Re: DC relay latching voltage

On 2024-08-25 11:27 p.m., Don Y wrote:

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!]

It is still cheaper than sending a repair person out.

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.

Pinball games are more service trouble than most operators want to deal
with. Anything we can do to make them more reliable is part of our
business model.

And I hate repeat service calls.

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!)

Ah, but you see that is the point of doing repairs that are better than
(improve upon) the original design. Back in the 80s I figured out why
one manufacturer's games were blowing up their driver transistors
randomly and published the answer in the trade journals of the day. It
was a ground issue, where the commons were done through Molex pins and
as the pins aged the ground connections generated resistance, which led
to transistors not fully biasing off and burning out. The fix was to
beef up the ground connections and all subsequent games we serviced
never blew the driver transistors any more and customers were happy!

Need I mention the factory never acknowledged the error and did NOT
implement my recommendations.
By the 80s they gained a reputation for unreliability that plagued them
until they finally closed shop in the mid-90s. Oddly enough the same
company was considered the Rolls-Royce of pinball up to the Solid State
machines. Their solid state games looked and played great until the
ground issues started randomly occurring (fried coils and driver
transistors) and then operators would get rid of that manufacturer's
games and buy other brand's machines.

There are two cases I deal with - commercial and home clients. Both
would rather see less of my shop then they have to and I am happy to
oblige them because I hate fixing simple things that could be made more
durable. Also we've gained a bit of a reputation of machines lasting
longer after being serviced by us and I wish to improve that score.

So, reducing contact arcing is very important to me and my customers!

We only get more business the better we fix the games!!

John :-#)#



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