The electrical resistance of an object is a measure of its opposition to the flow of electric current. Voltage is applied to make the current flowing. For a fixed current the required voltage depends on the resistance offered by the material. The relationship between voltage, current and resistance is given by Ohm’s Law. It states that $ Resistance = \frac{Voltage}{Current} $ , $ R = \frac{V}{I} $. In other words, current is proportional to the applied voltage.

Measuring Current

Current is measured by an Ammeter in the following manner. Ammeter has a shunt resistor between the terminals of it. The measured current flows through the shunt resistor and the generated voltage drop is measured, amplified and displayed in the current scale. The value of the shunt resistor in an ammeter will be small ($ < 1\Omega $) so that it’s effect on the circuit current is small.

In this experiment we use a larger resistor, along with the voltmeter A1, to measure current. Larger value of the shunt resistance results in a voltage that can be measured without amplification. This method works well because we are dealing with small currents, in milli Ampere range.

We are connecting two resistors, R1 and R2, in series so that the same amount of current flows through both of them. The voltage across R1 is given by A1, and PV1-A1 gives the voltage across R2. Verifying $ \frac{V1}{R1}=\frac{V2}{R2} $ establishes the Ohm’s law.

Procedure

• Connect two resistors R1 and R2 in series from PV1 to ground
• Connect the junction to A1. R1 is 1000 Ohms, and R1 and A1 together acts as the Ammeter.
• Voltage across R1 ia A1
• Voltage across R2 is PV1-A1.
• Use the displayed circuit current to calculate the value of R2.

Repeat the experiment using different values of R2.

We assume that the same current flows through R1 and R2. The effect of the input impedance of A1 (1MOhm) can be neglected while working with resistors below 10 KOhm.

Ammeters

Most of the ammeters have a shunt resistor conencted between the terminals and the voltage drop across the shunt is amplified and displayed, on a calibrated scale. For example, take a digital ammeter having a 0.1 Ohm shunt resistor. Assume the analog to digital converter range as 0 to 1 volt. To get 1 volt across 0.1 Ohm 10 A current is required, so the current range will be 0 to 10 A, without any amplification stage. If you want to implement a 0 to 10 mA range ammeter, the voltage across the shunt should be amplified by a factor of 1000. The shunt should be very small (zero in the ideal case) so that the insertion of meter does not affect the value f the current.

During the design f ExpEYES, integrating a current meter IC like INA219 was considered. But we decided to use the voltage drop across a 1 kOhm resistor to measure current, to reduce the complexity and cost to some extend. The method of using a 1 kOhm resistor is suitable for most of the experiments documented.

It is also possible to use the INA219 I2C module for current measurements.