Choosing the Right Relay for Your HHO Generator: Ratings, Types, and Wiring

Why Relay Selection Matters

The HHO relay must reliably handle the cell's maximum current draw for years of continuous duty cycling. An undersized relay runs hot, contacts oxidize, and eventually the relay fails to make contact — your HHO system stops working without any obvious sign. An oversized relay is always acceptable; an undersized one eventually fails.

Current Rating

Select a relay rated for at least 150% of your HHO cell's maximum current. A cell drawing 15A maximum needs a 30A relay minimum. Standard Bosch-type 30A relays (used in automotive applications everywhere) are the correct choice for most HHO installations.

Relay Types

• Standard Bosch 4-pin relay: SPST (normally open). Terminals 85, 86 (coil), 30 (power in), 87 (power out). Most common, readily available, $5–$10.
• 5-pin relay (SPDT): Adds terminal 87a (normally closed). Useful for triggering an LED indicator when HHO is active (power to LED from 87a when HHO is off, off when HHO relay activates).
• High-current relay (40A+): For large dual-cell systems drawing 25A+. Usually a larger body — check physical fit in your relay socket.

Relay Socket Mounting

Mount the relay in a waterproof relay holder with a DIN rail or direct mount option. A relay flopping on its wires eventually stresses the socket connections and fails. Secure mounting is as important as correct rating.

Coil Voltage

Standard automotive relays use a 12V coil triggered by the ignition switch. The coil draws approximately 150–200mA — well within any ignition circuit's capability. No relay driver transistor is needed for standard HHO switching.