Brand:

• AMX

Models:

• General

Overview:

The information in this article applies to:

Relays and IOs on any AMX product, connected to (controlling) an inductive load such as an external relay*

*Also applicable to other inductive devices: solenoids, such as in a doorbell or electronic water valve; motors; and transformers

Symptoms:

• Incorrect return status of control system relay channel
• Premature failure of controller hardware

A relay on an AMX controller is wired to complete a low-voltage circuit consisting of a power supply and an external relay.  The second relay may be part of another device or is necessary to switch higher voltage and/or current than the limits of the AMX part.

With the external relay disconnected, the AMX relay operates correctly: It turns on and off as the programming dictates, and channel status of the relay accurately reflects the relay's state.  (DEFINE_MUTUALLY_EXCLUSIVE and DEFINE_LATCHING behavior modifiers notwithstanding)

With the external relay connected, however, the AMX relay becomes unreliable or worse: It does not follow the commands given by the programming, or the status reports incorrectly, or catastrophic hardware failure results.  Worse, the connection of this relay may seem to affect the behavior of adjacent relays.

Cause:

The simplest and most common control system implementation is for a DC relay, shown here:

When the control system relay opens and the relay coil is turned off, its inductive energy can generate a voltage spike of hundreds or thousands of Volts.  This spike may wreak havoc on control system logic causing apparent failures in the programming or catastrophic failure of the hardware itself.

Resolution:

These problems can be averted by including electronic components to suppress the transients.

The energy, in this case, is dissipated in the circuit created by the coil and the small rectangle, rather than being fed back to the control system.

There are numerous common ways to suppress coil transients.  Three most common are given here:

The easiest and most common solution for the described symptoms in this application is a general-purpose diode (1N4001 or similar) reverse-biased (cathode to +) across inductive load, as shown below:

Although this is widely used, this configuration has a disadvantage that the external relay will release more slowly, which could ultimately shorten the relay's life.  An alternative adds a Zener diode as shown:

This configuration eliminates the problem of the external relay releasing more slowly, but does add additional cost and the complexity of finding the correct component (Zener should be rated at approximately the voltage of the power supply) and also installing the additional component (while a single diode can be installed across the existing relay terminals, the addition of a second diode requires another terminal point).

Both preceding configurations have the disadvantage of operating only on DC circuits.  Using two “back-to-back" Zener diodes, the same solution may be applied to an AC circuit.​