MOSFET to uC Guided Learning Q6: – What factors determine the turn on and off times for a relay?

Semiconductor Integrated Circuits (ICs)
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This article is part of a guided learning series to explore real-world applications of MOSFETS and microcontrollers.

:pushpin: Canonical Article: How to Interface a Microcontroller with a Relay Using a MOSFET
:blue_book: Learning Companion (Q&A): Explore All Questions

You are reading: Question 6

What factors determine the turn on and off times for a relay?

Answer

The speed at which a relay opens and closes is governed by several factors including

  • Electrical time constant: The electrical time constant has a profound impact on the speed at which a relay opens and closes. The system is governed by the elementary tau = L/R equation, where inductance if the coils inductance and R is the coils intrinsic resistance plus the resistance of external circuitry. The circuit designer has some (limited) control over the electrical time constant.

  • Mechanical time constant: The mechanical time constant is a fixed property of the relay. It reflects the strength of the magnetic field, the distance the contacts must travel, and the general construction of the relay and associated magnetic fields.

  • Driver time constant: For the MOSFET, this time constant is a dominated by the RC time constant of the series current limiting resistor and the MOSFET’s gate.

:writing_hand: Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey