Originally Posted by: H0 
...The 74490 has such a bad reputation, I presume they had to improve the endurance for everybody.
Spot on Tom, that is what I think too.
Originally Posted by: jeehring 
Originally Posted by: arconell 
Hi Darren, others,
What you shouldn´t do (sorry Fred) is try to put capacitors or diodes into the switches in an effort to suppress the contact sparks.
...(...)....
Regards, Robert
...however, on the last generation of 74490 - at least since april 2010- may be before - according to some pictures, it seems that it's what Marklin did : they have installed a small capacitor on the PCB at each end of micro switches...
Moreover on a French forum , one guy said that he had been doing it a long time ago on older turnout motors : he has installed a 0.1 µf capacitor with the goal to prevent micro switches being dirty too quickly (
because of micro sparks caused by "ruptured current"...sorry...it's my English...), then he made some endurance tests and obtained very good results.
-
Well, that is always the trouble with these endurance tests, you do a large number of switching actions in quick succession and it works beautifully. However if he´d run a blind test parallel to the test with the caps, it would have given equally good results.
To quote myself in an earlier post (sorry):
Quote:In reality there is a delicate balance between the speed of the moving plunger and the moment the solenoid is interrupted by the microswitch
The way M has constructed 74490 is such that the plunger has an extremely long travel. For a reliable switching action it therefore needs powerful solenoids and a long metal insert in the plunger. That makes the plunger heavy. At the same time, they had to consider heat management of the coils under power, which comes at a premium because of the height restriction so as to fit into the C-rail turnouts and it being completely surrounded by insulating plastic. This and the requirement for both digital (short pulses) and analog switching results in a design that:
A- cannot tolerate power-up periods of longer than about 0,5 a second unless you intend to fry eggs
B- the long travel dictates a high current to actually get the plunger from one end to the other, which occurs at high velocity.
C- at high velocity, the heavy plunger develops a high kinetic energy at the end of its travel which must be destroyed in about 2 microsecond just before reaching the end of its travel, if you don´t want it to bounce back again
D- this is also where the microswitch comes in, it cuts the supply just at the right point of the travel, but only when the power is sufficient to get the plunger all the way across.
That means that the power rating of the microswitch must be relatively high, about one Amp. In fact it is only 500 mAmps, with gold plated contacts. There is a good reason for the underrated switch, because that way M had hoped that the relatively high current will keep the contacts clean by sparking, so long as the voltage is 16V or higher. A higher contact rating would theoretically give an increased life span but not without regular problems with dirty contacts. A well-known phenomena in switching contact design. M sacrificed longevity here for reliable switching. However in many cases the switching voltage is well under 16 Volts and thus no sparking at all occurs and collected dust and other contaminants don´t burn off resulting in malfunctions.
Frankly, M hasn´t been able to get to grips with all these requirements and during the time since 74490 was introduced they experimented with at least a dozen different modifications, involving the (placement of) the switches, the power of the coils, adding resistors, modifying the plunger and what not. Regrettably all with very limited succes...
I guess we all hope that 74491/2 will finally solve our problems...
Best regards, Robert