Advanced relay drivers with programmable output-driver configurations may allow configuration of the flyback energy clamps used to dissipate a coil’s magnetic energy. This engineering brief demonstrates how a simple register setting can profoundly impact a contactor’s turn-off speed. When driving a +24 VDC motor starter’s coil, we see that a single bit can change the Analog Devices MAX14906’s turn off-time from 156 ms to 32 ms.
The theory of operation for the active clamp mentioned in this article is described in one of my previous articles. This prerequisite article describes the limitations of the conventional flyback diode. It also presents a schematic and working breadboard example of the active clamp.
Key Takeaways
- This discussion is best viewed through the lens of extinguishing the electrical arc in a motor starter operating under load. Fast contact release is a design decision to reduce arc duration and extend system life.
- A single bit change results in a 79% reduction in contactor release time for the hardware shown in Figure 1. It’s the difference between a snap and a sluggish release.
- Faster turn-off times are associated with a higher coil flyback voltage. This dissipates the coil’s stored energy in both the semiconductor clamp and the intrinsic resistance of the coil.
- Performance is measured from the falling edge of coil drive to the initial bounce of a normally closed auxiliary contact.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Applied Theory
Difficulty: 
Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey, Author bio.
Last update: 10 Sep 2026
Figure 1: Image of the MAX14906 on a MIKROE CLICK board with the Schneider contactor in the background.
Datasheet Description of the Programmable Digital Output Modes
The MAX14906 may be programmed as a high-side switch or a push-pull driver. This push-pull driver has two operating modes which are the focus of this article. According to the datasheet:
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Simple: “Simple push-pull mode uses a push-pull pair in which the DOI voltage switches rail-to-rail inside the VDD_-to-GND range even when an inductive load is present.”
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Active Clamp: “When an inductive load is turned off by opening the high-side switch, the DOI_ pins go to a negative voltage in both high side and active-clamp push-pull modes. The MAX14906 features an internal active clamp that clamps inductive energy at -55V (typ) relative to VDD_, which allows inductive load currents to decay quickly.”
Programmable Output Modes
The MAX14906 digital output modes are selected by changing the ConfigDO register. Each channel is controlled using the two DOModeX bits. According to the datasheet:
- 00: High-side
- 01: High-side with 2x inrush current for
tINRUSHtime - 10: Active-clamp push-pull
- 11: Simple push-pull
Measured Results for Simple Mode
In simple mode the output is bounded between the rails as shown in Figure 2. Here, the MAX14906 is driving a Schneider +24 VDC motor starter. The orange trace represents the falling edge of the coil drive. The blue trace shows the closure of a normally closed auxiliary contact. The motor starter’s release time is approximately 156 ms.
Note that the Schneider Electric DPE09BL coil is rated for 2.4 W (100 mA at 24 VDC). This is well within the 1.2 A capability of the MAX14906 and the 600 mA (unlimited) safe demagnetization limitation (see theory below).
Figure 2: Waveform for the MAX14906 in simple push-pull mode driving an inductive load.
Measured Results for Active Clamp Mode
The results for the active clamp mode are shown in Figure 3. The flyback voltage is clamped at approximately -55 VDC relative to the +24 VDC rail. The observed release time has been reduced to approximately 32 ms. This is a 79% decrease in release time (a 4.9 factor of reduction).
The observed difference is striking. The motor starter now releases with a snap, whereas before the prolonged release sounded like apologetically opening a door.
Figure 3: Waveform for the MAX14906 in active-clamp mode driving an inductive load.
Note that the contactor’s TVS diode as shown in Figure 1 has a higher activation voltage than the MAX14906. Presence or absence of the Schneider LAD4TBDL has no impact on the Figure 3 waveforms.
Why Turn-Off Time Matters
The MAX14906 is designed to drive the coil of midsized relays and contactors in a 24 VDC industrial automation ecosystem. In this application we can expect it to control three-phase motors using devices like the featured Schneider motor starter. This entire discussion only makes sense when we consider the electrical arc that occurs when the contacts open under load. In this case, faster contact separation reduces the arc duration. Simply put, slow contact opening will eat the contacts.
Theory
The theory of operation for the active clamp is contained in this article. It describes the design tradeoffs associated with flyback suppression techniques and the dissipation of the coil’s stored energy.
A conventional diode such as the 1N4002 is a poor choice when we consider the release speed and the resulting contact arc. With the diode, the coil’s energy is dissipated in the coil’s intrinsic resistance plus a tiny amount of energy due to the forward voltage drop in the diode. By contrast, the active clamp dissipates the energy in the MOSFET’s 55 VDC drop as well as the coil’s intrinsic resistance.
The challenge is that all associated components must withstand the higher voltage. Also, for the active clamp, the MOSFET is operating in a linear region. It must have a low thermal resistance to dissipate the energy and prevent overheating. It must also stay within its SOA as it is simultaneously handling a relatively high voltage at a moderately high current. It’s relatively easy for a MOSFET to handle 100 mA when fully turned on. It’s an altogether different story to handle 100 mA with a 55 VDC drop. That’s a 5.5 W decaying transient in a 7 mm by 7 mm QFN package. The event may be brief, but the MOSFET is still being hit with a hammer.
Tech Tip: The Analog Devices literature describes safe demagnetization circuitry specifically designed to protect the internal MOSFETs against thermal stress. This is necessary as simultaneous deactivation of multiple inductive loads can stress the driver MOSFETs. Referring back to Figure 2, we see distortion in the coil’s voltage waveform. This is NOT safe demagnetization in action. Instead, we are observing the changing reluctance of the magnetic circuit which is essentially the same as saying the inductance of the coil changes. Here, we have stored energy in the magnetic field that interacts with the changing inductance (increasing air gap size) as the iron armature moves to its open position.
Note:
- The Figure 2 waveform becomes a smooth discharge curve when the armature is physically held in position.
- I attempted to activate the SafeDemag circuitry. I programmed the microcontroller to toggle the contactor every 50 ms and I even used a hairdryer to add additional heat. While I couldn’t touch the chip, I was unable to activate the safeDemag circuitry. Perhaps I will try again using larger inductive loads or a heat gun. Then again, I don’t want to desolder the chip.
About this Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.