Why You Need USB Isolation for Industrial I/O – 8500901

USB Isolation and Industrial I/O

The USB port has become the most popular method for connecting virtually anything to a computer; from simple flash drives to complex industrial I/O equipment. Most computers built in the last five to ten years have at least one USB port; with newer computers having upwards of six or more. For most home users, having an isolated USB connection is not an issue. However, isolating a USB connection can be a necessity for applications such as; industrial/remote I/O, data acquisition, IT or medical equipment. This paper examines the advantages of using USB; what isolation means, types of isolation, and when USB connections should be isolated.

Why You Need USB Isolation for Industrial I/O

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What is Isolation?

Isolation is achieved by removing any direct electrical connection between two devices or portions of a circuit; but having them work as if they were still electrically connected. Isolation is generally rated by how high a low-current AC voltage can be set before there’s arcing across the isolation barrier. Some common isolation terminologies are:

  • Isolation Rating: The transient overvoltage that an isolation barrier can withstand. Usually expressed in terms of AC voltage. Some common ratings are:
    • 500 Vrms at 1 minute for industrial equipment
    • 2500 Vrms (IEC 60950-1) at 1 minute for IT equipment
    • 5000 Vrms (IEC 60601-1) at 1 minute for medical equipment
  • Working Voltage: Continuous voltage that can be applied across an isolation barrier, for the life of the device, without breaking down. Like the isolation rating, it’s usually expressed as an AC voltage.
  • Reinforced Isolation: Usually required for medical systems. This isolation rating equals two separate isolation barriers. In medical systems, this means the isolation barrier would have to withstand a short duration of 10,000 Vrms.
  • Creepage: The shortest distance between two lines on a surface layer of a PCB; on either side of the isolation barrier.
  • Clearance: The shortest air gap between two lines on a PCB.

Methods of Isolation

There are several methods of isolation. The method used depends largely on; what is being isolated and the bandwidth of the signals being isolated; as well as whether or not power must be transferred between the two sides of the isolation barrier. Some of the common methods are:

  • Optoisolators: Combine an LED and a phototransistor in the same package. As the LED emits light, the phototransistor will change states. These are well-suited for isolating digital signals, but cannot transfer power across the barrier.
  • Transformers: Use magnetic coupling. Additionally, transformers are very good for passing power and fast switching signals across the isolation barrier.
  • Differential Capacitor Coupling (DCC): This method makes use of a capacitors ability to pass AC signals while blocking DC. DCC can be used for high-speed digital data.

Learn More

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