How should surge protection be designed for Human-Machine Interfaces (HMIs)?

Designing surge protection circuits for HMIs requires determining the protection scheme based on the applicable surge test level for the product. Due to the high surge energy, a multi-stage protection architecture is typically employed. The first stage (coarse protection) uses gas discharge tubes or varistors installed at the power or signal line inlets to dissipate most of the surge current; for example, a 14D511K varistor is used at the AC power inlet. The second stage (fine protection) uses TVS diodes or varistors to clamp the voltage to a safe level; for example, an SMCJ24CA TVS diode is used on the DC side. Decoupling components, such as inductors, resistors, or fuses, are usually connected in series between the two stages to achieve energy coordination and ensure the first stage activates first.

For signal ports such as Ethernet and RS485, dedicated surge protection modules integrating GDTs and TVSs can be used. The selection of protection devices must consider their current-carrying capacity, clamping voltage, and response time. The ground wires of all protection devices must be very short and thick, connected to a separate "dirty ground" or chassis ground; this grounding path must be able to withstand surge current without generating a large potential rise. In PCB layout, surge protection areas should be clearly isolated from other circuits. Eintech offers a complete range of surge protection devices, from GDTs and varistors to TVSs, along with application guides to help engineers design reliable surge protection circuits that meet standards such as IEC 61000-4-5.