Current Probes and Logic Trigger Probes


Basically, the current probe is a transformer of which the primary winding is the test lead through which the current is measured. The probe head consists of a ferrox-cube core and the secondary windings of the transformer. The core can be split into two parts to clip it simply around the measuring lead. The white-colored part of the probe head can be moved backward and forwards to clip it around the lead. A voltage is developed in the transformer secondary windings by the magnetic field around the measuring lead. This voltage is fed to an amplifier box, the output of which is fed to the oscilloscope. The output cable from the amplifier must be terminated with 50 fl at the oscilloscope end (low-ohmic system for 75-MHz bandwidth).  Furthermore, if the oscilloscope is set to 50-mV/ div sensitivity, the amplifier box provides calibrated outputs ranging from 1 mA/ div on the screen.



Applications for current probes include:

  • Switching currents in power supplies, turn-on and turn-off currents of systems, transistors, and silicon controlled rectifiers (SCRs). The current probe may reveal more information about overload conditions than voltage transients do.
  • Circuits where minimum loading by the measuring device is necessary. The current probe only loads the lead under test between 0.5 and 2 pF, depending upon its diameter. For most applications, the impedance inserted into the lead by the probe can be disregarded.
  • Current difference. This is achieved by feeding two leads through the probe head such that the currents flow in opposite directions. The difference between the currents is measured at a high rejection rate for currents that are common mode. Sensitivity can be increased simply by feeding more turns of wire through the clip-on head. The upper range can be extended by splitting the current path into several parallel wires and feeding only one current path through the probe head.



In the more sophisticated digital practice, it frequently happens that the engineer wants to trigger an oscilloscope on a word or combination of bits (simultaneously) present at several lines, for example, in a BCD parallel output of a frequency counter. Word recognizers, suitable for use in conjunction with oscilloscopes, are readily available as a kind of probe. Usually, they have several inputs complying with tetrad or octad length (4 or 8). A binary word can be set by switches, and if this word is detected, the unit initiates a trigger which can be used to start a sweep. Alternatively, the trigger can be used to initiate a sweep after a certain delay time by making use of the digital or analog delayed­sweep facility. In this way, the display can be started on a recognizable byte, character, or any unique combination of data in logic circuits. For example, the start of the display can be made to coincide with a control byte coming from a peripheral or a BCD output of a decade counter.