The Hall effect probes can fail in a number of ways: Shock: The sensor is built on to a flat piece of ferrite, which is then glued to an L shaped piece of ferrite to form the C core. You see one of the ends of the C when you open the slider. The sensor is wire bonded after the adhesive that forms the core is cured. Then the AC winding is added and everything is placed into and potted in the dual mu-metal "can". Dropping the probe can put shock on the core, which either stresses the joint in the ferrite, or can break the bond completely. It can also break lose the wire bonds. In addition to being a magnetic field sensor, these thin film Hall devices also act like strain gauges and temperature transducers! The stress from shock that results from a drop stimulates the strain gauge effect, which gives you the high offset.
Over voltage: The sensor is electrically isolated from the ferrite by a thin layer of silicon oxide, similar to an IC passivation. Because it is only a few dozen angstroms thick, its dielectric strength is not that high - maybe 50 volts or so. There are users who probe bare conductors, and apply the probe while the circuit is live. (Bad idea for several reasons). If the voltage is high enough, and somehow the conductor touches the face of the C core without hitting the shield, it can break over the silicon oxide and vaporize the sensor. (the drive and sense side electronics in the amp are relatively low impedance to ground) In some cases, this even damages the amplifier. What is surprising to me is how the conductor hits the core face without hitting the transformer shield can, which is grounded. In any case it happens relatively often, and Tek replaces quite a few transformers which this damage. Sometimes, replacement of the DC offset resistor in the probe (early) or adjustment of the DC offset pot in the probe can adjust for changes in the sensor, sometimes not. Broken connections in either the probe head or the large connector! Failure to mate: It is important that the lid of the transformer can be precisely lined up with the edges of the can when the slider is completely closed. Otherwise you get an air gap, which will mess up the probe calibration and AC response. This problem can be caused by a number of faults in the slider mechanism, or by excess potting compoud oozing up round the C core edges. Above information courtesy Steve Ditters. Dave -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Henk ten Pierick Sent: 04 January 2010 19:26 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Tektronix AM503X current probes Hi, Tektronix made two types of current probes, AC probes and Dc probes. The AC probes go from some Hz or kHz upto Mhz or GHz. They are robust and not so expensive. The DC probes are ac probes with an additional hall sensor in it. The go from DC to say 100MHz. The DC probes are expensive and the hall sensor is easily damaged by a drop on the floor. At least, that is my experience. If you want to by a current probe and need a DC probe, take care that the hall sensor is not damaged. regards, Henk _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
