WiremanWireman · Buck-boost giving the wrong voltage: troubleshooting for electricians · 2023 NEC ·
Buck-boost giving the wrong voltage
Check the direction, the winding connections and the input voltage against the maker’s diagram.
Before you start
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How it works
A buck-boost transformer is a small transformer, often with a 120/240 V primary and a 12/24 V or 16/32 V secondary, connected as an autotransformer so the low-voltage winding adds to (boosts) or subtracts from (bucks) the supply. Only the difference passes through the windings, so a small unit can handle a big load. The output depends on how the windings are connected and their polarity: one wrong jumper turns a boost into a buck or changes the amount. For example, a 208 V supply on a 240 V primary with a 24 V secondary boosts by 208 × 24 ÷ 240 = 20.8 V, to about 229 V.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What is the output doing?
- Moves the wrong way (bucks instead of boosts, or the reverse)Low-voltage winding polarity reversed
- Right direction, wrong amountCompare the connections to the manufacturer’s diagram for your input and output. What doesn’t match?
- Primary windings in series vs parallelPrimary connection wrong
- Secondary windings in series vs parallelSecondary connection wrong
- Everything matchesTest: Measure the actual input voltageExpect: The input voltage used to select the connection.
- Input differs from what was assumedInput voltage isn’t what was assumed
- Input as expected, output sags under loadUnit too small or voltage drop
- Output equals inputTest: Measure across the low-voltage windingExpect: About the winding’s rated voltage for that connection (for example 12, 16, 24 or 32 V).
- 0 VPrimary not energized
- Correct winding voltageOutput taken from the wrong point