Switching Current
The current measurement path within the NI 4065 and NI 4070/4071/4072 utilizes an electromechanical relay that provides very low resistance 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 4065 and NI 4070/4071/4072 into the current measurement function before applying the current. Switching inductive current sources generally creates flyback voltages that stress the relay and, if done on a frequent basis, can shorten the reliability of the relay. Also, avoid interrupting the current by switching out of the current measurement function or resetting the DMM when currents are flowing through the circuit.
While the internal shunts are ideal for many applications, in applications where more than one current needs to be measured using the same DMM and the conditions require that the current cannot be interrupted across the load, you can use a break-before-make switch in multiplexer topology and place external shunts across the channels. Connect the shunts in series with the circuit under test and then measure the voltage drop across the shunts.
If the conditions allow interrupting the current flow through the circuit under test, it is still recommended to use external shunts. It is not common to find switches that can switch currents higher than 1 A, and switching currents can diminish the life of the switch.
The benefits of using the break-before-make switch in multiplexer topology with external shunts across the channels are as follows:
- Allows uninterrupted switching of current sources because the multiplexer switches voltages developed across the shunts instead of routing currents; ensuring that the current loop is not interrupted is generally helpful to avoid disturbing the current sources
- Extends the switch life, especially if the switch consists of electromechanical relays
The tradeoffs involved may be as follows:
- Reduces accuracy, although using the DMM to measure the resistance of shunts periodically (in 4-wire) can address this issue
- Introduces a leakage current from the switch that could significantly affect low-level current measurements; refer to your switch documentation for methods to reduce leakage currents
- Introduces errors due to resistor self-heating in high-level current measurements if the shunt resistor characteristics are not good enough
You should choose a shunt resistor whose characteristics and value help you reduce noise and errors, as follows:
- Consider the tradeoff 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.
- Look for a shunt resistor with low temperature coefficient (preferably not more than 10 ppm/°C) and absolute tolerance matched to your application requirement.
Recall that voltage offsets introduced by the switch could affect your measurement, because they add an additional burden voltage to the one specified by the ammeter itself. For the burden voltage per range, refer to Related Documentation for the specifications documents for the DMMs.
In cases where you must use the built-in current measurement capability of the NI 4065 and NI 4070/4071/4072, use a make-before-break setup for the switches for ensuring uninterrupted current loops. You may also choose to use specially designed current routing switches for this application.
Refer to DC and AC Current for the NI 4070/4072 or the NI 4065 and NI 4071 for general information about current measurements with the NI 4065 and NI 4070/4071/4072 and for recommendations when taking low and high level current measurements.