BK Precision 4011A Instrucciones de operaciones descargar pdf (Pagina 53)

Appendix B: Equipment and Instrument Circuits

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In the d’Arsonval galvanometer, current through a coil of fine wire develops a magnetic field

that opposes the field of a permanent magnet, and so rotates a needle across a scale that is

marked off in units of the measured variable. This type of movement is used extensively in DC

analog instruments. For AC measurement, however, the d’Arsonval movement is not sufficient

by itself, but must be adapted in some way:

 it must be used in conjunction with a rectifier diode that converts the AC into a waveform

with a DC level to which the meter can respond, or

 it must use an electromagnet instead of the permanent magnet in the standard d’Arsonval

movement, or

 it must use iron vanes [B-3].

The electromagnet form of the galvanometer is employed in the power measuring instruments

(wattmeters) that you will use in this lab course.

Figure B.10 (a) shows an equivalent circuit representation of the dual-coil electrodynamometer

wattmeters you will use to measure 60 Hz AC power in this laboratory. Because the meter de-

flection is proportional to the product of the current through the current-sensing coil (in series

with the load) and the current through the voltage sensing coil (across the load) the response is

proportional to the product of the load current and the load voltage drop. The size of the resis-

tance in series with the voltage coil determines the voltage range of the instrument. Figure B.10

(b) shows the equivalent circuit representation of the analog voltmeter. The size of the resistor

determines the voltmeter range. Figure B.10 (c) shows the equivalent circuit of the analog amme-

ter. The size of the shunt (parallel) resistor determines the range of the ammeter. Note that the

analog instruments on the Hampden ACVA-100 console and the Hampden ACWM-100 wattme-

ter console all have non-linear scales.

(a) (b) (c)

Figure B.10: (a) Equivalent circuit for analog wattmeter. (b) Equivalent circuit

for analog voltmeter. (c) Equivalent circuit for analog ammeter.

Digital Multimeter

A multimeter is an electronic device that measures a multitude of electrical values, usually in-

cluding at least AC and DC voltage and current, as well as resistance. The digital multimeter

(DMM) has a digital display. The DMMs in this laboratory are powered from the non-isolated

120 V, 60-Hz supply. Figure B.11 (a) shows an equivalent circuit for the DMM when it is used

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BK Precision 4011A Instrucciones de operaciones Pagina 53