DC and AC Current Measurements

This tutorial recommends tips and techniques for using a National Instruments digital multimter (DMM) to perform DC and AC current measurements.

For more information return to the Complete Digital Multimeter Measurement Tutorial.

1. Overview
2. Input Protection
3. Function Switching
4. Low and High Current

Overview

The NI 4070 Digital Multimeter allows you to measure currents (amps) ranging from a few microamperes to an ampere with a sensitivity of tens of nanoamperes. The DC and AC current (amps) functions have three ranges each: 20 mA, 200 mA, and 1 A for DC current; 10 mA, 100 mA, and 1 A for AC rms current. The AC current (amps) function uses the same fast measurement technique as the DC-coupled AC volts function.

The current (amps) you measure flows through a stable onboard 0.5 Ω precision shunt resistor for all ranges. Consequently, the current (amps) loop remains uninterrupted during autoranging. An ultra-low noise circuit amplifies the resulting voltage developed across the shunt and routes the voltage to the high-resolution ADC.

Input Protection

The NI 4070 Digital Multimeter can measure up to 1 A DC or 1 A AC_rms with up to 2 A peak currents. Higher current (amps) triggers the built-in protection circuit resulting in the need to replace the fuse. Exposing the current (amps) measurement connectors to sources higher than 1 V while the DC or AC current function is selected may also result in the need to replace the fuse. For more information on fuse replacement, refer to the NI Digital Multimeters Getting Started Guide .

Function Switching

Current (amps) measurement switching utilizes an electromechanical relay that provides very low resistance (ohms) to assure a low burden voltage. While this relay has an exceptionally long life, you can take special precautions to ensure reliable operation. Whenever possible, switch the NI 4070 Digital Multimeter into the current (amps) measurement function before applying the current. Switching inductive current (amps) sources generally creates flyback voltages that stress the relay and, if done on a frequent basis, these events can affect the reliability of the relay. Also, avoid interrupting the current (amps) by switching out of the current measurement function when currents are flowing through the circuit.
While the onboard shunt is ideal for many applications, for large-channel count applications and applications intolerant of breaking the current (amps) loop, you can use an external shunt in front of your switching for each wired channel. When choosing a shunt resistor value, consider the tradeoff you are making between creating a large enough voltage across the shunt, for resolution, while at the same time keeping the shunt voltage small enough to prevent high heat dissipation or burden voltage errors during the measurement. The phenomenon is often referred to as resistor self-heating  and can cause significant errors in the measurements.

Low and High Current

Low Current
You can use the NI 4070 Digital Multimeter to measure very low current (amps) values. In the 20 mA range, you can measure microampere level currents with a resolution of tens of nanoamperes. Take the following precautions to assure measurement integrity:

• Maintain a relative humidity less than 80%.
• Maintain an operating temperature less than 28 ºC.
• Use shielded cabling made of high–resistance (ohms) insulation and nonmoisture absorbing material such as Teflon.
• If you are using external switches for routing low current (amps) levels, consult your switch documentation on how to reduce leakage errors.
• Use software filtering and noise reduction techniques such as increasing the measurement aperture or DC noise rejection.
  

High Current
When measuring high current (amps) values, refer to resistor self-heating.

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