to the 150 VDC limit.
     
    Last edited: Jan 17, 2012

Share This Page