Re: DC relay latching voltage

On Fri, 23 Aug 2024 23:29:48 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

On 8/23/2024 10:05 PM, John Robertson wrote:

On 2024-08-23 12:43 p.m., Don Y wrote:

On 8/23/2024 7:36 AM, John Robertson wrote:

I assume you have the normal back diode across the coil for clamping the
ringing voltage.

If you care about the life of the relay (and how well it "opens"),
a better approach is to add a zener rated at the coil voltage in
series with that diode.  The diode, by itself, delays opening of
the contacts and leads to more opportunities for arcing.  The
zener speeds up this transition.

 
Don't you mean that one could use a zener diode in place of the back-EMF diode
- or parallel with it?

>
In series.  It speeds up the decay of the magnetic field (and, thus, the
opening of the armature) by allowing a higher potential to exist across
the coil while it is "opening".  The ideal condition is with NO catch diode
(but that tends to fry solid state switches!  :> )  You're making a
snubber, of sorts.

Essentially all modern discrete mosfets avalanche safely at ballpark
120% of rated abs max drain voltage. Try it.

The damage limit becomes average power dissipated, which would be low
in most cases of driving a small relay.

I recently designed a board brickwalled with about 130 power relays.
Each has a sot-23 logic-level mosfet on the bottom of the board,
snuggled between the thru-hole relay pins. No catch diodes.

https://www.dropbox.com/scl/fi/fiy73ou127rd5hi23jkw9/P948_A6.jpg?rlkey=symijy4b0veq1s5ho982r0sa0&raw=1

>

We've been using 1N400X (and 3A in some cases)diodes since the 70s for
protecting pinball driver transistors - not too worried about the decay time so
haven't really looked deeper into it before.

>
It really only matters if you are really concerned over the actual opening
time (delay) of the relay and/or how long you want the relay (contacts) to
last.  (many of my designs have to have service lives of decades or more.)
>
<https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN>
will give you a quick overview.
I have better notes from (relay) application engineers but I'm busy baking,
tonight, so can only take quick pokes at my mail, etc.

Baking what? I'm getting whining that we are out of both biscuits and
bread pudding, so I won't get much electronics done today.