Mastering Relay Flyback Voltage with the TVS Diode

The Transient Voltage Suppressor (TVS) diode is a two-terminal semiconductor device designed to absorb a surge of electrical energy. They have a wide variety of applications ranging from input circuitry protection for Electrostatic Discharge (ESD) to “catching” the inductive kick (flyback voltage) of a relay. The TVS semiconductor is a unique device that clamps at a predetermined voltage. The example featured in this article clamps at 48 VDC. This clamping action protects sensitive downstream electronics. A bidirectional TVS diode can be visualized as two Zener diodes installed back-to-back. In contrast, a unidirectional TVS diode may be visualized as a Zener diode in series with a conventional diode.

In this engineering brief we will explore the Schneider LCD09BD three-phase contactor as shown in Figure 1 with special emphasis on the integral LAD4TBDL TVS diode module shown in Figure 2. We will examine the turn off transient voltage, and then explain why this type of contactor is challenging to drive directly using a transistor. These concepts will be further explored in an upcoming article where the large Schneider contactor is driven by a 3.3 VDC Arduino microcontroller.

You are encouraged to read this related article, which suggests using an interposing relay to accommodate the higher voltage associated with the TVS diode. Note that the previous article assumes the contactor is used in a sourcing system with one side of its coil referenced to ground. The article you are currently reading assumes a sinking configuration where one side of the coil is referenced to the positive power supply.

This sinking configuration will be readily recognized by members of the Arduino community who have likely used a ground-referenced (common emitter configuration) NPN transistor to activate a relay. For an introduction to the terms sinking and sourcing, please review this article.

Figure 1: Picture of the Schneider Electric LC1D09BD relay. The integral LAD4TBDL TVS diode is visible as the white assembly on the lower right-hand side of the contactor.

Figure 2: Close up image of the Schneider Electric LAD4TBDL “Bidirectional Diode.” The schematic symbol for the TVS diode can be seen in the center of the clip-in module.

Why use a TVS diode instead of a regular diode in a relay flyback circuit?

Relay opening speed is the primary reason for using a TVS diode, especially when compared to a conventional diode. To understand this statement, we look back to our circuit theory class. We remember that the kinetic energy in an inductor is inseparable from the magnetic field. This stored energy is defined by the equation:

Energy_{Inductor} = 0.5 LI^2

We eliminate this energy by dissipating it across the coil resistance plus an insignificant amount across a conventional diode. With a TVS we dissipate the coil energy across the coil resistance plus a significant amount in the TVS diode. This speed is a function of the TVS diode’s clamping voltage; a higher voltage rating burn the coil energy faster. The result is a snappy opening of the relay or contactor.

Tech Tip: You may have noticed that many switches have a snap action. This fast-spring action is essential for arc suppression. Recall that an inductive load is accompanied by a transient flyback voltage. As a rule, a fast-opening switch prevents this transient arc from forming a sustained arc. So too is the situation with relays, contactors, and circuit breakers. A fast-opening mechanism is an essential element of fire and explosion prevention.

Example comparing a conventional and a TVS diode

The advantage of the TVS diode is easily understood by considering Figures 3 and 4. Here we see the contactor turn off time using the integral TVS diode (Figure 3). We also see the turn off time for the same contactor when a conventional diode is used (Figure 4). The results are striking, with the TVS diode taking about 30 ms to dissipate the inductor’s energy while the conventional diode takes over 250 ms. Clearly dissipating energy in the TVS diode is advantageous for responsive relay / contact opening.

We could improve the speed by using a TVS diode with a higher voltage rating. This may be a good idea for some application, but presents a challenge especially when we consider the V_{DS} or V_{CE} ratings of the semiconductor used to turn the relay / contactor on an off.

Tech Tip: To better understand the advantage of the TVS diode, consider the inductive properties of a relay’s coil. Recall that an inductor resists changes in current. It will generate whatever voltage is necessary to keep the current constant across a transient such as the turn off event. Consequently, the coil will clamp at whatever voltage is defined by the flyback diode. Knowing these facts, we can view the TVS diode as a power dissipating element with power defined by the P = IE equation.

Figure 3: The TVS diode clamps at approximately 48 VDC relative to the 24 VDC rail. The total turn off time is approximately 30 ms.

Figure 4: The conventional diode clamps at approximately 0.6 VDC relative to the 24 VDC rail. The total turn off time is approximated as a sluggish 250 ms.

