Control Transformer and 24 V DC Auxiliary Supply Selection
control transformer: A control transformer separates the relay, contactor coil and signal circuits inside a panel from the power circuit and supplies them, usually at 230 V or 24 V. Sizing is governed not by the steady sealed power but by the peak power drawn while contactor coils pick up; the secondary voltage must not fall below 85 % of the coil rated voltage at that instant.
Contents
The auxiliary supply is the least discussed circuit in a switchboard and the one that produces the most faults. A contactor that fails to pick up, a PLC input that flickers at random or a safety relay that trips unexpectedly usually starts not in the power circuit but in the control supply. This article covers control transformer sizing, the choice between 230 V AC and 24 V DC, secondary protection and earthing rules, and voltage drop calculation from the point of view of panel design.
Why is the auxiliary voltage built as a separate circuit?
Separating the control circuit from the power circuit serves three purposes: touch safety, noise immunity and fault containment. A control circuit fed through an isolating transformer is unaffected by an earth fault on the power side and keeps its own earthing reference. Switching noise from the power circuit is also less likely to reach signal circuits, which makes a noticeable difference in panels carrying analogue measurement and communication lines. Third, protecting the control circuit separately means that a blown fuse takes out only the function concerned rather than the whole board.
How is a control transformer sized?
Two power figures matter: the steady sealed power of the coils and the peak power at pick-up. Contactor coils draw several times their sealed current while picking up, and that current produces a sudden voltage dip on the transformer secondary. The acceptance criterion is that under the least favourable case, that is when the largest contactor picks up under the fullest loading, the secondary voltage must not fall below 85 % of the coil rated voltage. Catalogues therefore quote an inrush power figure alongside the continuous rating, and the transformer is chosen against that second figure. Summing the coils that can pick up simultaneously with a coincidence factor avoids unnecessary oversizing.
230 V AC control or 24 V DC?
230 V AC control remains common because it creates no voltage drop problem over long cable runs and suits classic relay logic, but it needs extra measures for touch safety and is not directly compatible with modern PLC input cards. A 24 V DC supply is the natural operating voltage of PLCs, sensors, safety relays and communication devices, and remaining in the safety extra-low voltage band reduces touch risk. In exchange, cable cross-sections grow because of voltage drop and the short-circuit behaviour of a switch-mode supply makes fuse discrimination harder. Mixed solutions are widespread: power switching with 230 V AC coils and the logic and signal layer on 24 V DC.
Secondary protection, earthing and voltage drop
| Criterion | 230 V AC control transformer | 24 V DC power supply |
|---|---|---|
| Touch safety | Requires extra measures and labelling | Stays within the safety extra-low voltage band |
| Long cable runs | Voltage drop is not an issue | Cross-section increased, voltage drop calculated |
| PLC and sensor compatibility | Interposing relay or converter needed | Directly compatible |
| Behaviour at pick-up | Coil inrush power dictates the transformer | Peak current capability of the supply dictates |
| Redundancy | Second transformer or transfer circuit | Diode module, buffer module or battery |
The secondary circuit is protected by a circuit breaker or fuse at the transformer output, selected to cover both a short circuit and transformer overload. Earthing the control circuit at one point makes the protective device operate on an insulation fault and prevents a circuit floating at undefined potential from carrying a hidden fault; earthing must be at a single point, since multiple points create circulating currents. In 24 V DC circuits the voltage drop calculation is critical: a valve coil at the end of a long field cable may not see enough voltage even when 24 V is measured at the source. A practical rule is to keep the drop below five per cent at the furthest load and to include the return conductor in the calculation.
Redundancy, fault management and monitoring
In critical applications two power supplies are paralleled through a diode module, or a buffer module bridges brief interruptions. In battery-backed solutions the rectifier, battery bank and low-voltage disconnect are planned together and battery health is tested periodically. An auxiliary contact or digital input on the supply output turns loss of supply into an alarm through the PLC, which is the simplest way to catch a safety function that has quietly dropped out. During commissioning, secondary voltage should be measured and recorded both unloaded and at full load.
Key takeaways: control transformer
- The transformer is chosen on inrush power — the continuous sealed rating alone is not a sufficient criterion.
- Secondary voltage must stay above 85 % — otherwise contactors fail to pick up or chatter.
- Earth the control circuit at one point only — multiple earths create circulating currents and hidden faults.
- Calculate voltage drop in 24 V DC circuits — five per cent at the furthest load is a practical target.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
My contactor sometimes fails to pick up, can the transformer be the cause?
Yes, it is one of the most frequent causes. The peak current drawn by coils at pick-up produces a voltage dip on the transformer secondary, and pick-up becomes unreliable once the voltage falls below 85 % of the coil rating. Add up the coils that can pick up simultaneously and check the transformer inrush rating.
Is earthing the control circuit mandatory?
Not mandatory, but strongly recommended. In an unearthed control circuit the first insulation fault goes unnoticed while a second fault can cause unintended operation. Alternatively, in installations using an insulation monitoring device the circuit is deliberately left isolated and protected by monitoring instead.
Can a single 24 V supply feed the whole panel?
Technically yes, but the impact of a fault grows. Splitting safety functions, field sensors and communication devices into separate circuits with separate protection stops one short circuit from halting the entire board. In critical applications supply redundancy is added as well.
Related articles
- PLC Automation Panel Design: I/O Structure, 24 V DC Supply and Signal Segregation
- DC Distribution Board: Rectifier, Battery Bank and 110/220 VDC Supply
- Internal Panel Wiring: Sizing, Colour Codes and Trunking
- Shunt Trip (MX) and Undervoltage Release (MN): Which One and Where?
- All technical articles
In Devpan panels the auxiliary supply architecture is designed by calculating contactor inrush powers and field cable lengths, with secondary protection and earthing shown on the diagram. We can plan your control circuit architecture together. Related Devpan solution: electrical panel design services.
