How to configure surge suppression for HMI (Human Machine Interface) interfaces?

Configuring surge suppression circuitry for HMI interfaces requires design based on the interface type, cable length, installation environment, and required surge test levels. Common interfaces requiring surge protection include Ethernet, RS485, CAN, and analog inputs. Basic protection circuitry typically employs two or three stages. The first stage uses a high-current-carrying gas discharge tube or varistor, installed at the interface inlet to discharge most of the surge current. The second stage uses a faster-responding TVS diode for precise voltage clamping. Decoupling components, such as resistors, inductors, or fuses, are needed in series between the two stages to achieve energy matching and ensure the GDT conducts first.

For example, for Ethernet ports, an integrated RJ45 with surge protection can be used. For RS485 interfaces, dedicated three-terminal protection devices or a GDT+TVS combination can be used on the A/B lines. During configuration, the current-carrying capacity and clamping voltage of the protection devices must match the withstand capability of the backend chips. All protective devices should be placed as close as possible to the interface connector, and their grounding pins must be connected to a separate "protective ground" or directly to the chassis via a very short, thick trace. This grounding path must be able to withstand large surge currents. Eintratronix offers a wide range of GDTs, varistors, and TVS products, as well as surge protection reference designs for various interfaces, helping engineers quickly configure interface surge suppression.