How to Measure Current

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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 is one of the basic electrical / electronic parameters, and therefore it is often necessary to measure the current flowing in the ciricuit to check its operation.

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 - they are also likely to be available in any hoe workshop or professional electronics laboratory.

Current measurement: basics

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.

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.

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 analogue multimeter it is possible to follow a number of simple steps:

1. Insert the probes into the correct connections - this is required because there may be a number of different connections that can be used. Be sure to get the right connections as there may be separate connections for very low or very high current ranges.
2. Set switch to the correct measurement type (i.e. to measure current) and range for the measurement to be made. When selecting the range, ensure that the maximum for the particular range chosen is above that anticipated. The range on the multimeter can be reduced later if necessary. However by selecting a range that is too high, it prevents the meter being overloaded and any possible damage to the movement of the meter itself.
3. When taking the reading, optimise the range for the best reading. If possible adjust it so that the maximum deflection of the meter can be gained. In this way the most accurate reading will be gained.
4. Once the reading is complete, it is a wise precaution to place the probes into the voltage measurement sockets and turn the range to maximum voltage position. In this way if the meter is accidentally connected without thought for the range to be used, there is little chance of damage to the meter. This may not be true if it left set for a current reading, and the meter is accidentally connected across a high voltage point!

How to measure current with a digital multimeter

To measure current with a digital multimeter it is possible to follow a few simple steps:

1. Turn the meter on
2. Insert the probes into the correct connections - in many meters there are a number of different connections for the probes. Often one labelled common into which the black probe is normally placed. The other probe should be entered into the correct socket for the current measurement to be made. Sometimes there is a special connection for current measurements, and sometimes a separate one for either low or high current measurements. Select the correct one for the current measurement to be made.
3. Set main selector switch on the meter switch to the correct measurement type, (i.e. current) and range for the measurement to be made. When selecting the range, ensure that the maximum range is above the expected reading anticipated. The range on the DMM can then be reduced as necessary. However by selecting a range that is too high, it prevents the meter being overloaded.
4. When the measuring the current, optimise the range for the best reading. If possible enable all the leading digits to not read zero, and in this way the greatest number of significant digits can be read.
5. Once the reading is complete, it is a wise precaution to place the probes into the voltage measurement sockets and turn the range to maximum voltage. In this way if the meter is accidentally connected without thought for the range used, there is little chance of damage to the meter. This may not be true if it left set for a current reading, and the meter is accidentally connected across a high voltage point!

Following these steps it is very easy to measure current using any digital multimeter.

Alternative methods of measuring current

The most obvious method to measure current with a multimeter is to break the circuit and apace a meter actually within the circuit. However it is not the only method that can be used.

There are some methods that can be implemented that do not require the circuit to be broken and a meter placed in series.

These methods are often used where it is important not to break the circuit, and methods which sense the current in one way or another are used.

The accuracy may often be almost as good as placing a meter in circuit, but they may require components to already be in place, or different types of sensor to be used.

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 .

When using this method of measuring current, the value of the resistor must be sufficiently accurate for the measurements to be made. Any tolerance into e resistor will give a similar tolerance not he measurement. Fortunately many measurements in this situation do not need extreme accuracy, and therefore even 10% resistors will be sufficiently accurate - 2% may also be adequate depending upon the tolerances needed.

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.

How to measure ac current with a multimeter

It is often necessary to measure AC current. Although the same basic steps are used for taking the AC current measurement as when a normal DC measurement is taken, there are a few additional points to note.

• AC setting required:   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.
• Analogue meters require rectifier:   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.

Despite these extra requirements, virtually all digital multimeters and most analogue multimeters have a facility to enable AC measurements to be made. For analogue meters there will be a separate AC scale as current cannot be made using a totally linear scale because of the voltage characteristics of the diodes used in the rectifier.

Although it is not as common to measure electrical current as it is to measure voltage, it is nevertheless a very important ability to be able to measure current.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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