What do the pull in voltage and release voltage of a relay mean?

Welcome to our complete guide on key relay electrical parameters. We'll cover the core concepts of relay operation that matter for reliable system design.

Pull-in voltage is also called 'must operate' voltage. It's the minimum coil voltage needed to energize the relay. This moves its contacts from their normal position to the actuated position.

Release voltage is the 'must release' voltage. It's the maximum voltage where an energized relay will definitely de-energize. This lets its contacts return to their normal, resting state.

You must understand these two parameters to design predictable, robust, and failure-resistant electronic circuits. This guide goes beyond simple definitions. We'll explore practical implications, environmental factors that influence these values, and how to read them correctly from datasheets.

In this guide, we will cover:

The fundamental definitions and physics behind pull-in and release voltage.

The critical concept of hysteresis and why it ensures stable switching.

Real-world factors like temperature and power supply variations that alter relay performance.

How to read and interpret datasheet specifications for reliable circuit design.

Actionable design rules and a practical case study.

The Fundamentals

What is Pull-In Voltage?

Pull-in voltage triggers the mechanical action of the relay. When voltage is applied across the relay's coil, current flows and creates an electromagnetic field.

The pull-in voltage is the specific point where magnetic force becomes strong enough. It must overcome the combined opposing forces. These include the tension of the armature's return spring and the static pressure of the normally closed contacts.

At this voltage, the armature is pulled toward the coil's core. This causes the movable contacts to switch their state. Normally open contacts will close, and normally closed contacts will open.

The value on a datasheet is a guaranteed maximum. The actual pull-in voltage of any individual relay unit may be lower. The datasheet guarantees that the relay will operate at or below this specified voltage. For example, a "must operate voltage of ≤ 9.6V".

For most standard DC relays, pull-in voltage is specified as a percentage of the nominal coil voltage. This is typically 70% or 80%. For a common 12VDC relay, the guaranteed pull-in voltage would be specified as less than or equal to 9.6VDC (80% of 12V).

What is Release Voltage?

Release voltage represents the threshold for de-actuation. As voltage across an energized coil is reduced, current flow decreases. The magnetic field weakens.

The release voltage is the point where magnetic force becomes too weak. It can no longer hold the armature against the mechanical force of the return spring.

At this voltage, the spring's restoring force overcomes the residual magnetic attraction. This pulls the armature away from the core. This action returns the contacts to their normal, de-energized state.

Like pull-in voltage, the release voltage on a datasheet is a guaranteed minimum. The relay is guaranteed to have released once the coil voltage drops to this level or below. A typical specification might read "must release voltage of ≥ 1.2V".

This value is also often expressed as a percentage of the nominal voltage. For DC relays, release voltage is commonly specified as 10% or more of the nominal rating. For a 12VDC relay, the guaranteed release voltage would be greater than or equal to 1.2VDC.

The Concept of Hysteresis

What do the pull in voltage and release voltage of a relay mean for stable operation? The pull-in voltage of a relay is always significantly higher than its release voltage. This difference between the two thresholds is known as hysteresis.

This characteristic is not a flaw. It is a fundamental and necessary feature for stable relay operation. Hysteresis prevents the relay from "chattering"-rapidly oscillating on and off-when the control voltage hovers near a single switching point.

Imagine if the pull-in and release voltages were identical. Any small noise or fluctuation in the control signal around that single threshold would cause the relay to rapidly turn on and off. This leads to contact arcing, premature wear, and unpredictable system behavior.

The hysteresis loop provides a deadband. Once the relay has pulled in, the voltage must drop significantly before it will release. This ensures a clean and decisive switching action.

We can visualize this with a simple graph plotting the relay's state against the coil voltage.

This graph clearly shows two separate paths for turning on and turning off. They form a loop that represents the relay's hysteresis.

Real-World Influencing Factors

Coil Temperature's Role

The single most influential factor affecting a relay's pull-in and release voltages in a real-world circuit is the temperature of its coil.

The physics are straightforward. A relay coil is wound with copper wire, which has a positive temperature coefficient of resistance. For copper, this value is approximately +0.4% per degree Celsius.

As the coil's temperature rises, its DC resistance increases. This happens either from a high ambient environment or from self-heating due to continuous operation. According to Ohm's Law (V=IR), if resistance (R) increases, a higher voltage (V) is required to produce the same level of current (I) needed to generate the actuating magnetic field.

As the coil gets hotter, its effective pull-in voltage increases. A relay that reliably pulls in at 9V in a 25°C lab might require 11V or more to pull in when operating inside a hot enclosure at 80°C.

