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Measuring Current with an Oscilliscope — Parallax Forums

Measuring Current with an Oscilliscope

poorieuserpoorieuser Posts: 7
edited 2007-10-29 23:38 in BASIC Stamp
Ok. So I'm creating a circuit to test batteries in a camera. The circuit is basically representing a cameras load and flash. The problem I'm having is using an oscilliscope to record current reeadings onto my computer. I've read about using current sense resistors but I'm really not sure how this works. If it helps I can post a schematic of the circuit itself after I finalize it this afternoon.


Thanks a lot,
PoorIEuser

Comments

  • FranklinFranklin Posts: 4,747
    edited 2007-10-29 16:43
    To measure current you actually measure voltage across a known resistance and calculate the current.

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    - Stephen
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-10-29 16:50
    Which is made difficult with many O'scopes, because the 'ground' lead of the o'scope tends to be tied to 'safety' ground at the o'scope.

    Meaning, you can't just put the scope probe across any old resistor. Instead you need to put the ground lead somewhere near 'ground'. This isn't a show-stopper, it just means you need to be careful.

    Having said that, a paper clip, straightened out and covered with heat-shrink, makes a pretty good 0.1 ohm current sensing resistor.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-10-29 17:00
    A current-sense resistor is a small-valued resistor, typically less than one ohm, that's put in series with the load. By measuring the voltage across the resistor and dividing your reading by the resistance, you obtain the current through the resistor and, hence, through your load.

    To view the instantaneous current over time, you can connect your scope leads across the resistor. You have to be careful when doing this, however. Some hookups require that the circuit under test be isolated from all other grounds. Here's an example of a bad hookup:

    You have your sense resistor on the positive side of the load. Your circuit under test is also connected to, and shares a common ground with, your PC. You connect your scope's negative lead to the negative end of the sense resistor. ZAP! What just happened? Many scopes have their negative inputs connected to chassis ground, which also connects to the ground prong on the AC power cord. PCs do the same: logic ground is connected to the AC line cord's ground prong. By using the above hookup, you've inadvertently connected your positive supply, attenuated only by the sense resistor, to ground!

    There are two ways to remedy this situation, if you must keep a PC (or other grounded apparatus, such as a an AC power supply) connected:

    1. (Okay in some situations) Switch the sense resistor to the bottom of the load, so one side is grounded in common with your PC's ground. This is not always possible, though, since the voltage drop through the sense resistor may cause an undesirable logic-level shift in your circuit's PC interface.

    2. (Better) Use a dual-channel scope. Most dual channel scopes let you display a trace which shows the difference between two input voltages. Therefore you can connect two probes, one to each end of the sense resistor, and both shields to your common ground.

    -Phil

    Post Edited (Phil Pilgrim (PhiPi)) : 10/29/2007 5:13:03 PM GMT
  • metron9metron9 Posts: 1,100
    edited 2007-10-29 17:02
    allanlane5, I never thought about that, how about using a dual scope and measuring the voltage on both sides of the resistor to ground, subtract for the difference. Would that work?

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  • poorieuserpoorieuser Posts: 7
    edited 2007-10-29 17:17
    Hmm I have a Parallax O'scope I plan on using so that's two channels. What I'm picturing with this set up is what metron9 is describing but I'm not sure how that gives me current from the whole circuit.
  • Professor ChaosProfessor Chaos Posts: 36
    edited 2007-10-29 18:30
    I have always been confused about this aspect of oscilloscope operation. If you are using a USB oscilloscope:

    - Are you connected to AC ground if your computer is running off a transformer with a two-prong plug? (e.g., a laptop with a power brick).

    - What if your PC is a laptop running on battery power?


    In these cases, can you safely measure low voltages between two non-ground points in a circuit?


    For that matter, since nearly all of our DC circuits run on wall-warts or batteries, I have never been clear on what the connection to AC or earth ground really is....
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-10-29 23:38
    Yes, if you connect probe A's "Ground" to signal ground, then probe B's "Ground" to signal ground, then connect probe A to one side, the Probe B to the other side, then run them in difference mode, then yes, you'll get a voltage proportional to the current through the sense resistor.
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