What is AVR? How do transformer taps correct the voltage?

The reason for the 'click' in a UPS. When the grid drops to 185 V, how does the UPS lift the output to 213 V without touching the battery? Answer: different taps on one winding, and relays that pick them.

What does it do? AVR (Automatic Voltage Regulator) keeps the output near 230 V when the grid is low or high. In UPSs it's done with a tapped autotransformer: one winding with several taps. Whichever tap the relays connect the grid to, the output rises or falls by that ratio. Since no battery is used, the battery is spared during short and long sags. Wall-mounted 'voltage stabilisers' use the same idea; the big ones move a sliding contact with a servo motor instead of relays (stepless).

🎚 Change the grid voltage and watch the AVR

The transformer is one long winding with several taps. The relay picks which tap the grid goes into; the output always comes from the same tap. Drag the slider.

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Grid voltage185 V

batt.

boost

normal

buck

batt.

140160195250285300

Boost

The grid is low. The relay connects the input to the 200-turn tap; since the output comes from the 230-turn tap, the voltage rises by 230/200 = 1.15×. No battery is used.

Why does it work?

In a transformer every turn carries the same voltage. With 185 V across 200 turns, each turn has 0.925 V; 230 turns then give 230 × 0.925 ≈ 213 V. Change the number of turns the grid is spread over, and you change the output.

The thresholds (160 / 195 / 250 / 285 V) are typical; every model is different. Real units also add a few volts of hysteresis so the relay doesn't chatter.

🔲 What does an AVR relay look like inside?

A small blue or black box on the board, usually 12 V coil and 10–16 A contacts. Inside: a coil, a moving iron plate (armature) and a springy contact blade. Give the coil 12 V and listen for the click.

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⚡ Give the coil 12 V

Coil off: COM → NC

No current, no magnetism: the spring holds the blade up, so COM touches NC (normally closed). In the AVR this is the normal tap.

Reading the label

12VDC · 10A 250VAC

12VDC: coil voltage. 10A 250VAC: what the contacts can switch. Replace with the same coil voltage and at least the same current rating.

🔧 Testing it

Coil: ≈250–400 Ω for a 12 V relay; OL means an open coil. Contacts: COM–NC ≈0 Ω at rest; give the coil 12 V from a supply, hear the click and check COM–NO ≈0 Ω. A few ohms or a wandering reading means burnt contacts.

📷 Real relays

Fotoğraf: Retired electrician, CC0, Wikimedia CommonsFotoğraf: Raimond Spekking, CC BY-SA 4.0, Wikimedia Commons

🔍 How to spot it on a board

• •In a UPS, many coloured wires from the transformer to the board (each one a winding tap)
• •1–3 small relays on the board next to the transformer wires (the AVR relays)
• •An 'AVR' or 'Boost/Buck' LED on the front; a 'click' when the tap changes

🔧 How to measure it

• •Unplugged: measure Ω between transformer taps; the outermost ends give the highest reading, the middle taps fall in between. None should be open (OL).
• •Running (with an isolation transformer): measure the grid and the output together; the ratio tells you the tap (≈1.15 boost, ≈1 normal, ≈0.87 buck).
• •If you have a variac, lower the input slowly and note the voltage at which the relay clicks.

⚠ Common faults

• •Burnt AVR relay contact: at a certain grid voltage the output drops, or the UPS goes to battery for no reason.
• •The relay keeps clicking: the grid is right at the threshold, or the sensing circuit (voltage divider, capacitor) is faulty.
• •An open transformer tap: no output when that tap is selected; it reads OL on Ω.

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