3.3.2 Coil Malfunction Detection

One failure mode not directly covered in this setup is a burned-out or shorted coil inside the relay.

Note: In many cases, this failure may cause the relay to switch incorrectly, which would be detectable.

The simplest way to detect this failure mode is to monitor the current flow through the coil. If the coil is open, then no (or minimal) current is flowing. If it is shorted, then too much current is flowing. A simple way of implementing a current amplifier is with an operational amplifier and a current shunt resistor, as shown in Figure 3-3.

Figure 3-3. Current Amplifier