Technical Insights > Benchtop

How to Make a DC Power Measurement with Digital Multimeters

2018-12-13  |  7 min read 

Digital multimeters (DMMs) are incredibly versatile for almost all forms of voltage, current, and resistance measurements. Using simple math, you can measure power in watts too. While many dedicated instruments have been introduced in the past few years for measuring DC power, DMMs continue to be useful.

Most bench DMMs are single channel instruments. In order to measure the DC power of a device under test (DUT), the general method is to use two DMMs to measure the DC voltage and the DC current, and multiply both results together to obtain the DC power.

Diagram represents a typical DCV and DCI measurement configuration using two DMMs
Figure 1. Diagram represents a typical DCV and DCI measurement configuration using two DMMs.

In this blog post, we will explore how to measure a simple DC power measurement of a DUT using a DMM. Some DMMs have multiple channel inputs but they are more expensive and generally take up larger footprints in your test environment. For the purpose of this post, we will use a typical single channel DMM, Keysight’s Truevolt 34465A DMM. Most product test benches have just one DMM.

There are two possible conditions when measuring your DUT’s DC power:

  • You have the control to set either the voltage or current across the DUT to be constant
  • You have no control to set either the voltage or current across the DUT to be constant

Here are the methods to address each of the conditions respectively.

Method 1: Measure DC power when either the voltage or current is constant

Very often, you have the control to source either constant voltage or a constant current to your DUT. Let’s take an example of sourcing a constant across a DUT as shown in Figure 2 below. If we know the voltage across the DUT, all we need to know is the current. To measure the current, place a DMM in series with the DUT on the return path.

Diagram showing DMM DCI measurement
Figure 2. Diagram shows DMM DC current measurement of a DUT.

Modern DMMs have math scaling capability. For example, Figure 3 shows a primary and a secondary measurement display. The secondary measurement is a pre-math display in blue and shows the raw electric current measurement by the DMM without any math scaling applied to it. The primary measurement display in yellow, shows the power consumption of the DUT in milli-watts (mW) after a math scaling has been applied. In this case, a gain was applied to the readings.

DMM_screen_power_measurement
Figure 3. Keysight’s 34465A DMM screen shows power measurement by multiplying a constant voltage of 3 V to the current measured.

In order to get DC power readings, you can use the function Mx – B where:

  • x is your raw current measurement
  • M is your gain or in this case, the constant voltage source
  • B is your offset

By using M as your constant voltage applied to the DUT, you can use the function to multiply it with x, which is your raw current measurement, you get your DC power readings.

Method 2: Measure DC power when both voltage or current are not constant

There is also a possible condition where you have no control to set either the voltage or current across the DUT to be constant. In this case, you will need to measure the voltage and the current across the DUT simultaneously to get the power consumed by the DUT. Some single channel DMMs allow you to make secondary DC voltage measurement through the HI/LO sense inputs.

Diagram showing two DMM DCV measurements
Figure 4. Keysight’s 34460A and 34470A series DMMs allow secondary DC voltage measurements when sensing voltages are not used.

For the second method, you will need an external shunt resistor to measure the electric current as shown in Figure 5. The DMM will measure the voltage across the external shunt and you can convert the DCV2 reading into current by dividing the voltage reading by the value of the shunt resistor using Ohm’s law (I=V/R).

Circuit diagram showing two DMM DCV measurements with external shunt
Figure 5. The diagram shows an external shunt used to measure current on the return path of the DUT.

To obtain DC power measurement, the DCV1 reading is multiplied to the current converted from the DCV2 reading via Ohm’s law. Figure 6 shows the DCV ratio measurement of Input/Reference where:

  • Input is the DCV1 voltage measurement across the DUT
  • Reference is the DCV2 voltage measurement across the external shunt

DMM screen showing DC voltage ratio
Figure 6. Keysight’s 34465A DMM shows DC voltage ratio measurement.

Summary

This blog post shares two methods on how to make simple DC power measurements with a single DMM. The first method involves setting a constant voltage or current source to the DUT, and then measuring power using voltage multiplied by current used by the DUT. The second method depends on your DMM’s capability as to whether it can measure two different voltages simultaneously or not. If it can, you will be able to use a current shunt to convert one of the DC voltage measurements to the current consumed by the DUT.

To find out more about Keysight’s 34460A and 34470A Truevolt series DMMs, please visit our website at www.keysight.com/find/truevolt.

To learn more before you purchase your next DMM, read 10 Things You Must Know Before Buying Your Next Benchtop Digital Multimeter.