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How to measure current
- an overview or tutorial about how to measure electrical current with a digital multimeter (DMM) or an analogue multimeter. This includes how to direct electrical current and how to measure ac current with a multimeter.
This test meter tutorial is split into several pages: Analogue multimeter
 DMM digital multimeter
 Basic instructions on how to use a multimeter
 Multimeter tutorial and video
 Making a multimeter voltage measurement
 How to measure current with a multimeter
 Making a multimeter resistance measurement
 Simple diode and transistor test
 Testing and fault finding a transistor circuit
It is often necessary to know how to measure current using a multimeter. Current measurements are easy to make, but they are done in a slightly different way to the way in which voltage and other measurements are made. However current measurements often need to be made to find out whether a circuit is operating correctly, or to discover other facts associated with its current consumption.
Current measurements can be made with a variety of test instruments, but the most widely used pieces of test equipment for making current measurements is a digital multimeter. These items of test equipment are widely available and at very reasonable prices.
Basics of current measurement
Current measurements are made in a different way to voltage and other measurements. Current consists of a flow of electrons around a circuit, and it is necessary to be able to monitor the overall flow of electrons. In very simple circuit is shown below. In this there is a battery, a bulb which can be used as an indicator and a resistor. To change the level of current flowing in the circuit it is possible to change the resistance, and the amount of current flowing can be gauged by the brightness of the bulb.
A simple circuit in which to measure current
When using a multimeter to measure current, the only way that can be used to detect the level of current flowing is to break into the circuit so that the current passes through the meter. Although this can be difficult at times, it is the best option. A typical current measurement can be made as shown below. From this it can be seen that the circuit in which the current is flowing has to be broken and the multimeter inserted into the circuit. In some circuits where current may often need to be measured, terminals with a shorting link may be added to facilitate the current measurement.
How to measure current using a multimeter
In order that the multimeter does not alter the operation of the circuit when it is used to measure current, the resistance of the meter must be as low as possible. For measurements of around an amp, the resistance of a meter should be much less than an ohm. For example if a meter had a resistance of one ohm, and a current of one amp was flowing, then it would develop a voltage of one volt across it. For most measurements this would be unacceptably high. Therefore resistances of meters used to measure current are normally very low.
How to measure current with an analogue multimeter
It is quite easy to use an analogue meter to measure electrical current. There are a few minor differences in way that current measurements are made, but the same basic principles are used.
When using the meter it is possible to follow a number of simple steps:
How to measure current with a digital multimeter
To measure current with a digital multimeter it is possible to follow a few simple steps:
Following these steps it is very easy to measure current using any digital multimeter.
How to measure ac current with a multimeter
It is often necessary to measure AC current. Although the same basic steps are sued for taking the AC current measurement as when a normal DC measurement is taken, there are a few additional points to note.
The differences in the measurement result from the fact that the multimeter has to rectify the alternating waveform to enable it to measure AC current. For a digital multimeter the main difference is that the measurement type switch must be set to measure AC current rather than DC current..
For an analogue multimeter the situation is a little different. As an analogue multimeter does not contain any active electronics, the diode rectifier used to rectify the alternating waveform has a certain turn on voltage and this will affect the low voltage end of some scales. Some meters may not be able to measure AC current, or they will have very restricted ranges..
Although it is not as common to measure electrical current as it is to measure voltage, it is nevertheless and important ability to be able to measure current. Also knowing how to measure current to gain the best of the multimeter is also important.