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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: DC relay latching voltage
Date: Sun, 25 Aug 2024 07:40:49 -0000 (UTC)
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John Robertson <jrr@flippers.com> wrote:
> On 2024-08-23 11:29 p.m., Don Y 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.
>> 
>>> 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.
>> 
> 
> Good reading! It appears counterintuitive, but if I warp (!) my head 
> around it I should be able to sort it out.
> 
> Thanks, that is very helpful.
> 
> John :-#)#
> 

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.

-- 
piglet