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The most accurate method of measuring current in a circuit is to place a multimeter into the circuit itself.
The disadvantage of this method is that it is necessary to break into the circuit. This can be achieved reasonably easily in many instances, but in others it is not always convenient.
When this is the case, there are other current measurement techniques that can be used. Each one has its own advantages and disadvantages.
Additional current measurement techniques
The most straightforward technique for measuring current with a multimeter is to insert the multimeter in series with the circuit..
This technique may not always be suitable and therefore other techniques may be more suitable. The two main other techniques include:
- Use a series resistor
- Use a current sensor / coil
Using a series resistor to measure current
This method for measuring current can bring some advantages to some circumstances when it is foreseen that current may need to be measured on a regular basis within a circuit.
This current measurement technique entails placing a small resistor of a convenient value into the circuit. Normally one end of the resistor is at ground potential to avoid the risk of either high voltages of accidental shorts to ground while making the test.
By measuring the voltage across the resistor the current can easily be calculated.
For example of a 10Ω resistor is placed in circuit and a reading of 100 mV is detected across it, then using Ohms Law it can be deduced that the current is V / R = 0.1 / 10 = 10 mA .
In the case shown, the series resistor used for the current measurement is placed close to ground, and also it is bypassed with a capacitor to bypass any signal to ground. This is particularly important if the circuit is used at radio frequencies, RF as it will help prevent any signal being radiated along the test meter leads.
Current measurement technique using a current sensor / coil
If it is not possible to break into the circuit in any way, it is possible to use a current sensor.
Current sensors normally come in the form of a sensor that is placed around the current carrying conductor. It is able to detect the current flowing in the conductor and in this way give a reading.
These sensors often come as part of a complete meter, so it is often not possible to use a standard multimeter for this type of test.
There are several different types of sensor / meter that can be used this current measurement technique.
- Current transformer: One of the most common forms of current sensor is referred to as a current clamp. It consists of a split ferrite or soft iron ring onto which a coil is wound - one on each half. The core is passed over the conductor in which the current needs to be measured and the two halves of the core clamped in place. In this way, the assembly acts like a transformer, the clamp coils picking up the magnetic field from the current flowing in the conductor. As the overall assembly is effectively a transformer, this current measurement technique only works for AC. Also meters using this normally come as a separate "Clamp Meter".
- Hall effect sensor: The Hall Effect sensor using a different technology. It is able to measure both AC and DC flowing in a conductor. It is often used in conjunction with oscilloscopes, and high-end digital multimeters, although their use is becoming more widespread.
There are other similar current measurement techniques using sensors, but the current clamp and the Hall effect sensors are the most common.
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