Re: DC relay latching voltage

On Sat, 24 Aug 2024 23:22:01 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

On 8/24/2024 11:21 PM, Don Y wrote:

On 8/24/2024 10:03 PM, John Robertson wrote:

Good reading! It appears counterintuitive, but if I warp (!) my head around
it I should be able to sort it out.

 
Think about the extremes:
 
- with NO diode  or snuber, there is nothing to keep the magnetic
   field intact when the switch opens.  So, the spring force is the
   sole actor in play, pulling the contacts open
- with a diode, some current continues to "recirculate" in the coil
   thus keeping the magnetic field active -- so it is trying to hold
   the relay closed while the spring is trying to open it.  *When*
   the recirculating current falls, the field collapses and the relay
   can open.
 
The zener lets the circuit look more like the "no diode" case.

>
i.e., imagine that case as having an infinite voltage zener (open
circuit) in series with the diode -- NO diode!
>

The losses in a real relay will limit the peak voltage, which will
actually be a damped ringing thing.

L changes substantially as the parts move around, decreasing as the
magnetic loop goes from closed/seated to open/air gap. Envision some
weird waveforms.

That's the reason that AC-coil relays are prefered over DC relays in
industrial control systems.