Pick-up voltage is the minimum coil voltage required to initially pull in a relay or contactor armature and close its contacts. If you are searching to confirm whether pick up voltage is the same as the nominal coil rating or the drop-out threshold, the direct answer is no: it is a distinct, lower threshold (typically 70% to 85% of nominal) dictated by the physical air gap of the unenergized magnetic circuit.
The Physics of the Coil: Why Pick-Up, Nominal, and Drop-Out Differ
To understand why these thresholds are different, you have to look at the magnetic circuit. When a relay or contactor is de-energized, there is a physical air gap between the stationary core and the moving armature. Air has high magnetic reluctance (resistance to magnetic flux). To generate enough magnetic force to pull the armature across this gap, the coil requires a high number of ampere-turns, which translates to a higher initial voltage and current surge.
Once the armature slams shut, the air gap drops to nearly zero. The magnetic circuit is now highly efficient, requiring a fraction of the original energy to hold the contacts closed. This creates three distinct voltage specifications on any quality datasheet:
- Nominal Voltage: The design voltage the coil is optimized to run at continuously without overheating (e.g., 24V DC).
- Pick-Up Voltage: The minimum voltage required to overcome the initial air gap and spring tension to close the contacts.
- Drop-Out Voltage: The maximum voltage at which the magnetic field weakens enough for the return spring to force the armature open again.
Typical 24V DC Relay Pick-Up: 16.8V to 19.2V (70-80% of Nominal)
According to standard testing guidelines outlined by All About Circuits, the drop-out voltage is often as low as 10% to 20% of nominal because the closed magnetic gap requires very little flux to maintain the hold.
Worked Numeric Example: Sizing a 24V AC Control Circuit
Where does pick-up voltage actually change your installation decisions? It dictates your wire sizing and control transformer capacity to prevent voltage sag during the inrush current spike.
Let's look at a standard industrial installation using a Schneider Electric TeSys D (LC1D09) 9-amp contactor with a 24V AC, 50/60Hz coil.
| Parameter | Value | Notes |
|---|---|---|
| Nominal Coil Voltage | 24V AC | Standard control circuit voltage |
| Inrush Power | 70 VA | Power required to pull in the armature |
| Inrush Current | 2.91 A | Calculated: 70 VA / 24V |
| Pick-Up Threshold | 20.4V AC | 85% of nominal per IEC 60947-4-1 |
The Scenario: Your PLC is located in a control cabinet, but the contactor is mounted on a motor starter bucket 100 feet away. You plan to run 14 AWG copper control wire.
- Calculate Wire Resistance: 14 AWG copper has a resistance of roughly 2.525 ohms per 1,000 feet. A 100-foot run requires a 200-foot loop (out and back). Total wire resistance = 0.505 ohms.
- Calculate Inrush Voltage Drop: Using Ohm's Law (V = I × R), the voltage drop during the initial pull-in surge is 2.91 A × 0.505 ohms = 1.47V.
- Determine Minimum Source Voltage: To guarantee the contactor picks up, the voltage arriving at the coil must be at least 20.4V. Therefore, your control transformer must supply at least 20.4V + 1.47V = 21.87V at the moment of energization.
If your control transformer is undersized and sags to 21V under load, the contactor will fail to pick up. It will hum loudly, draw locked-rotor inrush current continuously, and burn out the coil in minutes. Understanding the pick-up threshold prevents this exact jobsite failure.
Where You Meet This in Practice
You will run into pick-up voltage constraints in three common scenarios:
- Automotive 12V Systems: When a car engine cranks, the battery voltage can sag to 9V or lower. Automotive relays (like the standard Bosch ISO mini) are specifically designed with a pick-up voltage around 6V to 7.2V so they remain engaged or can successfully engage during the cranking cycle.
- Microcontroller Driver Circuits: If you are driving a 5V relay module from an ESP32 or Arduino via a MOSFET, your 5V rail might sag when the coil energizes. If the rail drops from 5.0V to 3.2V, and your relay's pick-up threshold is 3.5V (70% of 5V), the relay will click weakly but fail to close the high-current contacts.
- Long-Run DC Control Wiring: In solar or off-grid battery systems, running 12V or 24V DC control wires over long distances to heavy-duty contactors requires thick gauge wire to ensure the massive inrush current doesn't drop the voltage below the pick-up threshold at the coil terminals.
What People Commonly Confuse Pick-Up Voltage With
When reading datasheets from manufacturers like Omron, Eaton, or standard electromagnetic guides, beginners frequently mix up these terms:
- Confusion 1: Nominal Voltage. Nominal is the target operating voltage. Pick-up is the absolute minimum floor to initiate movement. Applying exactly the pick-up voltage to a relay will not result in reliable, long-term operation.
- Confusion 2: Drop-Out Voltage. Because the magnetic gap is closed during operation, the drop-out voltage is drastically lower than the pick-up voltage. A 24V relay might pick up at 18V, but won't drop out until the voltage falls below 2.4V.
- Confusion 3: Dielectric Withstand Voltage. This is an insulation rating (often 2,000V to 5,000V) that defines how much voltage the coil can withstand relative to the contacts before arcing internally. It has nothing to do with the coil's magnetic operation.
Frequently Asked Questions
Is the pick up voltage the same as the nominal coil voltage?
No. The nominal coil voltage is the manufacturer's rated design voltage for continuous, thermally stable operation (e.g., 24V DC). The pick-up voltage is the minimum threshold required to initially overcome the mechanical spring tension and magnetic air gap (e.g., 18V DC). Operating a relay continuously at its pick-up voltage rather than its nominal voltage will result in weak contact pressure, excessive contact bounce, and premature arcing failure.
Why is pick up voltage always higher than drop out voltage?
This is due to magnetic reluctance. When the relay is open, there is a physical air gap between the core and the armature. Air is a poor conductor of magnetic flux, requiring a high voltage (and resulting high current) to generate enough magnetic pull to close the gap. Once closed, the air gap is eliminated, the magnetic circuit becomes highly efficient, and it takes significantly less voltage to maintain the hold than it did to initiate the pull-in.
What happens if I operate a contactor exactly at its pick up voltage?
If you supply only the minimum pick-up voltage, the armature will move, but it will lack the kinetic energy to slam the contacts shut firmly. This results in 'contact chatter' or bounce. The contacts will lightly touch, arc heavily due to the high current of the load, and eventually weld themselves together. Furthermore, any minor vibration or temperature-induced increase in coil resistance will cause the contactor to drop out unexpectedly.
Can I use a 24V DC relay on a 12V DC circuit if it switches on at 12V?
While a 24V relay might physically click and pull in when fed 12V (since 12V is roughly 50%, which is near some drop-out/pick-up margins), you should never do this. The magnetic force holding the armature closed will be far below the design specification. Under normal mechanical vibration, the relay will drop out. More importantly, the contact pressure will be too low to handle the rated current, leading to severe internal arcing and fire hazards. Always match the nominal voltage to your supply.
