Lab 02: Digital Multimeter: DC Voltage and DC Current Measurement


  • Measurement of DC current and voltage
  • Measurement of voltmeter and ammeter internal resistance




An ideal voltmeter has infinite resistance, which can be considered as an open circuit. Although it is impossible to make a physical voltmeter with infinite resistance, a well-designed voltmeter exhibits a very large internal input resistance.

To determine the internal resistance of the voltmeter, set up the circuit shown in Figure 1. The voltmeter reads the voltage across itself, which includes its internal resistor. The current is given by the equation:

\(I = \frac{{{V_s} - {V_M}}}{R}\)

Figure 1: Circuit for Measuring the Resistance of the Voltmeter

Since the circuit has only a single branch, the current flowing through the resistor also flows through the voltmeter. From Ohm's Law, if we know the current (I) and the voltage (VM), we can compute RV:

\({R_V} = \frac{{{V_M}}}{I} = \frac{{{V_M}}}{{\left( {\frac{{{V_S} - {V_M}}}{R}} \right)}} = \frac{{R \times {V_M}}}{{{V_S} - {V_M}}}\)



An ideal ammeter has zero resistance so that the circuit in which it has been placed is not disturbed, and it can be considered as a short circuit. However, all ammeters have some small internal resistance. One way to measure the internal resistance of an ammeter in the circuits is shown as below:

Figure 2: Current Divider Circuit



  1. Define "resolution", "accuracy", and "precision" and explain the differences (if any) among these three terms.
  2. What is the difference between the "nominal" and the "actual" value of an electronic component?
  3. What is the input resistance of an ideal current-measuring DMM?
  4. What is the input resistance of an ideal voltage-measuring DMM?
  5. How must a DMM be connected in a circuit to measure current: In series or in parallel?
  6. How must a DMM be connected in a circuit to measure voltage: In series or in parallel?
  7. Under what conditions will the voltage burden or the input resistance of the DMM produce a significant error in current measurement?
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