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Appendix B: Equipment and Instrument Circuits

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Figure B.7 illustrates another thing you will need to watch out for when taking measurements

using a scope. Both Channel 1 and Channel 2 on the scope have ground leads, and those leads

are physically connected together at the scope itself. What would happen if you tried to measure

both resistor voltages using the set-up shown here? The grounds on Channel 1 and Channel 2 are

at the same potential and so would short the 200 Ω resistor. This would again cause a large

ground loop current to flow through the scope’s ground leads, causing a circuit breaker to trip, a

fuse to blow if an autotransformer were also included in the circuit, and/or damage to the scope’s

ground leads. If you use both channels on the scope, you must make sure that the measurements

will be taken with both channel grounds at the same reference point. Slight differences between

probe ground leads that are supposed to be at the same potential will cause ground loop currents

to flow and perhaps cause errors in your results and damage to equipment.

Figure B.7 Illustration of invalid circuit measurements when using isolation transformer.

Finally, another common mistake is made in the use of the scope’s ground connections. When

taking scope measurements on an isolated circuit, you must use the probe’s ground clip from one

channel in order for the measurements to make sense. At first, it might seem that you would not

get any measurements at all if the ground connection on the probe were missing, as in Figure

B.8. However, there is always a small amount of stray capacitance in a circuit that will tie the

circuit under test back to the measuring device. Unfortunately the impedance of this ground tie is

large and it will give you unpredictable and erroneous measurements.

Figure B.8: Common problem with using isolation transformer.

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BK Precision 4011A Betriebsanleitung Seite 50