Question

A 0-25 mA ammeter having resistance 10 $\Omega$ is to be converted into voltmeter with a range 0-25 V. The resistance connected in series is

• A. 900 $\Omega$
• B. 990 $\Omega$
• C. 1000 $\Omega$
• D. 1100 $\Omega$

Answer 1

Answer: (B)

990 $\Omega$

Answer 2

Here's how to calculate the series resistance:

1. Full-scale current:

   $I = 25 \text{ mA} = 0.025 \text{ A}$

2. Voltage drop across the ammeter:

   $V_a = I \times 10\,\Omega = 0.025 \times 10 = 0.25\,\text{V}$

3. Extra voltage to be developed by the series resistor:

   $V_s = 25\,\text{V} - 0.25\,\text{V} = 24.75\,\text{V}$

4. Calculate series resistance:

   $R_s = \dfrac{V_s}{I} = \dfrac{24.75}{0.025} = 990\,\Omega$