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Identifying and Managing Inductive Loads

Let’s start with a inductive loads and how they interfere with USB connectivity:  Inductive loads are high voltage or high current loads that involve a magnetic field.  A motor, solenoid, valve, transformer, pump, or any other device that causes motion or generates a magnetic field is an inductive load.  Inductive loads are particularly difficult to control for ANY USB relay board, as the electromagnetic interference does not stay confined and isolated to the relay as one might assume (though solid-state relays are superior to mechanical relays in terms of isolation).

With regard to mechanical relays, USB relay boards may have to deal with induction from two sources: the coil of the relay and whatever happens to be connected to the relay.  The relay coil is well known for its inductive tendencies.  But the device connected to the relay is often ignored because there is an assumption of isolation since the coil of the relay is not electrically connected to the contacts.  In fact, nothing could be further from the truth.

When a relay is activated, current begins to flow through the contacts.  This current is flowing right next to the coil built into the relay.  The coil acts as a high-ratio transformer as current flows, so the driver circuit must be strong enough to handle the current (often in the form of electromagnetic interference) that flows back into the driver circuit.  The higher the voltage, the more noise is inducted onto the coil of the relay.  Many driver circuits are designed to handle this current flow, as flyback protection is usually built into most relay controllers.  Flyback protection alone is not enough to halt the electromagnetic interference from affecting the logic logic, power supply, and drive circuits.

When the USB port receives a command to deactivate the relay, the magnetic field inducted on the coil collapses, and a high voltage spike is sent right back into the control electronics, which can result in a malfunction or even a disconnect from the USB driver (more on this later).  This spike cannot be suppressed with a reverse polarity diode as one might expect.

Not only are the electronic relay drivers and logic affected, but most inductive loads also adversely affect the contacts of the relay.  Each time a relay is activated or deactivated with an inductive load attached, micro-pits begin to form in the relay contacts, causing excessive wear.  Mechanical relays will display micro-pits under a powerful microscope.  Some pits are so large they can be seen without any magnification.

Solid state relays are not immune to the effects of induction, in fact, they tend to be more fragile and less tolerant of highly inductive load sources.  Solid-state relays are; however, superior isolators, and will effectively block inductive loads from reaching the logic or control circuits of the relay board.  So at first glance, they appear to be more reliable since they do not reveal the symptoms of induction.

Solid state relays handle induction in a slightly different way.  Solid state relays are rated over a voltage range.  Induction introduces a spike that can frequently exceed the voltage rating of the solid state relay (even when working with small loads).  Frequent switching can cause excessive heat and over-voltage conditions for a solid-state relay, leading to failure.

Loss of Communications with the USB Relay Board

The High Voltage Spikes released by inductive loads are NOT compatible with USB communications.  Frequently, these spikes will travel through the USB port directly back to your computer, causing your motherboard to disconnect the USB device from the list of available USB devices.  The only way to recover from this condition is to remove the USB device from the computer and plug it back in.  In extreme cases, the inductive spikes can cause damage to the USB port of your computer.

It’s easy to blame a controller for malfunctioning when working with inductive loads, but in reality, the fact that a controller (regardless of manufacturer) malfunctions when working with these loads does have one benefit:

A malfunctioning USB relay controller is a sure way to indicate to the user that something is terribly wrong with the installation, and it must be properly handled to ensure a long life span.  Inductive loads MUST be managed externally, away from the relay board, regardless of how good your controller is.  We put together a tutorial on Controlling Inductive Devices which demonstrates the problem.  Inductive loads are typically managed using external components, as shown in the tutorial.  When a controller malfunctions, it’s letting you know the problem has not been properly managed.  When a controller is working properly, induction is properly managed, and the relay controller can give you years of reliable service.  Unmanaged problems can lead to early failure of the relay controller and potentially damage to a computer.  The USB port is a fragile bus, and the fact that it kicks off devices that misbehave is an ideal way to protect the computer from the serious electronic damage that can be caused by induction.

In our USB relay boards, we galvanically separate the USB circuit from the relay control PCB.  This provides greater noise immunity on the USB port, allowing the controller to tolerate a little more induction before your motherboard kicks the device off the USB bus.  Extreme induction will still cause our controllers to malfunction, but this is easily managed with an external components.  Properly suppressing induction requires external components to be installed as close to the inductive source as possible, which is why it is not possible for us to include these components on any of our relay controllers.

Symptoms of Unmanaged Inductive Loads Include: