DC Voltmeters and Ammeters

DC Voltmeters and Ammeters

By Rachel Wampler

Ammeters

• Ammeter is a device used to measure current in a circuit.

• Analog (non-digital) ammeters use a galvanometer​ to measure current.

  • Made of magnets, wire coils, springs, pointer, and a calibrated scale.

  • current ​flows through the coil creating a magnetic field.

• Analog ​ammeters (galvanometers) are best suited for small mA currents.

Ammeters

• ​Connected in series to measure current.

• Low resistance to not cause a ​significant voltage drop.

• Coil usually measures small currents.

• Has Shunt resistors connected in parallel to galvanometer so excess current can bypass

  • A knob lets you select with shunt resistor is in use.​

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Problems with Ammeters

• The resistance of the coils and shunt resistors add to the resistance of the circuit

• The added resistance reduces the current in the circuit

• Ideally an ammeters has no resistance

  • Real-life, quality ammeters have small resistance so as only cause a negligible change in current

Voltmeters

Problems with Voltmeters

• The voltmeter takes some of the voltage out of the circuit.

• Ideal voltmeters would have an infinitely large resistance as to draw the smallest current possible

• Good voltmeter has large enough resistance as to make the current draw (and voltage drop) negligible. ​