NI Circuit Design Community Blog
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Ask a PowerPro: Power Dissipation
In last week's Facebook trivia question, we asked for the power dissipation through resistor R4. Congrats to Ruel for being the first to answer correctly! Let's take a look at a few different methods for solving this problem - first using hand calculations, then using Multisim to calculate the answer a little quicker.
Method 1: Solving by Hand
There are a couple different approaches that can be taken to calculate the power dissipation of resistor R4 - in this case, let's simplify the circuit, determine the current through resistor R4, and then use this information to determine the power dissipated. We'll begin by replacing resistors R3 and R4 with a parallel equivalent resistance, yielding a resistor value of 2.4 ohms. This new equivalent resistor is in series with resistor R2 (1.6 ohms), producing a new equivalent resistance of 4 ohms. Our new equivalent circuit is shown below.
We can then determine the current through the 4 ohm resistor:
Therefore, the current through R4 is:
And finally, the power dissiptated through R4 is:
Method 2: The Wattmeter
To use the Wattmeter in Multisim, simply place it on the schematic in the following configuration and then begin the simulation. Double-click on the front panel to view the results. (Reminder: voltage measurements are taken in parallel with a device, whereas current measurements are taken in series).
Method 3: DC Operating Point
Another method to calculate power dissipation for DC circuits is to run a DC Operating Point analysis, selecting P(R4) as our output variable of interest. To do this:
- Navigate to Simulate > Analyses > DC Operating Point
- In the Output tab, select variable P(R4)
- Click Simulate to generate the results as shown below
Method 4: The Probe
The probe has the ability to annotate a variety of attributes directly on the schematic. In this case, we use the voltage and current values provided by the probe within the following equation:
If you would like to try out any of these methods for yourself, download the current divider circuit attached. (Note: This is a Multisim 11 file. To download the latest version of Multisim, click here). Also, be sure to join the Multisim Facebook group to keep up to date with our latest trivia questions!
