Lab 4.pdf - Experiment 4 Voltmeter and Ammeter Ana L...

This preview shows page 1 - 3 out of 9 pages.

Experiment 4 Voltmeter and Ammeter Ana L Sanchez Jomolca Experiment Date : 7/12/2021 Submission Date : 7/16/2021 Lab Section : PHY2049L 001 Lab Instructor: Ananya Adhikari Purpose: The purpose of this experiment is to build a lab-made voltmeter and ammeter using a galvanometer and external resistors. The lab-made voltmeter has a scale of 0-10 V and the ammeter has a scale of 0 -100 mA. Equipment: Galvanometer (500 μA), resistance box, R m (= R 1 or R 2 or R 3 , multiplier resistors for a lab-made voltmeter), ruler (30 cm), steel wire (shunt resistor for a lab-made Ammeter), digital multimeter (DMM), potentiometer, DC power supply (HY152A) Theory: To construct a lab-made voltmeter or ammeter, a galvanometer and external resistors are used appropriately. A galvanometer is a basic deflection type meter that has a needle that can detect when a current passes through it. The needle is connected to a coil of wire mounted on bearing between two poles of a magnet. When current passes through, a torque occurs and rotates the needle along a scale. This deflection is proportional to the current and can be read on the scale. For our lab experiment, we decided that the maximum scale reading for the lab-made voltmeter and ammeter are Vmax = 10V and Imax = 100 mA. The galvanometer has an internal coil resistance (r = 0.123 k Ω ) and a maximum permitted current (Ic = 500 μ A). With these two values known, the additional resistance needed to make the voltmeter or ammeter can be calculated. For the lab-made DC voltmeter, the desired maximum scale reading is 10 V. To create this Vmax a resistor needs to be added in such a way that the resistance of the galvanometer and the resistor increases, allowing the lab-made voltmeter to measure 0V - 10V. A large enough resistor must be added. A multiplier resistor (R m ) is added in series to the galvanometer because it multiples the voltage range of the galvanometer. Connecting them in a series adds up their resistances and increases the equivalent resistance. Rm can be calculated using Ohm’s law in the equation R m = (V max / I c ) - r. This shows that the larger the Rm value, the larger the Vmax. A voltmeter is always connected in parallel with a circuit component (such as R) to measure the potential difference/ voltage drop across the component as shown in this figure
The circuit does not need to be broken in order to measure the voltage drop. After adding the resistor Rm, the voltage range is 0V-10V but the needle is pointing to the current from the galvanometer. In this case, the current translates into the voltage where 500 μ A 10V, 400 μ A 8V, 300 μ A 6V, etc. For the lab-made ammeter, the desired maximum scale reading is Imax = 100 mA. The galvanometer only permits Ic = 500 μ A, so a very small resistor must be added. This resistor is called a shunt resistor, Rsh, and is placed in parallel with the internal resistance r of the galvanometer shown in this figure This resistor is called a shunt because it directs most of the current away from the galvanometer.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture