Control Transformers for Panels — Sizing VA, Choosing 12 or 24 V, DIN-Rail versus Wall Mount
Selection guide compiled by cBallast. 3-page PDF; the full content is on this page.
Compiled by
cBallast
Subject
Control circuit transformers
Document type
Selection guide
Reference
2026 edition
Issued
2026-10-03
Pages
3
Format
Web page and PDF
Sizing a control transformer for refrigeration and automation panels: steady versus inrush VA, 12 V and 24 V secondaries, DIN-rail and wall-mount formats in stock (3–20 VA), fusing both sides and the earthing convention for control circuits.
Why a dedicated control transformer
Controllers, EEV drivers, sensors and pilot devices want a clean low-voltage supply that doesn't sag when contactors pull in and doesn't share a fuse with the loads it supervises. A control transformer provides galvanic separation from the 230 V side, a defined fault path, and — correctly sized — voltage stability through coil inrush. In refrigeration panels it typically feeds the electronic controller, the EEV driver and the probe/transmitter rail.
Sizing: steady VA plus inrush headroom
Sum the steady burden: controller + driver + transmitters + indicator lamps (data-sheet VA or W each).
Add the switched burden that must not dip the rail: relay and small-contactor coils at their inrush VA (several times holding VA).
Select the next standard size with ~25 % margin. For a SmartCella-class room: 3–6 VA suffices; a panel with an EVD driver and valve wants the 20 VA class — the stocked 230/24 V 20 VA DIN-rail (E2V-driver class) exists for exactly that load.
12 V secondaries serve the 12 V controller families (e.g. Eliwell ICPlus 11I, 12 V); 24 V is the automation default (drivers, relays, sensors). Never run a 12 V controller from a 24 V rail "briefly" — electronics counts in milliseconds.
Protection and earthing
Primary fuse per the transformer's data (slow type at rated primary current class) — the panel's main MCB does not protect a 4 VA winding.
Secondary protection sized to the winding; a bolted fault on an unfused 20 VA secondary is a slow smoulder, not a trip.
Earthing convention: control circuits commonly earth one secondary pole (grounded control circuit) so a single earth fault blows the fuse instead of silently energising coils; follow the plant's existing convention consistently.
Separation: route 230 V and extra-low-voltage wiring apart; probes and stepper cables (see the EEV guide) do not share trunking with contactor wiring.
Diagnosing the rail
Controller reboots when the compressor starts → measure secondary voltage during pull-in: a dip below the electronics' minimum means undersized VA or shared wiring runs. Warm transformer at idle is normal; smelling varnish is not — check for a shorted turn after nearby lightning events. When replacing, match secondary voltage and VA at least; stepping VA up one size is always acceptable, stepping voltage never is.