Practical exercises with LTSpice
Voltmeter design
Exercise 1:
Create a simulation in LTSpice that illustrates technical resistance measurement of analog ampere meter. To do this, draw the following diagram in LTSpice, where the resistor R2 simulates an analog ampere meter:
Based on the graph, read the resistance of the ampere meter, which will be equal to the resistance of R1 when the current drops by half (i.e. from 100mA to 50mA).
When you're done, compare the result with the video below:
Exercise 2:
Design a voltmeter that has the following ranges: 0.5V, 2V, 5V, 20V and 50V. Knowing that the maximum deflection is at 100 mA and the internal resistance of the ammeter is \(0.7 \Omega \), calculate the resistors that should be connected to the ammeter to select the specific operating range of the voltmeter. As a reminder, below is a schematic diagram of a voltmeter:
Once you have determined the resistors, create simulations in LTSpice and read the current on the ammeter. On its basis, the voltage on the voltmeter is determined according to the formula:
\( U=(R_1 + R_2) \cdot I \)
where:
\(R_1\) is the resistance of the ammeter equal to \(0.7 \Omega \),
\(R_2\) is the resistance of the resistor connected to the ammeter.
Check if the voltmeter returns the value correctly. When you're done, compare the result with the video below:
Exercise 3:
The second instrument has parameters such as the swing current \( 40 \mu A \) and the internal resistance \( 5.8k\Omega \). Repeat exercise 2 with the new parameters. Express the resistances of both instruments in Ohms/V. The answer to the question, which of the instruments will be better suited as a voltmeter?
Hint: Look at the voltmeter's internal resistance.