# 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
1
0
sir,
how can i measure the power dissipation of a circuit as a function of time using cro .

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Power is measure in watts, which are a function of voltage and current. Power metering is usually done with a wattmeter - http://en.wikipedia.org/wiki/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
111
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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 value.so, its always better to measure power with watt meter.

4. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,630
468
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.

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