Using a Digital Multimeter to Detect Intermittent Faults

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Using a Digital Multimeter to Detect Intermittent Faults [Copy link]

Intermittent electrical faults are the most difficult thing for all troubleshooters to find, mainly because of the timing. If the fault phenomenon can be observed when the problem occurs, it will greatly simplify the fault diagnosis process. However, as the saying goes, "hasty tofu can't be eaten", and this also applies to intermittent electrical faults. When the fault exists, being able to show it and being able to make electrical measurements are the two biggest challenges in tracking down intermittent faults.

There are many test tools that can make the diagnosis of intermittent faults simple. These tools include sophisticated signal analyzers and storage oscilloscopes, as well as handheld digital multimeters (DMMs). Of course, maybe you don't have these tools available, or the location of the fault makes it difficult to use a large analyzer to the fault point. With a digital multimeter, you can get a lot of information about intermittent faults without having to go back to the office to carry a bulky storage oscilloscope to the work site. This article describes how to use a digital multimeter as a diagnostic tool for intermittent faults.

DMM Functions That Can Be Used to Capture Intermittent Faults

Combining the basic measurement functions of a DMM (AC voltage, DC voltage, and resistance) with a measurement recorder function, the user can detect the symptoms of intermittent faults. Not too long ago, you could buy voltage or current measurement tools that used a mechanical strip recorder. Simply connect a voltage to the input or clamp a current transformer on the wire, and the recorder would record progressive marks on a paper tape under the pen. The maximum length of the record depended on the amount of paper on the roll. The same strip recorder function is now integrated into some DMMs.

Fluke's 170 and 180 series DMMs have a feature called Min/Max/Average recording mode. Like a strip recorder, a DMM reads the input at regular intervals. But instead of saving each reading, it compares the reading to the two previously saved readings to determine if the value is higher than the previous maximum or lower than the previous minimum. If so, the new reading replaces the value previously stored in the high or low reading register. After a period of recording, you can call up the registers to display the maximum and minimum values during the recording time. As an additional feature, these DMMs can also calculate and store the average of all readings during a given period of time.

To use the Min/Max/Average recording mode, follow these steps:

1. Select the appropriate function for the measurement (AC Volts, DC Volts, Resistance, AC Current, DC Current, and Frequency).
2. Connect the DMM's test leads to the circuit under test. Fluke's SureGrip test leads and probes provide a variety of circuit connection methods. Be sure to connect the test circuit before activating the Min/Max/Average function, otherwise the minimum reading will always be the ambient value before the test leads were connected. This will affect the analysis of the recorded data after the recording period ends.
3. Press the RANGE button to switch to manual ranging. If necessary, press RANGE repeatedly until the DMM's display shows the correct range. This step is necessary because the DMM automatically switches to manual ranging mode when you set it to Min/Max/Average recording mode. Once the Min/Max/Average function is activated, you cannot change the range setting.
4. Press the MIN MAX button to activate the Min/Max/Average recording mode. On the Fluke 170 series, the maximum reading is indicated on the display and a beep sounds when a new maximum or minimum value is detected; on the Fluke 180 series, the primary display shows the maximum reading and the secondary display shows the current measurement.
You can leave the DMM in place to make measurements while you focus on other tasks, knowing that the DMM will not be tampered with and will not pose a safety hazard to anyone. At any time during the recording cycle, you can review the stored readings using the method described in the next step, or press the HOLD button to pause the recording mode without deleting the stored readings. Press the HOLD button again to resume recording.
5. Press the MIN MAX button to review the stored readings. Each time the button is pressed, the stored values (minimum, maximum, and average) are displayed sequentially on the DMM's display.

Recording time stamps

of the times when the minimum and maximum values were detected are very useful information in determining the cause of intermittent faults. Fluke 180 Series DMMs in Min/Max/Average recording mode store the amount of time from the start of recording until a new minimum, maximum, or average value is saved. Therefore, each saved minimum, maximum, and average value has a "time stamp" associated with it.

By recording the time you activated the Min/Max/Average recording mode, you can easily calculate the actual time when the DMM detected the reading. For example, if you activated the recording mode at 3:07:00 PM, and the maximum reading displayed has a time stamp of 47:05, you can simply add the time stamp and the start time to determine the time when the maximum value was recorded. In this case, the maximum value would have been recorded at 3:54:05 PM on the same day.

The Min/Max/Average recording mode of the 170 and 180 Series DMMs is very effective in diagnosing intermittent faults. However, it assumes that the circuit point to which the fault is connected is capable of exhibiting a maximum or minimum value; if the intermittent fault causes readings between the maximum and minimum values, the Min/Max/Average function will not be very helpful in determining the cause of the intermittent fault.

Tip:

Do not disconnect the test leads from the circuit under test until you have pressed the HOLD button to stop logging, or until you have reviewed and archived all the stored values. Disconnecting the test leads while logging will cause the DMM to process the values that appear on the disconnected test leads and affect the average values that were stored when the test leads were connected, and may affect the stored minimum or maximum values.

Advanced Min/Max Logging

The Fluke 189 True RMS Multimeter not only has the standard Min/Max/Average logging features described above, but also combines them with another feature called AutoHOLD and a larger memory to create Event Logging.

The AutoHOLD feature senses when the measurement signal becomes unstable and when it stabilizes again. Using the AutoHOLD feature to trigger the start and stop of the Min/Max logging feature allows the DMM to detect more than just those faults that produce minimum or maximum values. To

use Event Logging, you need the Fluke 189 True RMS Multimeter, FlukeView Chart Archiving Software, and a computer to download and view the logged data. The DMM can be taken to the site where logging is required and logged during the time period when intermittent faults are likely to occur. Only after logging is complete, the software and PC are required. Set up
the DMM by following the setup steps for Min/Max/Average logging mode described above, selecting the appropriate measurement function, and manually setting the range. Connect the DMM input to the measurement point and activate the event logging function.

As with the Min/Max/Average recording mode, the DMM begins storing the minimum and maximum values in memory. As long as the measured signal is within a certain percentage of the selected range, the DMM will update the same set of stored minimum and maximum values as in the Min/Max/Average recording mode; however, if the input signal changes by more than a certain percentage of the range, the DMM will save the acquired minimum and maximum values, along with their relative time stamps, and begin acquiring another set of minimum/maximum values until the measured signal stabilizes. Once the signal stabilizes, the DMM saves the minimum and maximum readings for that period and begins acquiring a new set of minimum/maximum values for the stable period. The DMM will continue to repeat this process as long as the recording task is ongoing, or until the memory overflows. Depending on the DMM settings, the DMM memory can store up to 3 days of minimum/maximum data pairs. When you are finished

recording, you can take the DMM back to the office and transfer its stored data to a PC using FlukeView Chart Archiving Software. Using the PC, you can perform detailed analysis of each stable and unstable event. Not only can you view the minimum and maximum values for each stable and unstable period, but you can also view the start and end time of each period. In addition, the average value for each period is recorded.

On a PC, the recorded data can be displayed in a tabular format as shown in Table 1 or in a graphical format as shown in Figure 1. Looking at row 3 of Table 1, you can see that the third period began at 9:53:30 AM on July 4, 2000. The duration of the event was 1 minute and 20.6 seconds. During this period, the maximum value was 8.1 amps, the average of all readings was 7.7 amps, and the minimum value was 7.5 amps. This was a stable event that ended at 9:54:51 AM. As we have seen in this example, the event recording function can capture a large amount of data during intermittent periods.