[time-nuts] Tektronix AM503X current probes

[David C. Partridge]

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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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