Transient spikes can create someserious problems when theyoccur in sensitive electronicequipment. Not only can voltagespikes cause electronic compo-nents to fail, but also circuitrycan be falsely triggered and pro-duce unexpected results. And ifthat’s not bad enough, the tran-sient’s unpredictable naturepresents a considerable chal-lenge to the technician trying todetect the elusive signal andlocate its source.

In the past, transient problemsrequired a trained eye and acomplicated oscilloscope fordetection — not always a con-venient or practical solution.But today, technicians canemploy a high-performance digi-tal multimeter (DMM) with thelatest recording features to trackdown even the most difficulttransient problems.

Not your daddy’s DMMBoth the Fluke 87 and Fluke 189offer a MIN MAX function, arecording feature that allows theDMM to record the highest andlowest values. When the functionis enabled, the DMM stores thefirst reading it takes into tworegisters. One register for highvalues (MAX) and one for low(MIN). From this point on, eachmeasurement of the DMM iscompared to the values in thesetwo registers. Whenever a meas-ured value is higher than thevalue in the MAX register orlower than the value in the MINregister, the DMM beeps and thenew value is stored.

In order to track down tran-sients, the DMM has to be ableto “capture” a very short changein measured value and still pro-duce a reading that representsthe magnitude or peak value ofthat change. For example, if theDMM is measuring a steady dcvalue, and a transient voltagespike is placed on the dc value,the DMM needs to display themaximum voltage deviationcaused by the transient. Alsokeep in mind, the transient couldeither increase or decrease thesteady dc voltage.

Voltage measurementsmade easy

Only a matter of timeTransients can be pretty short induration. How short a signal theDMM can detect and measuredepends on the DMM’s responsetime, which is the length of timean input must stay at a newvalue to be recorded. Obviously,a shorter response time capturesshorter events.

Both the Fluke 87 and 189DMMs have selectable MIN MAXresponse times. For trackingdown transient problems, a DMMneeds to have very fast response

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Application Note

2 Fluke Corporation Voltage measurements made easy

times. When using a Fluke 87, thefastest MIN MAX response time iscaptured when the “PEAK MINMAX” mode is enabled. Thesame feature is labeled “FASTMN MX” on the Fluke 189. Inboth these meters, these selec-tions set the DMM’s responsetime to 250 microseconds.

Step by stepTo demonstrate the use of theMIN MAX function in detectingand measuring transients, con-sider the example shown in Fig-ure one. The circuit is part of acontrol circuit that, among otherthings, controls some dc relays.The diode across the coils of therelays is used to short out anyelectrical energy created by thecollapsing magnetic field in therelay coil when the voltage isremoved. Assume for this exam-ple that one of the diodes hasopened. Without the diode toshort out the energy released bythe collapsing field around thecoil, the energy must be dissi-pated by the remaining circuitryof our control circuit. With thecoil now acting like a source, thevoltage it creates will be addedto the supply voltage, possiblycausing a spike on the dc supplyline. As this transient propagatesthrough the circuitry, it maycause erratic and intermittentbehavior in our control circuit.

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Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The NetherlandsFor more information call:In the U.S.A. (800) 443-5853 or Fax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 or Fax (31 40) 2 675 222In Canada (800) 36-FLUKE or Fax (905) 890-6866From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116Web access: http://www.fluke.com

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At this point, the MIN MAXrecording mode has been acti-vated and the DMM is comparingeach new measurement againstthe MIN MAX registers and stor-ing any new values it detects.When the relay with the opendiode de-energizes, a transientmay be detected by the DMM.

To see the recorded values,perform the following.For the Fluke 87:Press the to see themaximum value measured. Pressit again to see the minimumvalue measured.For the Fluke 189:With the maximum value alreadydisplayed in the primary display,each press of stepsthrough the minimum (MIN),average (AVG) and back to themaximum (MAX) reading.

Again, the present measuredvalue is always displayed in thesecondary display while cyclingthrough the MIN MAX values.

It may also be helpful to knowthat the amount of time that haselapsed since the MIN MAXmode was activated to the lastrecorded MAX or MIN value isalso displayed.

The MIN MAX function is avery handy addition to a DMM.You will find it very helpful foridentifying intermittent problems,such as transients. The Peak orFast MIN MAX functions greatlyenhance DMMs, making yourwork easier and more productive.

F i gure 1.

To capture and measure thistransient, set up the DMM as follows:1. Select the dc voltage mode.2. Plug the red test lead into the

V jack and the black testlead in the COM jack.

3. Connect the black test lead tothe dc common of our controlcircuit and the red test leadon the 12-volt power linegoing to the relays.

4. Press the until thedesired range is selected. Thisshould be done to get thebest accuracy before enteringthe MIN MAX mode.

5. Activate the MIN MAX mode.For the Fluke 87:Press the .

Press the to enter thePEAK MIN MAX mode until 1 mS is displayed.For the Fluke 189:Activate FAST MN MX by

pressing

then press .The maximum stored value is presented in the primarydisplay while the secondarydisplay indicates the presentmeasured value.