Measuring the Voltage of Floating Outputs

September 26, 2017
Tech Tuesday

All Functional Devices’ AC power supplies, stepdown transformers and isolation transformers have a floating output. The outputs are said to be floating because they are isolated from the input and have no internal connection to it. Since that is the case, when measuring the output voltage of any of those devices, ground cannot be used as the reference for that measurement; doing so in this situation will lead to erroneous readings. To accurately measure the voltage of a floating output, the meter probes should be connected directly to the output terminals.

Our AC power supply and transformer outputs provide galvanic isolation for connected equipment. Separating the input and output circuits prevents ground loops from forming, which can cause potentially damaging currents to flow in a system. It can also help to reduce noise on the line.

While the above listed devices do come from our factory with the outputs isolated, it is possible to connect the primary and secondary when it is required or desired. The diagram below shows how the primary and secondary can be connected. When connecting the primary to a source with a neutral, that neutral can be connected to the “Com” output of the transformer. It is important to make this connection properly, especially when dealing with phase-dependent applications. If the incorrect output is connected to the primary neutral, the output will be out-of-phase with the input, which can adversely affect operation. On Functional Devices Power Supplies and Transformers the primary “Neutral” or “Com” will be in phase with the secondary “Com” terminal or wire.

As always, when installing any Functional Devices products, make sure to follow all national and local codes. If you have any questions about or interest in our products, give us a call or an email.

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About the Author

Henry Smith is a 34-year-old engineer at Functional Devices, Inc. He has a BSEET obtained from Purdue University in 2014 and a lifelong interest in electronics. As an engineer at Functional Devices, he gets to provide Technical Support to our customers, from distributors to specifying engineers and installers.

Henry enjoys providing tech support, as it allows our company to assist at every level of our product’s lifecycle and exposes us to interesting and unique applications. While not every technical question is unique, even answering a simple question or providing someone with the information in a timely manner can go a long way to helping him or her meet a deadline.