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Tests and Measurements

Current Measurements

Current measurements can usually be taken only if the ammeter is placed in series with the circuit under test. These measurements require that a circuit connection be unsoldered or otherwise opened to insert the meter in series with the circuit. An easier method you may use to obtain a current measurement is to take a voltage measurement across a known resistance and calculate the current with Ohm's law. The accuracy of current measurements depends on the internal resistance of the meter as compared with the resistance of the external circuit (load). Greater accuracy is obtained if the meter resistance is considerably less than the load resistance.

Multimeter Method

The multimeter contains circuitry that allows it to be used as an ammeter. It is usually capable of measuring both AC and DC currents of up to several amperes.

Oscilloscope Method

Current can be measured with an oscilloscope by shunting the input terminals with a low-value resistor. The input terminals must then be connected in series with the circuit being tested. The value of the resistor must be small enough not to interfere with the operation of the circuit under test. At the same time, it must be large enough that the voltage developed will cause adequate deflection of the oscilloscope trace.

Current Probes

Current probes are primarily designed to be used with an oscilloscope or ammeter for measuring current. The primary advantage in using a current probe is that it does not need to be in series with the current being measured. Unsoldering wires or connections to terminals is not necessary; current probes are designed to be clamped onto insulated conductors. They are able to sense, through inductive action, the magnitude of the current flowing in the conductor. Current probes are designed for performing AC current measurements. They can be divided into three basic types: passive, active, and Hall effect. Each type has advantages and disadvantages peculiar to its method of operation.

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