Step 8 / 14

How does a voltage regulator work?

The box that feeds the home's appliances close to 220 V even when the grid is low or high. Inside there's one big autotransformer, relays that pick its taps and a processor that decides. Change the grid voltage at the top and watch which tap comes into play.

Grid input

The home's mains. In rural areas, at the end of a line or at peak hours it can drop to 170–180 V, and sometimes exceeds 250 V.

Oscilloscope

Multimeter≈220 V AC

Tap a block to see what it does and what you'd measure there.

🔧 Inside the regulator: what's where?

This drawing isn't a photo; it follows the typical layout of relay-type home voltage regulators. Open the cover and you first see the big doughnut-shaped autotransformer with rows of relays beside it. Pick a grid voltage and watch which tap wire and which relay come into play.

Energy path:

Tap a region to see what it does and how to spot it on the board.

⚙ Servo regulator: stepless correction

Large regulators (10 kVA and up) use a small motor instead of relays to slide a carbon brush over the autotransformer's bare copper winding. Change the grid and watch the motor slowly move the brush to bring the output to 220 V.

Grid voltage190 V

Relay type vs servo type

	Relay	Servo
Correction	in steps (±5–8%)	stepless (±1–3%)
Speed	fast (≈20 ms)	slow (1–3 s)
Wear	relay contacts	carbon brush, motor
Typical	1–10 kVA	10–100 kVA

🔧 Common servo faults

A worn carbon brush (output crackles, sparks visible on the winding); a burnt track on the winding where the brush sits most; the motor or its limit switch failing (the brush stops at one end and the output stays high or low).

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⚠ Safety: The inside of a regulator is entirely at mains voltage; an autotransformer gives no isolation from the grid (the output is connected to the mains too). Unplug it or switch off its supply breaker before opening the cover. Relay contacts and cable lugs under load can get very hot.

Next step: Step 9How does a hybrid (off-grid) inverter work?→

Go further

Repair Workshop: find the regulator fault by measuring Concept: AVR, how transformer taps work The same idea in a UPS: AVR How the 7-segment display works Component: relay