BK Precision 4011A Instrucciones de operaciones descargar pdf (Pagina 49)

Appendix B: Equipment and Instrument Circuits

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Oscilloscope Grounding Errors

The purpose of the ISOLATED supply on your workbench is to enable you to ground points in

your test circuits in order to make measurements without drawing a large current through the

ground connection and tripping a circuit breaker or “zapping” an instrument. Note that for per-

sonal safety, the oscilloscope (“scope”) should be plugged into a NON-ISOLATED outlet. Consider

now the scenario shown in Figure B.5, in which an oscilloscope is used to measure a voltage on a

circuit also plugged into the NON-ISOLATED outlet. The probe ground on the oscilloscope is con-

nected to the case ground and to the ground lead on the scope’s power plug. This is the same

ground lead that is connected to the neutral lead at the lab circuit breaker box. If we attempt to

make a differential voltage measurement as shown in the figure, then a number of bad things can

happen. First, the ground probe being connected to the circuit will create a ground path that will

effectively short out the 200 Ω resistor, giving us an erroneous measurement. Second, the scope

ground lead becomes part of the path that carries current, which in this example is 24 amps. This

will at least cause a circuit breaker to trip and possibly blow out the scope lead. This is one rea-

son why some of the scope leads you pull off the rack don’t work properly.

Figure B.5: Illustration of improper circuit measurement.

If instead we use an ISOLATED supply for the circuit, as illustrated in Figure B.6, then we won’t

have this problem. Since the resistor circuit is electrically isolated from the supply ground, the

addition of the scope’s ground lead has no impact. Note that if you use the isolation transformer

to “float” your circuit, then you must make sure to use the scope ground lead to take the meas-

urement. Otherwise you will get a measurement that will not make any sense.

Figure B.6: Illustration of valid circuit measurement due to the use of isolation transformer.

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BK Precision 4011A Instrucciones de operaciones Pagina 49