Understanding AC vs DC Coils: A Practical Guide for Control Panels

Coil selection is one of those topics that feels straightforward on the surface, yet it is a surprisingly common source of panel issues. A relay that chatters, a contactor that buzzes, or a coil that runs hotter than expected often leads back to one root cause: the wrong type of coil for the application.

AC and DC coils may look identical, but electrically they behave very differently. Understanding these differences helps panel builders, machine builders, and maintenance engineers avoid a long list of avoidable problems.

What A Coil Actually Does

At its core, a coil is a length of copper conductor wound around an iron core, designed to create a magnetic field when energised. When the magnetic field is strong enough, it pulls in an armature, closes contacts, and allows current to flow to a load such as a motor, lamp, or solenoid.

The type of current used – AC or DC – drastically changes:

• Magnetic field behaviour
• Pull in strength
• Heat generation
• Power consumption
• Voltage sensitivity
• Long term reliability

This is why coil selection is more than a tick box on a datasheet. It directly affects how reliable a machine or control panel will be in real world conditions.

DC Coils: Stable, Smooth, And Predictable

DC coils operate using a constant voltage, which produces a steady magnetic field. Once energised, the field does not rise and fall with time in the same way an AC field does. This stability makes DC coils a natural fit for modern control circuits.

Key characteristics of DC coils

• No waveform zero crossings
• No cyclic collapse of the magnetic field
• No inherent vibrational buzz
• Lower inrush current than equivalent AC coils
• Consistent pull in force once energised
• More predictable temperature rise

Because of their smooth magnetic behaviour, DC coils are an excellent choice for:

• PLC output driven circuits
• Low noise environments
• Long duty cycles
• Battery powered or 24 VDC systems
• Applications sensitive to vibration or chatter

Where DC coils can become problematic

The main limitation is voltage drop, especially on long cable runs or heavily loaded DC supplies. If a DC supply falls significantly below its rated value, the magnetic field may not be strong enough to pull in or hold the armature.

Typical DC coil issues include:

• Relays that pull in on the bench but not at the end of a long cable
• Coils that drop out when other DC loads start
• Non repeatable behaviour when supply voltage is marginal

AC Coils: More Powerful, But More Complex

AC coils operate on a sinusoidal voltage. In the UK, the mains supply is 50 Hz, which means the voltage crosses zero 100 times per second. Every zero crossing causes the magnetic field to collapse momentarily.

If the design relied only on the raw AC flux, the armature would relax slightly at every zero crossing. The result would be mechanical buzz, noisy operation, and potentially unreliable contact pressure.

The role of the shading ring

To overcome this, AC coils use a shading ring. This is typically a copper ring embedded in the pole face of the iron core. It delays part of the magnetic flux, maintaining enough pull during the zero crossings to hold the armature firmly.

It works extremely well, but it creates a different set of electrical and thermal characteristics.

Key characteristics of AC coils

• High inrush current at energisation
• Higher steady state heat output than DC coils of similar size
• Slight mechanical vibration, even when healthy
• Strong initial pull in force at waveform peaks
• Sensitivity to voltage dips and brownouts
• Often larger physical size in contactors and larger relays

Where AC coils make sense

AC coils are often the right choice in:

• Mains voltage control circuits (115 VAC or 230 VAC)
• Systems where a DC power supply is not available or not desirable
• Heavy duty applications needing strong pull in force
• Panels where slight noise and vibration are acceptable
• Simpler schemes where adding an extra DC supply would add cost and complexity

Common Real World Failure Modes

Across industry, the same coil related problems appear again and again. Most of them are avoidable with correct selection and an understanding of how coils behave in the field.

1. Chattering on AC coils

Chattering is a classic symptom of marginal conditions on an AC coil. Common causes include:

• Undersized control transformer
• Voltage drop under load
• Long cable runs with significant impedance
• A weak or damaged shading ring
• Incorrect coil fitted during maintenance

Chatter is not just a noise issue. It increases mechanical wear, heats the armature, accelerates contact erosion, and can lead to intermittent operation that is hard to trace.

2. Overheating on DC coils

DC coils can overheat when:

• The supply voltage is higher than the coil rating
• A PSU is running at the top end of its tolerance band
• The panel ambient temperature is higher than expected
• The coil is energised continuously when only intermittent duty was assumed

The result is accelerated insulation ageing and, eventually, coil failure.

3. Early dropout during voltage dips

AC coils are particularly vulnerable to voltage dips and brownouts. A short reduction in supply voltage can cause:

• Contactors to drop out while a motor is running
• Control relays to release mid cycle
• Unexpected machine shutdowns

These events can be intermittent and difficult to reproduce, which is why understanding dropout behaviour is important in critical applications.

