Regulator rectifier

The regulator rectifier is a finned metal box, usually under the seat, behind a side cover or on the frame where air can reach it. It has a plug with wires from the stator and wires to the battery and earth. The K-Light 125 uses a three-phase, full-wave type.

What it does

The stator makes AC (alternating current), which rises with engine speed. The rectifier part turns it into DC (direct current) to charge the battery. The regulator part stops the voltage rising too high by dumping extra power as heat. That is why it has fins and gets hot. Heat, loose plugs and corroded connectors damage it; when it fails it either stops charging or lets the voltage run too high.

Signs it is worn or failing

• Battery not charging while riding.
• Battery overcharges and bulbs blow and bulbs keep blowing.
• Regulator very hot or connector melted.
• Battery boiling or swollen.

When to replace it

When charging voltage is out of range with a good battery, good stator readings and clean connections, or its diode test fails. Replace a melted plug with the regulator. Follow your bike's manual.

What to match when buying

• Match make, model and year, or the part number: single-phase or three-phase, number of wires and plug.
• Some newer types (MOSFET) run cooler; only fit one if it is made for your charging system.
• Cheap units can overcharge. Test charging voltage after fitting.
• On many newer bikes the rectifier and regulator are one unit. If either half fails, replace the whole unit.
• Some bikes with no battery have no regulator and blow bulbs at high revs. A suitable aftermarket regulator can fix this; check the system voltage first.

Fitting notes

• Home job: Test the regulator rectifier, Test the charging system.
• Do not unplug it with the engine running or the key on; the K-Light manual warns this can destroy it.
• Clean the mounting face so heat can pass to the frame, and make sure the earth is good: Clean and tighten earth connections.

Reference (K-Light 125)