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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: DC relay latching voltage
Date: Sun, 25 Aug 2024 02:03:55 -0700
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On 8/25/2024 12:40 AM, piglet wrote:
> An even simpler coil catcher is a resistor. Many times the extra power
> consumption is not an issue and resistors are even cheaper and more
> reliable than zener + diode.

A lot depends on the relay's (coil) duty cycle.

To limit the peak transient (to approximately the supply voltage) when
the coil opens, R needs to be on the order of the coil resistance.
If the this halves the load resistance that must be driven when the
coil is active.

Of course, if the duty cycle is low and 2X the driving current is
within the capabilities of the device you had already planned on
using for the switch, then this has minimal impact.

OTOH, if the coil current is higher, already taxing the capabilities
of the switch or the duty cycle has the coil energized more often
than not, then this can be a significant factor.

And, as R approaches the coil's resistance, the opening time/delay of
the relay *increases*.  I.e., you want R to approach infinity to get the
best opening transition but this also gives the highest switching
transient voltage -- often many times the supply voltage!