# How long does it take to close a relay?

## How fast can we close a relay?

There are so many factors that enter this question that no definitive answer can be given. However, there are some interesting learning opportunities regarding relay behavior and physics. This includes considerations of magnetic properties (inductance) as the armature makes physically contacts the solenoid core, time of flight, contact bounce, and the impact of supply voltage. Armed with this knowledge of relay dynamics, you can better interpret the relay’s datasheet and improve your next design.

## Test circuit to determine relay close time

To answer the question, we can perform an experiment using the setup shown in Figure 1 with accompanying schematic as Figure 2. The setup includes a representative industrial relay plus socket, the relay driver as shown in the schematic, and an Arduino Nano Every to toggle the driver on and off. A Digilent Analog Discovery 3, probe adapter, and 10 X probes are uses to record the signals.

Figure 1: Test setup to measure relay activation time.

The relay driver may appear to be overdesigned. However, the Q2 high-side driver (sourcing configuration) is necessary to reference the relay to ground. This allows a small value shunt resistor (R5) to be installed. With this resistor in the ground position, it is easy to measure the relay’s current as a small voltage drop across a known resistor.

The remainder of the circuit consists of level shifting transistor Q1 and a means of detecting the status of the relay via the normally closed (N.C) and normally open (N.O.) indicator LEDs.

Let’s not forget the D1 flyback diode placed across the relay coil. The diode is necessary to protect transistor Q2 when the relay is deactivated. Understand that this diode has no effect on the relay’s activation, yet it has a profound impact on relay closure. Perhaps in the future we can answer a related question about the time it takes to open a relay.

Figure 2: Schematic featuring a high-side PNP relay driver (Q2) and a current shunt (R5).

## Experiment results

The results are shown in Figure 3. There are three panels:

• upper: The orange trace (CH 1) is the relay activation voltage as measured at the collector of Q2. The blue trace (CH 2) is the relay current as measured across the R5 shunt resistor.

• middle: The blue trace (CH 2) is the voltage as measured on the relays normally closed contact.

• lower: The blue trace (CH 2) is the voltage as measured on the relay’s normally open contact.

Tech Tip: The Digilent Analog Discovery 3 operates as a dual channel oscilloscope. When equipped with 10 X probes it is capable of measuring signal up to +/- 250 VDC. The Figure 3 composite could have been constructed as a single screen capture if a 4-challel oscilloscope were used.

Figure 3: Activate waveforms for the relay including the coil’s current, normally closed, and normally open contacts.

Based on the Figure 3 data we observe:

• the armature movement is first observed at 4.7 ms when the N.C. contact toggles.

• there is a 2.9 ms time of flight from 4.7 to 7.6 ms. In this “flight time” neither the N.C. nor the N.O. contacts are connected to the circuit.

• first contact with the N.O. contact occur at 7.6 ms.

• the contacts bounce for 1.2 ms starting at 7.6 ms extending to 8.8 ms.

In addition to the to these contact changes, there is a subtle dip in the relay current. This occurs while the armature is in motion. Presumably, the relay inductance changes as the armature’s iron plate makes physical contact with the coil’s metal core. The sudden change in coil inductance disturbed the gently slope in relay current. Note that this disturbance does not occur if the armature is held in position against the coil.

## How can a relay’s closure time be reduced?

To reduce a relays closure time we can apply a few tricks typically associated with stepper motors. Recall that this is the same problem. Specifically, how do we force current into an inductor. Recall that the time constant (\tau) of an inductor defined as

\tau = \dfrac{L}{R}

### Reduce relay closure time using an external resistor to reduce the RL time constant

Our chosen inductor, be it a relay or stepper motor coil has a fixed inductance. It also has a fixed resistance. Yet, nothing prevents us from adding additional external resistance to lower tau by implementing a L/nR system where n is the multiplier of coil resistance. For example, we could double the series resistance. This L/2R system has cut \tau by a factor of 2. Likewise, L/4R system would reduce \tau by a factor of 4.

The penalty for this external resistance is additional voltage and wasted power. Our 24 VDC relay would require 48 VDC in a L/2R and 96 VDC in a L/4R. With an increase of 2 and 4x the relay power.

### Reduce relay closure time using high voltage Pulse Width Modulator (PWM) driver

Let’s pull back and recognize that a L/4R system for a relay is stepping into crazy territory. On the other hand, a PWM driver with 96 VDC is not out of the question. This would allow responsive magnetic field buildup with the ability to throttle the PWM to a low “holding” level after the initial burst.

## Experiment results obtained by reducing the time constant

With that said, let’s see how the situation improves with a L/2R system. For this experiment we will add R4 as shown in the Figure 2 schematic. We will also increase the supply voltage to 48 VDC. The results are shown in Figure 4.

Figure 4: Activate waveforms for the relay when operated in a L/2R environment.

Note that the steady state current for Figure 3 and figure 4 is the same. While we did double the voltage, the R4 series resistor is approximately the same as the coil resistance. Consequently, they form a balanced voltage divider.

The results presented in Figure 4 are a noticeable improvement from Figure 3. The most significant change is the time to change the N.C. contact. It has changed from 4.7 to 2.4 ms. This follows the 2X speedup suggested by the L/2R calculation. Observe that the flight time is reduced slightly and there is a slight but not consequential improvement in bounce time. This suggests that inductance (buildup of the magnetic field) is the limiting factor for relay activation. The physics of spring tension and metal-on-metal bounce are relatively consistent.

Once again, notice the dip in the relay’s current. This is attributed to an inductance change due to the proximity of the metal armature “attraction” plate to the relay’s coil.

## Conclusion

This was fun little experiment!

Did you learn something about inductance and relay operation?

More importantly, I trust you will be better able to read and a relay’s datasheet.

Best Wishes,

APDahlen

P.S. The “how long to open” continuation to this article has been posted.