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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 23:27:23 -0700
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On 8/25/2024 10:01 PM, John Robertson wrote:
>> Don't kill yourself worrying about this sort of thing for a pin table.
>> You're likely not as concerned with durability.  The board's already
>> (likely) designed.  And, it's already got an established (tolerable?)
>> level of EMI.
> 
> Actually the 2nd link you provided that speaks of Coil Suppression with DC 
> relays pointed out that the zener and regular diode in series had a drop -out 
> time of almost the same as an unprotected coil,

Yes.  What that implies is the contacts don't "linger" in close
proximity/partial contact.  The opening action is more "snappy",
as intended by the choice of armature spring.

> but the EMI was limited to the 
> zener's rating along with the diode voltage drop.

Yes.  You also have to remember that you are going to see a larger voltage
spike than the simple diode (that would have clamped it to the supply rail).
If the semiconductor switch can't handle this...

> What is interesting is if you 
> have a regular diode across the coil it takes around 5 times as long to decay. 

That's the point; the relay is operating in a mode that wasn't intended
(i.e., shit or get off the pot!)

> That may actually matter in pinball games - one could get snappier coil action 
> with the zener/diode combination (or zener across the driver transistor - 
> skipping the regular diode as mentioned in the note) and folks may be able to 

It depends on what you are talking about.  In an old electromechanical game
where the relays implemented the logic, the effects would largely be unnoticed.
Driving a pop bumper or flipper solenoid could see some improvement in the
"liveliness" of the action.

But, in the case of kicking targets, remember that the setting of the
sense contacts probably has a bigger impact on the feel of the game;
if the target reacts too soon or too late, the kinematics change as
the ball is in a different place with respect to the design of the
kicker.

> notice that. Hmm, 1.9ms vs 9.8ms - can humans detect that when playing 
> considering that for the most part 100ms is considered 'instantaneous'? I may 
> have to set up a game and see...

An easier way to do it would be to have a little piece of code that allows
you to vary the delay between contact sense and solenoid actuation.  Remember,
most kicking targets do their work on the "activation stroke"; thereafter, they
are just "recovering".

> "Many engineers use a rectifier diode alone to provide the transient 
> suppression for relay coils. While this is cost effective and fully eliminates 
> the transient voltage, its impact on relay performance can be devastating. 
> Problems of unexplained, random "tack welding" frequently occur in these systems."

On an old machine, such a failure would be pretty obvious.  I don't know if
newer (electronic) machines take any action to determine if their hammer
drivers are failed or coils faulty.

[I spent a shitload of time designing the interface to the irrigation
valves (solenoids) in my irrigation controller to avoid these sorts
of failures -- as well as detecting "forced" failures (e.g., user wired
the valves -- ir god knows what else! -- to the controller incorrectly).
But, the cost of a repair, there, far exceeds that of hauling a pin
table into the back room!]

>> Rather, it should be seen as a counter to the "old saw" that you *just* use
>> a recirculating diode without considering the consequences.  Just like
>> considering how to *drive* the coil based on how it will be used.
>>
>> [Remember the "pull in" coils and "end of stroke" (EOS) switches on flipper
>> solenoids?]
> 
> Oh, yes, we constantly deal with them, and pitted contacts on the EOS switches 
> in our shop.

But, you have the advantage of being able to pull the machine off the floor,
and burnish and regap the contacts.  And, you get some idea of how likely
the need base on how many plays it sees.

If the device in question is on a mountain in tibet...  (yes!  <frown>)

> Would zener diodes across the contacts help reduce pitting?

An RC snubber might work better.

But, *maintaining* a pin table is half the fun!  (unless, of course, you
are in a business to make money from them!)