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Copyright 2008, National Control Devices LLC

By Ryan Sheldon
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USB Devices CANNOT be used in inductive control applications.

Do not use USB relay controllers for these applications. USB Relay Controllers may be safely used for these applications.
Motors (Including Fans, Pumps, and Aquariem Aerators) Audio/Video Switching
Solenoids Telecommunications Switching
Valves Low-Power Signal Switching
Transformers AC/DC High-Voltage/Low Voltage Incandescent Lighting Control
Flourescent Lighting Audio Speaker Switching
As a general rule:  If it moves under electronic control, USB Relay Controllers should NOT be used.  Other more suitable devices are available.  Please CONTACT US for a recomended device. As a general rule:  If the device you are trying to control stays electrically quiet, and does not generate Electromagnetic Interference, then USB relay controllers may be safely employed.

Why USB Cannot be Used in Inductive Control Applications:

When you plug a USB device into your computer, the motherboard and our USB device establish a constant high-speed communication with each other.  This communication is very delicate in nature.  The USB Standard makes NO PROVISIONS for error checking and self correction, therefore, the data must ALWAYS stay clean and uncontaminated. 

While it is true that mechanical relays are electrically isolated, they are NOT EMI isolated.  This means they are able to transfer EMI back into the controller, and disrupt the delicate high-speed data stream between our device and the motherboard of your computer.  Many of our USB devices employ Galvanic Isolated, an isolation technique that electrically separates the USB interface from the rest of the controller (offering outstanding lightning protection).  However, Electromagnetic Interference is a lot more like a radio transmitter, and is capable of disrupting nearby microprocessors and other electronic devices.

An inductive control application is a lot like trying to get a 5V microprocessor to run flawlessly when a lightning strike occurs just inches away from the interface without interference.  It simply is not possible to effectively isolate a high voltage control application that is in such close proximity to a 5V high-speed microprocessor.

So What Happens if you Break the Rules:

You will get a USB Communications Error, your Program will be Halted, and the Motherboard of your computer will kick the USB device Off the interface (you will not be able to communicate to the device again until you physically remove the USB connection and then reconnect it).

What We are Doing About it:

We have discovered isolation techniques that allow us circumvent these problems completely.  We are currently developing a line of devices that will allow USB to coexist in high-induction control applications.  These new designs are currently under development, and will be released in the Fall of 2008.  as of February 18th, 2008, we have created our first prototype test devices.  We will be testing these devices for approximately 3 months before we migrate our new interface into the rest of our line of relay controllers.  Watch for a significant increase in product offerings Late 2008.

Related Articles:

The following article does NOT apply to NCD USB Devices unless otherwise noted, but is very useful when using other interface methods in conjunction with Inductive Control Applications: Controlling Inductive Loads: Managing Electromagnetic Interference