The effect on release voltage is similar. A hotter coil with higher resistance means the voltage must drop to a lower level to reduce the magnetic field sufficiently for the armature to release. Therefore, the effective release voltage also increases with temperature.

We once investigated a field failure where a control unit worked perfectly in the lab (25°C) but failed to actuate a critical safety relay in an outdoor enclosure under the summer sun (70°C). The design supplied a voltage just above the cold pull-in specification. It failed to account for the significant increase in pull-in voltage at high temperatures. The solution required redesigning the driver circuit to provide a higher guaranteed output voltage under all thermal conditions.

Power Supply Variations

A circuit's power supply is never perfect. Its variations directly impact relay operation. Designers must account for these realities.

First is the supply tolerance itself. A power supply rated for "12V" is often 12V ±5% or even ±10%. A designer must work with the worst-case scenario. If the supply can droop to 11.4V (-5%), this minimum voltage must still be comfortably above the relay's maximum possible pull-in voltage, considering full temperature effects.

Second is voltage drop, or I*R drop. The wires and PCB traces connecting the power source to the relay coil have resistance. Even a small resistance over a long wire run can cause a significant voltage drop when the coil draws current. The voltage at the relay coil will be lower than the voltage at the power supply terminals. This factor must be calculated and compensated for in the design.

Finally, voltage ripple is a concern, especially in simpler AC-DC power supplies. If the ripple on the DC voltage is large enough, the voltage trough could dip below the relay's hold or release voltage. This causes the relay to chatter or release unexpectedly. This is particularly problematic for circuits operating close to the release voltage threshold.

Coil Suppression Impact

When a transistor switches off the current to an inductive load like a relay coil, the collapsing magnetic field induces a large voltage spike (V = -L * di/dt). This spike can easily destroy the driving transistor if not suppressed.

The most common suppression method is a simple flyback diode placed in parallel with the coil. When the transistor turns off, the diode provides a safe path for the induced current to circulate and dissipate. This protects the driver.

However, this method has a significant trade-off. The circulating current prolongs the existence of the magnetic field. This holds the relay's armature in the energized state for longer. This dramatically increases the relay's release time.

This delay can be unacceptable in applications requiring fast switching. Furthermore, the slow decay of the magnetic field can also effectively raise the voltage at which the relay finally releases.

For applications where release time is critical, better suppression methods are required. A Zener diode in series with the standard diode, or a resistor-capacitor (RC) snubber network, can provide a faster decay path for the coil current.

Here is a comparison of common suppression techniques:

Choosing the right suppression method is a balance between protecting the driver component and achieving the required relay release performance.

Practical Application and Design

Reading a Datasheet

Translating theory into practice begins with correctly interpreting a relay's datasheet. The coil data section contains the critical voltage specifications that govern your driver circuit design.

Let's break down the key parameters you will find.

Nominal Coil Voltage: This is the standard, intended operating voltage for continuous use. The relay is designed for optimal performance and lifespan at this voltage.

Must Operate (Pull-in) Voltage: This is specified as a maximum value (e.g., ≤ 9.6V). Your circuit must supply at least this voltage under all conditions (worst-case low supply, max temperature) to guarantee actuation.

Must Release Voltage: This is specified as a minimum value (e.g., ≥ 1.2V). To guarantee the relay de-energizes, your circuit's "off" state voltage must be below this level.

Maximum Continuous Voltage: This is the highest voltage the coil can withstand indefinitely without overheating or suffering damage. Exceeding this can drastically shorten the relay's life.

Below is an example of a typical coil data table for a 12VDC "sugar cube" relay.

Careful reading is essential. Note that these core specifications are often given at a reference temperature, usually 25°C. As we've discussed, these values will change at different operating temperatures.

Design Case Study: UVLO

Let's apply these concepts to a practical design problem: creating a battery undervoltage lockout (UVLO) circuit.

The goal is to protect a 12V lead-acid battery from deep discharge. The circuit must automatically disconnect a load when the battery voltage falls to a critical level, for instance, 11.5V. It should only reconnect the load after the battery has been recharged to a healthier voltage, like 12.5V.

The challenge is to select an appropriate relay and design the control logic. The natural hysteresis of a relay seems perfect for this task.

Our analysis requires the relay to perform two actions based on voltage thresholds:

The relay must release (disconnect the load) when the system voltage drops to 11.5V.

The relay must pull-in (reconnect the load) when the system voltage rises to 12.5V.

In the selection process, we would start by looking for a standard 12VDC nominal relay. The datasheet specifications are our primary guide. The must release voltage must be well below our 11.5V disconnect threshold. The must operate voltage must be below our 12.5V reconnect threshold.

