DC relay latching voltage

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.

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, but the EMI was limited to the zener's rating along with the diode voltage drop. What is interesting is if you have a regular diode across the coil it takes around 5 times as long to decay. 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 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...

"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."

Oh, yes, we constantly deal with them, and pitted contacts on the EOS switches in our shop. Would zener diodes across the contacts help reduce pitting?

John :-#)#

It is still cheaper than sending a repair person out.

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.

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 :-#)#