Tech Tip: The waveform in Figures 3 and 4 were constructed using the Digilent Analog Discovery 3 Pro bundle. This includes the Discovery, adapter board and oscilloscope probes. The 10x probes allow the Discovery to measure up to 250 volts.

Next steps for semiconductor control of the contactor

There is a subtle and often overlooked problem when we attempt to design with the TVS diode. We forget that the corresponding MOSFET or Bipolar Junction Transistor (BJT) must have the necessary V_{DS} or V_{CE} to accommodate the higher clamp voltages. For example, if we used an NPN transistor in a common emitter configuration, it must have a V_{CE} rating of at least 100 VDC. This will accommodate the 48 + 24 VDC voltage (72 volt peak) as shown in Figure 3 as a plus provide some safety overhead.

This is no longer a trivial application, especially when we the elevated temperature derating factor. For an NPN transistor application the Venn diagram would likely include the TIP41. We can find similar specifications for MOSFETS such as the IRL510BF.

Stay tuned as we will explore how to control the featured Schneider contactor with a 3.3 VDC Arduino microcontroller in the near future. We will see that it’s not as simple as selecting a TIP41 BJT or a IRF510 MOSFET. Direct control with a 3.3 VDC logic signal is either on the edge or out of bounds for the BJT and MOSFET. We may resort to an interposing relay or a multistage transistor configuration.

Figure 5: Potential components for a future article to control the Schneider contactor with a 3.3 VDC Arduino microcontroller.

Conclusion

The TVS diode provides a simple to use method to handle the inductive kick (flyback voltage) associated with a relay or contactor turn off event. The TVS diode is preferred as it quickly dissipates the energy stored in the inductor. This turn off speed advantage comes with a cost in terms of higher clamping voltage. It requires an interposing relay or a semiconductor driver that can reliably withstand the higher voltages.

Do you think I’m being overly conservative with my semiconductor choice? What part would you choose for the featured Schneider contactor with integral TVS diode module? Please leave any questions, comments, or concerns in the space below. Applications involving the TVS diode are especially welcomed.

Best Wishes,

APDahlen

About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

Highlighted Experience

Leveraging his military engineering experience, Dahlen provides unique insights into rugged and reliable electronics solutions suited for extreme environments. His articles often reflect the practical, hands-on knowledge. For Dahlen’s related content please visit these index pages:

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Questions

  1. Sketch the schematic symbol for a TVS diode.

  2. How does a TVS diode decrease the opening speed of a relay? Will the TVS diode impact the closing speed?

  3. What information is being shown in Figures 3 and 4?

  4. Sketch the voltage / current characteristics of a convention diode, a Zener diode, and a bidirectional TVS diode.

  5. Research and then provide at least three alternative names for a TVS diode.

  6. Sketch a time domain representation of a relay coil’s inductive kick. Assume the relay does NOT have a clamp to “catch” the flyback voltage.

  7. Sketch a relay coil in a sinking system and then another in a sourcing system.

  8. Using the DigiKey parametric search tool, locate a bidirectional TVS diode with a clamp voltage of approximately 50 VDC.

  9. Describe the dynamic current / voltage characteristic of a relay coil. Hint: Focus on the transitions between on and off.

  10. Why does a TVS diode complicate the V_{DS} or V_{CE} requirements for the drive semiconductor?

Critical thinking questions

  1. Pulse width modulation may be used to reduce the energy consumed during a relay’s holding phase. Is the TVS diode suitable for this application? Hint: Where does the energy go in the PWM off phase?

  2. Suppose the TVD diode in the Schneider contactor were replaced with a unidirectional TVS diode. Identify the potential problem(s). Hint: What is the significance of A1 and A2 coil designations on an industrial relay.

  3. The author suggests 100 VDC provides a reasonable safety margin for the drive semiconductor in a system featuring a TVS diode that clamps at 70 VDC with respect to ground. Is this an appropriate assumption?

  4. Support or refute this statement. The relay coil driving transistor is operating as an on off switch while the TVS diode is operating as a linear element.

  5. Support or refute this statement. A moderate value resistor and conventional diode could be used in place of the TVS diode. Hint: Be sure to identify your design constraints.

  6. Describe at least three methods of extinguishing an arc in switches, relays, or circuit breakers. For each method comment on the importance of snappy opening of the contacts. What does this snappy movement have to do with the relay and TVS diode?