Both voltage and current are measured as voltages: an Ampere-Meter internally is a Volt-Meter, too.
Measuring Voltage
The voltmeter is connected in parallel to the circuit or device whose voltage is to be measured: the positive terminal of the voltmeter is connected to the point in the circuit where you want to measure voltage, and the negative terminal is connected to the reference point (usually ground).
Voltmeters have a high internal resistance. This is necessary because essentially, with a Voltmeter you are short-circuiting your circuit. Due to the high internal resistance, nothing bad happens, and only a very small current flows. This current is directly proportional to the voltage that drives it: the measured current can therefore directly be translated into the voltage.
Analog Voltmeters
Classic analog voltmeters use a coil or iron part that responds to the magnetic field produced by the electrical current. The amount of deflection of the pointer is proportional to the current flowing through the coil, which, in turn, is proportional to the voltage being measured. Since the internal resistance of the voltmeter is known, the amount of current flowing through it is directly proportional to the voltage:
- 5V: When the voltmeter is connected to 5V and has a known internal resistance of 500 Ohm, according to Ohms law the current then is U / R = I, thus 5V/500Ohm = 10mA. The current of 10mA produces a magnetic field of 4x10-8 Tesla which moves the indicator of the analog voltmeter.
- 10V: When the voltmeter is connected to 10V and continues to have a known internal resistance of 500 Ohm, according to Ohms law the current now is U / R = I, thus 10V/500Ohm = 20mA. The current of 20mA produces a magnetic field of double the strength: 8x10-8 Tesla which moves the indicator of the analog voltmeter twice as much.
Essentially, an analog voltmeter is using the magnetic field produced by a current to show the voltage. This is possible because the internal resistance of the voltmeter is known and fixed.
Digital Voltmeters
Today, digital voltmeters are more common. They add an ADC (Analog-Digital-Converter) to convert analog voltage to a digital signal that is then displayed.
Measuring Current
The example of the analog voltmeter has already illustrated that there is a close relationship between current and voltage.
Both together move electrons and produce various physical effects that can be measured. The principle of measuring current and voltage is the same: you look at appropriate physical effects that can be correlated to the flow of current or voltage. When some parts of the Ohms law formula are known or fixed, you can calculate the other parts.
There are two physical effects proportional to the flow of current:
- Voltage Drop: Every load consumes energy and causes a voltage drop: by inserting an artificial load (called Shunt resistor) with a very precise known resistance in series with the real load, the current can be measured via the voltage drop. The Shunt resistance must be very low in order to minimize energy loss through heat. For the same reasons, the shunt resistor must be capable of handling the total current. A shunt can measure current very precisely. It requires the insertion of a shunt resistance into the circuit, though, and the higher the currents are, the more strain is on the shunt resistor.
- Hall Effect: Every electrical current produces a magnetic field. In AC current, the magnetic field is alternating and can be measured using a cheap coil. With DC, the magnetic field is fixed but can still be measured using the hall effect. In a nutshell, the DC current produces a constant magnetic field. Hall effect sensors can detect these magnetic fields. By measuring the strength of the magnetic field with a hall sensor, you can calculate the DC current (provided there are no other sources of magnetic fields around). Hall sensors are more expensive than coils, especially those sensitive enough to reliably measure the relatively weak magnetic field of smaller currents. This is why i.e. clamp meters for DC are more expensive than those that can only measure AC. Coil and hall sensor are physically separated from the circuit and do not require insertion of a sensor. Even very high currents can be safely measured. There is no risk of thermal overload. However, magnetic fields generated in the proximity by other sources can influence the result.
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(content created Mar 10, 2024 - last updated May 17, 2024)