MOSFET to uC Guided Learning Q5: – What causes the dip in the relay’s current curve upon activation?

Category Archives Microcontroller
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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 5

What causes the dip in the relay’s current curve upon activation?

This post also answers these closely related questions:

  • How can we visualize changes in magnetic reluctance?
  • How do magnetic flux lines behave in a relay?

Clarification

Figure 1 show the waveforms for an activating relay. The 24 VDC input voltage (orange) is shown along with the corresponding coil current (blue). The waveform distortion is caused by a change in the magnetic reluctance as the armature moves into position.

Figure 1: Turn on command (orange) and the resulting relay current waveform (blue). The dip is caused by the changing magnetic properties and the armature closes thereby eliminating the air gap.
Figure 1: Turn on command (orange) and the resulting relay current waveform (blue). The dip is caused by the changing magnetic properties and the armature closes thereby eliminating the air gap.975×474 78 KB

Figure 1: Turn on command (orange) and the resulting relay current waveform (blue).

Answer

We expect a smooth 1st order response in current as the relay is activated. Instead, we see a dip in the current waveform near the 6 ms mark. This distortion is caused by the system’s changing magnetic properties.

This change in magnetic reluctance is best viewed as a series of steps with supporting illustration as Figure 2.

  • The coil is initially relaxed (no magnetic field) and the armature is relaxed in the normally open position.

  • A current is applied to the coil.

  • The coil’s magnetic field increases like a first-order system, based in the RL properties of the coil, yoke, and surrounding electrical circuit.

  • When the magnetic field increases, the armature is pulled to the coil thus changing the magnetic properties (inductance) of the coil.

  • The current is disturbed by the changing RL properties.

Technically, we say that the reluctance of the coil has changed. Originally, the relay’s coil had a large air gap. With the armature pulled in, the air gap (magnetic reluctance) is eliminated.

Figure 2: Picture showing the magnetic path for an initially energized but still open relay alongside an energized and closed relay.
Figure 2: Picture showing the magnetic path for an initially energized but still open relay alongside an energized and closed relay.1055×400 65.9 KB

Figure 2: Picture showing the magnetic path for an initially energized but still open relay alongside an energized and closed relay.

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