Wahl ST2200, Руководство пользователя

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ST2200 Soldering Iron Tester User Manual
Page 27 of 34
8.3.2 Resistance to Ground Measurements
All resistance measurements are made with respect to the ground reference of the
AC line cord. Therefore, the ST2200 line cord ground wire must be connected to
the ground reference as chosen by the user (either power ground or earth ground).
If a ground reference is chosen other than the ground available via the AC outlet
powering the soldering iron, then the ground reference of the ST2200 must be
connected to that ground to make tip resistance measurements.
The test current for resistance measurements is 10 milliamperes DC and the
ST2200 will display resistance with a resolution of 0.01 ohms. The operator
should be aware that it is possible for circulating ground currents in the AC power
distribution system to cause errors during ohms measurements. This will be
evidenced by a difference in displayed ohms when the soldering iron power cord
and ST2200 power cords are interchanged in their position of connection at the
AC power outlet. The reason for this is that a ground current is flowing in the
ground line and results in a voltage offset between the two adjacent outlets. In
one orientation this offset voltage adds to the ST2200 voltage generated during
ohms check and then subtracts if the orientation of the AC power cord
connections is reversed. Therefore, as little as 10 millivolts of either polarity will
cause a variance of the ST2200 digital display of
±
1.0 ohms as the scale factor is
1.0 ohms per 10 millivolts of the voltage measured.
Should this phenomenon be seen, it would indicate that there is excessive ground
current flowing (caused by a faulty piece of equipment connected to the AC line)
or excessive resistance between AC outlets and should be resolved if highest
accuracy measurements are to be made.
Methods of avoiding the errors due to differences in ground potential are:
1. Use the AC convenience outlet provided on the right side of the instrument.
2. Use a multiple AC outlet adapter in which both the ST2200 and the soldering
iron under test are connected. This essentially connects the ground of the ST2200
and the soldering iron under test to a single point ground reference.
8.3.3 Ground Integrity
The Resistance (ohms) measurement will be most easily affected by an
unsatisfactory ground connection of either the ST2200 or the soldering iron under
test. Contact resistances of the AC power plug and receptacle combination can
have a wide variability depending on the surface area of contact, pressure of the
contact interface, and cleanliness. An open ground connection would indicate
itself as an "1---" on the display. When a Resistance measurement fails, the
connections to the AC line should be investigated for intermittent contact.