Considerations When Using a Power Supply with a Switch


This article is part of the National Instruments Measurement Fundamentals series. Each article in this series will teach you a specific topic of common measurement applications, by explaining the theory and giving practical examples. This article describes how to properly use a power supply with a matrix or multiplexer switch.

For additional power supply only concepts, refer to the Power Supply Fundamentals main page.
For the complete list of related articles, return to the NI Measurement Fundamentals Main page.


Considerations when Switching Currents

It’s important to understand the current specifications and the relay connection type of a switch module before connecting the power supply. The maximum current specification for most switch modules is usually broken into maximum carry current and maximum switched current. Switching current is the maximum rated current that can flow through the switch as it makes or breaks a contact. Switching active currents results in arcing that can damage the contacts of electromechanical relays. Carry current is defined as the current that can be passed by previously closed relays.

A switch module has one of the following types of relay connections: break-before-make (BBM) and make-before-break (MBB). The make-before-break switches are the ideal relay type because the current signal path is never interrupted. However, you can prevent the currents transients caused by switching current with a break-before-make relay type by using the cold switching technique. Cold switching is the process of closing the relay contacts before turning on the power supply voltage and current, as well as turning off the power supply voltage and current before opening the contacts.

Connecting a Power Supply to a Multiplexer or Matrix Switch

Connecting a single power supply to a multiplexer or matrix is a cost-effective way of powering multiple devices under tests (DUTs). Before choosing your power supply, you should first determine the total current output requirement for your application. Total current output is calculated by multiplying the current requirement for each DUT by the total number of DUTs you want simultaneously power.  

For example:

  • A DUT requires 10V at 100mA
  • Test systems requires five DUTs powered at a time
  • Hence, you must program the power supply, such as the NI PXI-4110, to output 10V at 500mA, as shown in figure 1. 

Figure 1. Sharing the current between five channels of a multiplexer

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