measure power using cro

Discussion in 'General Electronics Chat' started by harrypot_007, May 29, 2010.

  1. harrypot_007

    Thread Starter New Member

    Apr 27, 2010
    how can i measure the power dissipation of a circuit as a function of time using cro .
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Power is measure in watts, which are a function of voltage and current. Power metering is usually done with a wattmeter -

    You might be able to set up one oscilloscope channel to register voltage and another for current. With a USB output, you could log the data over time, but the accuracy might be less than what you need.
  3. whale

    Active Member

    Dec 21, 2008
    in case of ac power there is a special parameter called power factor, so the power you manipulated by multiplying voltage and current will give higher power, its always better to measure power with watt meter.
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    First, you need an oscilloscope that can do trace math. It must be able to show a trace which is the instantaneous product of the voltages applied to two channels of the scope.

    Here are two images showing the result. I put a small resistor (.1 ohm) in series with a compact fluorescent lamp to measure the current. You must use an isolation transformer for this particular measurement for safety.

    Channel 1 of the scope (orange) is showing the line voltage. Channel 2 (blue) is showing the current through the lamp. The red trace is the product of the voltage at channel 1 and channel 2; it is the instantaneous power into the lamp.

    The first image is from a lamp with an large inductive ballast. Notice that the power goes negative for a small interval of time. The current drawn from the line is nearly sinusoidal, although the power factor is not 1.

    The second image is from a more typical lamp. The current drawn is quite non-sinusoidal, but the power just barely goes negative. The power factor is worse than the first lamp because the current is so distorted.