However, a professional engineer knows not to design directly to these wide-tolerance specifications. In practice, we would never rely on the relay's own imprecise pull-in and release voltages to set our critical UVLO thresholds. The pull in voltage and release voltage of the relay have inherent hysteresis that is too variable and heavily dependent on temperature.

Instead, we use the relay's specifications to create a safe operating window for a much more precise external trigger circuit. We would design a comparator circuit (using an op-amp or a dedicated supervisor IC) with a precise resistive divider to set the 11.5V and 12.5V thresholds. This comparator then drives a transistor, which in turn drives the relay coil.

For this design, our relay choice is guided by ensuring it does not interfere with our precision circuit. We would choose a relay with a Must Release Voltage of, for example, ≥ 1.2V and a Must Operate Voltage of ≤ 9.6V. This wide, guaranteed window (from 1.2V to 9.6V) gives us ample room. Our precision circuit can then operate reliably at 11.5V and 12.5V, completely immune to the relay's own wide tolerances and thermal drift.

This case study shows how the relay's properties are a crucial part of the system design. But they are managed and controlled by external intelligence rather than being relied upon for precision themselves.

Design Rules of Thumb

For robust relay driving, we follow a set of core principles summarized in this checklist.

Rule 1: Mind the Gaps. Always ensure your driver circuit's minimum guaranteed output voltage is greater than the relay's maximum specified pull-in voltage. Account for the highest possible operating temperature.

Rule 2: The Low End Matters. Ensure your driver's "off-state" leakage voltage, combined with any system noise, is always less than the relay's minimum specified release voltage. This prevents the relay from failing to de-energize.

Rule 3: Temperature is Not Your Friend. Always derate your voltage calculations for the worst-case thermal environment. A good practice is to budget for at least a 20-25% increase in pull-in voltage when moving from a 25°C lab environment to a hot 85°C industrial application.

Rule 4: Drive with Authority. Never drive a relay coil directly from a standard microcontroller I/O pin. These pins have limited current sourcing capability and high output impedance. Use a dedicated driver, like a BJT or MOSFET, that can provide clean, low-impedance power to the coil.

Rule 5: Suppress Wisely. A flyback diode across the coil is the minimum requirement for protecting the driver. If your application is sensitive to release time, invest the extra components for a Zener-diode or RC snubber solution.

Advanced Considerations

Latching Relays

It's important to differentiate standard non-latching relays from latching relays, which operate on a different paradigm.

Unlike a non-latching relay that requires continuous coil power to hold its state, a latching relay is bistable. It switches state with a brief voltage pulse and then remains in that new state with zero power consumption.

These relays don't have a traditional pull-in or release voltage. Instead, their datasheets specify a Set Voltage pulse to move the contacts to the energized position and a Reset Voltage pulse (often on a separate coil) to return them to the normal position.

The primary benefit is immense power savings. This makes latching relays ideal for battery-powered or energy-sensitive applications where a state must be maintained for long periods.

AC vs. DC Coils

The principles discussed throughout this guide apply primarily to DC-coil relays. AC-coil relays are designed differently.

The operation of a DC coil is governed by its DC resistance and the resulting current. The operation of an AC coil, however, is based on its impedance at the specified line frequency (e.g., 50Hz or 60Hz).

AC coils often incorporate a shading ring or shading coil. This is a single, shorted copper turn embedded in the face of the core. It creates a delayed magnetic flux that keeps the armature seated during the zero-crossings of the AC sine wave. This prevents audible buzz and contact chatter.

Their pull-in and release voltage specifications are consequently given in Volts AC (VAC) and must be evaluated within the context of the AC power source's stability.

Conclusion: The Foundation of Reliable Control

Pull-in and release voltage are more than just numbers on a datasheet. They define the fundamental operating window of an electromechanical relay. The hysteresis created by the gap between these two thresholds is the key to ensuring clean, stable, and chatter-free switching.

A successful design, however, must look beyond these static, room-temperature values. The central message of this guide is that a robust circuit must account for the dynamic, real-world factors that directly impact relay performance. This especially includes the significant effects of coil temperature and power supply variations.

By moving beyond the component datasheet and considering the entire system-from the power source and driver circuit to the ambient thermal environment-engineers can properly manage these critical parameters. This approach allows us to leverage the fundamental characteristics of relays to build control circuits that are not just functional on a lab bench, but are truly reliable in the field.

See also

Relay production process and testing flow

Application of Relays in Solar Power Generation Systems

How to distinguish between normally open and normally closed contacts of a relay

How to Choose the Right Automotive Relays and Fuse Boxes