47. GDT for 24VDC Industrial Field Wiring Surge Protection
1. Component Purpose
Long 24VDC field cables running across factory floors and outdoor cable trays act as antennas for lightning-induced surges. These surges can damage the controller’s input circuitry, TVS diodes, and ICs.
A GDT at the controller input acts as primary (first-stage) protection. It absorbs the high-energy portion of the surge. A downstream TVS diode (second stage) then clamps any residual voltage to a level safe for the board components.
The 75V spark-over is well above the 24VDC supply range (up to 30V with tolerance), so the GDT stays inactive during normal operation.
Required: 75V Spark-Over · ≥5kA (8/20µs) · 2-Pole · Surface Mount · Littelfuse · RoHS
2. How to Find It on DigiKey
Go to DigiKey.com → Circuit Protection → Gas Discharge Tube Arresters (GDT)
Apply these filters:
- Voltage - DC Spark Over (Nom) → 75V
- Impulse Discharge Current (8/20µs) → 5kA or higher
- Number of Poles → 2
- Mounting Type → Surface Mount
- Manufacturer → Littelfuse Inc.
- Part Status → Active
Tip: In a two-stage protection scheme, a decoupling element (usually a small inductor or resistor) between the GDT and TVS diode ensures proper energy coordination — the GDT fires first and handles the bulk energy, while the TVS handles the fast residual.
3. Key Specifications & What They Mean
| Specification | Required | Why |
|---|---|---|
| Spark-Over Voltage | 75V | Above 24VDC max (30V); surge-only firing. |
| Surge Current | ≥5kA (8/20µs) | Lightning protection for long field cables. |
| Number of Poles | 2 | Protects 24V line and return together. |
| Mounting | Surface Mount | Compact board layout. |
| Manufacturer | Littelfuse | Company AVL requirement. |
| RoHS | Yes | Compliance required. |
4. Selecting a Safe, Production-Ready Part
The filtered list will show matching parts. Before you pick one, check: Is it Active? Is stock available? Is the manufacturer reputable? Is a datasheet available?
Full checklist: How to Select a Safe, Production-Ready Component — EWskills Guide