Capturing Transient Signals with a Graphical MultiMeter

Introduction

What is a Transient?

Power lines running overhead or even underground can have voltage induced in them either by a direct or indirect strike of lightning. Due to its high energy and sporadic nature, this type of transient is rarely isolated using a handheld multimeter.

Transients caused by other electrical equipment are well within the capabilities of a hand-held multimeter. These types of transients are usually caused by the discharge of stored energy in inductive and capacitive components. Electric motors, such as those used in elevators, heating, air conditioning, refrigeration or other inductive loads, can create a continuous stream of 250V to 1000V transients. DC motor drives, variable speed AC motor drives, DC power supply switching, and portable power tools are other sources of transients.

A more elusive transient is one caused by the switching of inductive loads. A 250V to 3000V transient can occur with the interruption of an inductor's current. This type of transient occurs when opening and closing contacts to electric motors or fluorescent lighting using magnetic ballasts. Two other sources are arc welders and furnace ignitors. Electric wiring carrying signals from these sources can induce voltages in other conductors in close proximity.

The examples used in this application note actually occurred. When an indoor fluorescent lighting circuit was turned off, it caused outdoor security lighting to come on. The security lighting is controlled by a motion detector and is enabled, or active, only after sunset. The light from the fluorescent lighting is completely isolated from the security lights and no motion was apparent in the security area. The only thing that is common to both, is the AC branch circuit that feeds both lighting circuits. A Fluke Graphical MultiMeter was used to look for any transients that may be present on the branch circuit.

Setting up the GMM

Input Connections

Selecting the VIEW Mode

In the View mode, the GMM allows you to capture and examine a single waveform that has met the glitch trigger criteria. This is called Glitch Capture mode.

Setting the Range

To set the range:

1. Press the Range key to activate the range selection softkeys. The selected range is displayed just above the softkey labels in the lower right-hand corner of the display.
2. Press the up (2) or down (3) arrow softkeys to change the range. This application requires a range setting of 300V.
3. Press Exit (5) to return to the ACV function softkey labels.

Setting the Timebase

1. Press the Timebase (1) softkey.
2. Press the Slower (2) or Faster (3) softkey to set the desired Timebase. The GMM displays the timebase setting along the right edge of the display grid.
3. Press Exit (5) to return the softkeys to the View mode functions.

GMM Glitch Capture Design

Figure 1. Glitch Trigger Circuit

The filter is designed to pass signals that have a 10 µs to 1 ms pulse width while blocking the low frequency signals from getting to the trigger circuit. This circuit is optimized to capture transients while monitoring powerline (50 Hz and 60 Hz) signals.

The trigger circuit delivers a trigger to the meter whenever a signal coming through the filter exceeds the preset trigger level, either in a positive or negative (bipolar triggering) direction. The glitch trigger level is not adjustable. The waveform trigger levels set through the front panel to set the displayed waveform do NOT change the glitch trigger settings.

Figure 2. GMM Input

The glitch trigger circuit monitors the AC signal coming out of the input signal conditioning and range control (Figure 2), where the signal is maintained between 20 mV and 500 mV. With the trigger circuit placed after the range control, a signal greater than 20% of the selected range is required to trigger the meter. For example, on the 30V range, the transient must be 6V or greater to trigger the meter. One exception is the 1000V range where the trigger level is only 6% of the range, or 60V.

Capturing a Transient with the Glitch Capture Mode

First enable and arm the Glitch Capture mode by pressing the Glitch Capture (4) softkey. A timer next to the lower right corner of the grid indicates elapsed time in hours and minutes since arming the glitch capture mode.

In the Glitch Capture mode, the GMM acts like a video recorder. In the GMM, there is enough memory to store 25 time divisions worth of input data. When the memory is full, the GMM stores the next piece of input data over the oldest piece, always retaining the last 25 time divisions of data.

When armed, the GMM starts the "recorder", storing the input data in memory. When the input signal trips the glitch trigger circuit, the GMM records nineteen more time divisions worth of input and then displays the waveform.

Figure 3. Glitch Capture Waveform

At this point you can analyze the signal in the display, print the display to a printer, or store the display in the GMM's memory for later analysis.

Even though the GMM only displays ten time divisions worth of information, up to 6 time divisions before the trigger and 19 time divisions after the trigger are viewed using the left (2) and right (3) softkeys. This is analogous to the forward and reverse buttons on a video recorder.

Once you are through with the display, you can easily reset the GMM to capture the next glitch or transient. To capture another glitch with the same settings, simply press the Arm (1) soft-key. This resets the trigger circuitry and resets the elapsed time to 00:00. Waveform positioning set by the position softkeys maintains its position in subsequent arm cycles.

Conclusion:
For this application, the display in Figure 3 makes it apparent that the collapse of the magnetic field in the fluorescent lighting ballast is creating a voltage spike on the branch circuit. This spike, in turn, causes the motion detectors on the security lighting to trip and turn on the security lights. With the transient source identified, an appropriate solution can be implemented to fix the problem.

Storing Configurations and Waveforms

1. Press the PRINT/SAVE key.
2. Press the Save Screen (1) softkey.
3. Press the up (2) or down (3) softkey to move the highlight bar to one of the three waveform storage locations.
4. Press the Save (3) softkey. The label used for the stored screen is a combination of the display mode (Meter, Combo, View, etc.) and the measured value.

Storing the waveform captures an image of only what is within the confines of the display. This is analogous to placing a camera on the display and snapping a picture. The waveform information that is off screen is NOT saved in memory. Therefore, it is important to position the waveform in the display before saving or printing it.

To speed up the setting of the range, trigger levels, and timebase, you can save the GMM configuration settings as follows:

1. Press the PRINT/SAVE key.
2. Press the Save Config (4) softkey.
3. Move the highlight bar to one of the seven configuration memories by pressing the down (1) or up (2) arrows.
4. Press the Save (3) softkey.
5. Press Exit (5) to return to the selected display mode.

To recall the configuration:

1. Ensure that the rotary function switch is set to the function on which the configuration is based.
2. Press the PRINT/SAVE key.
3. Press the Recall (2) softkey.
4. Move the highlight bar to the desired configuration by pressing the down (1) or up (2) softkey.
5. Press the Load Config (4) softkey. The Load Config softkey will only appear when the function selected by the rotary switch is compatible with the highlighted configuration.

To print the display, first review the "Printing" section of the 860 user's manual, which describes how to properly setup the printer and connect it to the GMM. To print a screen on a properly connected printer:

1. Press the PRINT/SAVE key.
2. Press the Print (3) softkey.

You can stop the printing process by pressing the Stop Print (3) softkey.

To download the GMM screen data to a PC, you will need the FlukeView 860 PC software. Refer to the software user's manual for setup, connection, and downloading instructions.

Summary

There are two models in the 860 Series: the 863 and the full-featured 867B. Both of the models will make the measurements mentioned in this application note. Contact your nearest Fluke representative for further information.