DC Measurements and Meters Flashcards
Define measurand
The physical variable (e.g. voltage, current, mass, force, temperature, pressure) being measured by a measurement system is called the measurand.
What is dc?
A current that doesn’t change with time is said to be dc (direct current). The same applies to voltage; i.e. if it’s constant, it’s called a dc voltage.
how does a PMMC (Permanent Magnet Moving Coil) galvanometer work?
- a current-carrying coil is mounted on a pivot and placed between the poles of a permanent magnet;
- at zero current, the coil is held at zero deflection by the torque of a spring;
- when current flows, the magnetic field generated by the coil is at some angle relative to the permanent magnetic field, and this generates a torque against the spring;
- the coil rotates to the point where the two torques are in balance;
- the greater the current, the greater the needle deflection; deflection is
proportional to current.
What is a shunt resistor?
The shunt resistor simply allows a fraction of the current to “bypass” the coil.
How do you protect a multi-range ammeter?
Multi-range ammeters are constructed by having several built-in shunts, one of which is selected by a switch, which indicates the current measurement range. To protect the meter, it is important to use the largest range first, and switch to successively smaller ranges if the current does not produce a readable deflection.
Give a main advantage of digital meters over analogue ones.
A main advantage of digital meters is that the results can be easily and automatically stored, processed and analysed.
What is the basic function of a DVM(digital voltmeter)?
The basic function of a DVM is to take an analogue voltage input signal and display the amplitude of the voltage in digital form.
What happens in a DVM:
Analogue Voltage Input Signal -> Amplify -> Convert to digital form -> Display
Describe what happens in the “AMPLIFY” stage in a generic DVM:
The AMPLIFY stage/unit (amplifier or conditioning circuit) simply modifies the input signal’s amplitude so that it is within the correct input range for the CONVERT stage/unit. Hence, it also carries out attenuation where necessary.
Describe what happens in the “CONVERT” stage in a generic DVM:
The CONVERT stage (usually an Analogue-to-Digital (ADC) convertor) converts analogue input voltage to digital format (usually BCD form).
Describe what happens in the “DISPLAY” stage in a generic DVM:
The DISPLAY unit simply renders the converted digital format on display devices such as LED or LCD digits or screens.
What is a digital multimeter(DMM) ?
An instrument that measures both voltage and current digitally is called a Digital Multimeter (DMM).
What are the conditions for the shunt resistance to avoid disturbing the circuit when digitally measuring the current?
Rs must have a low value, to avoid disturbing the circuit that is being measured, but should also be large enough to pick voltage.
How does a true rms-to-dc converter work (state all steps)?
Input ac E -> Square -> Average -> Square Root -> Erms dc output