4. Coil burnout from incompatible outputs

Another common problem is driving coils from outputs that are not designed for them. Examples include:

• An AC coil driven from a small relay contact that cannot handle the inrush
• A DC coil driven from a PWM or highly noisy electronic signal
• Coils driven in a way that creates repetitive arcing on the control side

In these situations, either the coil or the driving device will eventually fail.

Why Coil Selection Matters For Shenler Relays

Shenler is the relay manufacturer that Charter Controls has chosen to partner with. One of the reasons is their consistent coil design and clear coding system, which makes it easier for engineers to specify the correct coil for the job.

Shenler coil voltage coding

For DC coils:

• 012 = 12 VDC
• 024 = 24 VDC
• 048 = 48 VDC
• 110 = 110 VDC
• 220 = 220 VDC

For AC coils, Shenler adds 500 to the DC code:

• 512 = 12 VAC
• 524 = 24 VAC
• 615 = 115 VAC
• 730 = 230 VAC

This logical system helps reduce errors and makes it very clear whether a relay has an AC or DC coil, provided the engineer understands the difference in behaviour between the two.

Practical Coil Selection Guide

Below is a practical way to approach coil selection when designing or modifying a panel.

Choose a DC coil when:

• The control signal comes from a PLC or other DC logic
• Power efficiency and lower heat are desirable
• Noise and vibration need to be minimised
• You are using transistor or solid state outputs
• You have a stable DC control supply already available

Choose an AC coil when:

• The control system already uses 115 VAC or 230 VAC
• You need strong pull in force for larger contactors
• Slight noise or vibration is acceptable
• You prefer a simpler power design with fewer supplies
• The coil is only energised intermittently

A simple rule of thumb is:

If your logic is DC, your coils will usually be DC. If your control voltage is AC, your coils will usually be AC. Then check the details, such as current, inrush, ambient temperature, duty cycle, and cable length.

Additional Engineering Considerations Often Overlooked

Cable length

On DC systems, voltage drop along the cable reduces the effective voltage seen by the coil. On AC systems, long runs can introduce both resistive and reactive effects that reduce the voltage at the load end.

Ambient temperature

Coils run hotter in warm panels. A coil that is perfectly happy at 20 degrees C may run very close to its limits at 50 to 55 degrees C. Always consider the real operating temperature, not just the room temperature when testing.

Duty cycle

Some coils are designed for continuous duty. Others are not. If a coil is energised continuously when only intermittent operation was assumed, it can run significantly hotter than expected.

Compatibility with switching devices

Driving an AC coil from a small relay contact can lead to welded contacts due to high inrush. Driving coils from semiconductor outputs requires careful checking of inrush, leakage current, and flyback protection.

Panel heat management

Coils are often the first components to show signs of poor ventilation. If you are seeing multiple coils running hotter than you expect, it may be a sign that the panel layout or cooling strategy needs attention.

Final Thoughts

The difference between AC and DC coils may sound like a basic distinction, but it is one of the most influential factors in relay and contactor behaviour. A well selected coil improves uptime, reduces nuisance faults, extends contact life, and makes a panel more predictable during voltage dips and transients.

This is why understanding coil behaviour is essential, and why Charter Controls works with Shenler. Consistent coil design, predictable performance, and reliable manufacturing at scale give engineers one less thing to worry about.

If you would like help reviewing coil selection in an existing design, or you are standardising on a relay platform, you can get in touch with the team at Charter Controls. We are always happy to talk through real world applications.

Frequently Asked Questions

What is the main difference between AC and DC coils?

DC coils operate with a constant magnetic field and typically have lower inrush and less noise. AC coils deal with a sinusoidal voltage, use shading rings to avoid chatter, and have higher inrush and heat. The choice affects reliability, noise, and how sensitive the device is to voltage dips.

Why do AC coils draw more inrush current?

When an AC coil is first energised, the magnetic circuit and current have not settled. Depending on the phase angle at switch on, the instantaneous current can be many times higher than the steady state current, leading to a high inrush that small contacts or undersized transformers may struggle with.

Are DC coils always better for PLC outputs?

In most cases yes. DC coils are usually better suited to PLC outputs because they have lower inrush and smoother magnetic behaviour. However, you still need to check the coil current against the PLC output rating and ensure suitable suppression is used where required.

How does Shenler identify AC and DC coil versions?

Shenler uses a clear coding system. DC coils use codes like 012, 024, 048, 110, and 220. AC coils add 500 to the DC code, so 512, 524, 615, and 730 indicate 12 VAC, 24 VAC, 115 VAC, and 230 VAC respectively. This makes it easy to see whether a relay has an AC or DC coil.

What should I check before swapping a relay or contactor coil?

Always confirm the coil voltage and type (AC or DC), the inrush and holding current, the duty cycle, the ambient temperature, and how the coil is driven. Do not rely solely on physical fit or appearance when selecting a replacement.