Measuring Current with a Digital Multimeter
- essential or key details of how to measure current with a digital multimeter.
Current measurements are one of the key measurements that can be made with a digital multimeter.
Using a digital multimeter to measure current is easy to achieve and produces some accurate results.
Basics of measuring current with a multimeter
Voltage measurements are easy to make with a digital multimeter, but using a digital multimeter to measure current is slightly more involved.
When measuring current with a multimeter it is necessary to place the multimeter in series with the circuit so that the current actually flows through the digital multimeter.
The example below shows how a current measurement may be made by breaking the emitter resistor connection and placing the digital multimeter in series with the circuit to make the current measurement.
A typical digital multimeter current measurement
In this way a direct current measurement using a digital multimeter can be made.
Precautions for measuring current with a multimeter
When measuring current with a multimeter, there are a few precautions that can be observed to ensure that the readings are as accurate as possible.
- Ensure test leads are secure: When measuring current with a multimeter it is necessary to ensure that the connections will remain in place for a while. It may not be appropriate to use ordinary test probes, because these will only remain in place while they are held. It may be more appropriate to have connections using crocodile / alligator clips or some other method that will keep the connections in place while the circuit is switched on for the test.
- Meter resistance: The leads and meter will introduce additional resistance. In most cases this will be very low and can be ignored, bit on some occasions it may be important.
- Lead length: The leads used for the multimeter may cause issues with some circuits, especially if they are long. If the circuit is carrying signals above 100kHz or is capable of operating at frequencies above this, the leads may introduce stray capacitance and inductance which can cause the circuit to malfunction. Also radiation of the signal may cause problems.
Some voltage regulator chips are known to oscillate if the lead length to the smoothing capacitor is too long. Local decoupling may therefore be required on some occasions.
Using links to break circuit
When it is known at the design stage that a current measurement may need to be taken, some designers may insert a link into the design. This link can be removed when a current measurement needs to be taken.
Typically test points would also be provided as shown.
Use of a removable link to facilitate current measurement
For designs using surface mount technology, components known as zero ohm links may be used. These are effectively short circuit resistors that can easily be removed from the circuit, enabling current to flow through a meter connected to the circuit.
Current measurement test point
Another method of facilitating current measurement with a multimeter is to use a small resistor in the circuit.
The circuit can be modified so that a resistor carrying current may be split so that it develops a small proportion of the voltage across it. This will be proportional to the current flowing. This if a 100Ω resistor is placed in the emitter circuit, then voltage across the resistor can be measured with a digital multimeter set to the volts range. Using Ohms Law, it is an easy matter to calculate the current.
Measuring current with a DMM set to measure voltage
In the circuit shown above, the 100Ω resistor would typically be much smaller than the emitter resistor, Re. If 0.10 volt is developed across the measurement resistor, then using Ohms Law it is possible to calculate the current, I, flowing is V/R = 0.10/100, i.e. 1 mA.
If the circuit is carrying high frequencies it is good practice to decouple the measurement t resistor to ensure that it does not carry any signal, which could be radiated and cause problems .
By Ian Poole
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