022 switches

[servoguy]

 Some time ago, there was a long post about 022 switches and problems with the new versions eroding away the contacts.  The solution which was proposed and used was to solder a piece of brass shim stock on top of the existing contact which was made from foil on a printed circuit board. 
The belief was that the eroding was caused by heating of the shim stock and burning it away because it had too little mass to dissipate the heat.  There was also a suggestion to add a transient voltage suppressor to the switch to limit the back EMF voltage from the coil when the contact opened.  Let me give my analysis of the problem and suggest what is needed to fix the problem.

I don't believe the transient voltage suppressor is necessary.  This is an AC circuit, and any plating done by the arc at one time will be reversed another time because it is an AC circuit.  There will not be any continuous plating of the metal from one contact to another by the current.   I have a total of 49 022 switches, all post war, which show no signs of contact erosion after many hours of operation.  There was talk of limiting the voltage used to operate the switch.  I think this is unnecessary for the above given reason.  I operate my switches on 20 volts, limited by the 18 volt lamps.  If the voltage is higher, the lamps melt the plastic switch lanterns. 

 The voltage across the contact when it opens is not the line voltage supplied to the switch.  The coils have a back EMF constant (Ke) which reduces the voltage due to the velocity of the armature inside the coil.  The faster the armature moves, the larger the back EMF voltage, and the smaller the voltage across the coil.  Motors also have a Ke which is why your engines don't keep going faster and faster when you put a constant voltage on the track.  The back EMF of the motor opposes the voltage supplied by the track, and the current through the motor reduces as the speed increases.  

Bruce

[lionelsoni]

Bruce, I don't know what is eroding the foil; but I wouldn't discount arcing.  Any back EMF would only reduce the open-circuit voltage, although I doubt that there is much of a back EMF anyway from the movement of the passive armature.  But the greater likelihood, to my mind, is an inductive voltage impulse produced by the collapse of the coil's magnetic field as the circuit is opened.  That would be well handled by a TVS.

[servoguy]

 Bob,

The problem with using a TVS is that its voltage has to be set so high that it doesn't suppress much.  If you run the switches from 18 volts, the TVS has to be about a 30 volt TVS, and I don't think that is going to make much difference.  I am going to try to get a scope on the contact to see what the voltages actually are.  As I said, I am not seeing any contact erosion in switches that are 50-60 years old.

 

[lionelsoni]

I just put my oscilloscope on a 5122 coil and measured a pretty consistent 200-volt spike using 16 volts from a 1033 transformer.

[Birds]

servoguy

...As I said, I am not seeing any contact erosion in switches that are 50-60 years old.

 

 

 

Bruce: Thank you for bringing this up again.  I was one of the people having problems with the foil etching, and the one who added the brass plates over the foil.

Just to clarify...

The etching was taking place in new O-22 switches made/purchased in the past 2-5 years, not post war switches.

These new O-22 switches are constructed differently than the post war O-22 switches.  I had a couple post war switches and the contacts are thick metal strips.  They are not foil mounted on a board as is the case with the new O-22 switches.  The metal strip contacts in post war O-22 switches are significantly thicker and heavier than the foil used in the modern O-22 switches.

I have switches made in the early 1990s (prior to the re-release of the O-22 switches), and those don't have problems with contacts etching.  These switches also use metal strips as the contacts and not foil.

I don't know the engineering behind TVS, solinoids, etc.  All I know is that the modern/new O-22 switches with thin foil contacts were etching, while the older switches that use thick metal strips as contact were not.

My switches are now permanently mounted on my layout, so I can't check how the brass strips are holding up, but they have been functioning very well.

I've been hoping someone would resurect this issue because I can't believe the few who posted about it were the only people having issues.

[Neil Poersch]

I am the person who started the original thread. As Birds explains the problem is with new O22 switches that only have etched foil contacts which erode very quickly. I also have some post-war O22s which have a thick metal strip for contacts and have no problem with contact erosion.

I solved my problem as Birds suggested. I sweat-soldered  0.006" thick brass strips onto the original foil contacts. I did not add a TVS across the solenoid coils. After about a year of use the brass strips show no sign of erosion. There is a dark trail where the moving contact slides along the fixed brass strip but no erosion.

I was using the full 18V of the fixed voltage post on my post-war KW to power the O22s. This probably contributed to the accelerated erosion that I experienced on the new switches whereas someone using 12-14V to power their switches may not experience the erosion as quickly. However I can't help but think that this is a looming problem that a lot of people are going to experience sooner or later.

 Neil