Ryaninspiron posted:stan2004 posted:...To be clear, the scope is showing the voltage between the center-rail and outer-rail of a DCS layout? That is, the track is power by 18V AC and this is driving a 12V AC relay coil via the insulated-rail section? I set the track power to 12v for testing this. Besides that the scope is hooked to the rails, the relay was driven using a pin wire jumper that I touched to the same point as the scope probe. The EMF we see is on the track power after I lift the relay power wire off the track.
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On your last point about why a light bulb does not create an equivalent spike, I thought a light bulb would be considered an almost purely resistive load and is quite a bit different from a relay coil in terms electron momentum.
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But that's exactly the point! I think we are in agreement that the EMF spike originates at the relay coil due to the sudden interruption of "electron momentum" when it is disconnected from the track. But since the relay coil is disconnected from the track at that point in time, how can that EMF spike be measured on the track-side? For example, if you have a chance, why not measure the EMF of the relay coil itself when you lift the power wire to the relay from the track. I believe you will see dynamics that are much slower than the nanosecond pulses you're seeing.
That's why I'm curious about EMF spikes originating from the interruption of "electron momentum" in the inductance of the track itself. In other words a straight, non-coiled, piece of wire or track has some inductance - assemble a few hundred feet of track and I can imagine the inductance being large enough that a sudden power interruption to any type of load (bulb, engine, etc.) can unleash the stored energy in that inductance.
Bottom line #1: I don't believe the EMF from a relay coil powered by an insulated rail section is an issue wrt damaging a TIU channel. Instead, it is the EMF from large engine loads that are suddenly interrupted from derailment or intermittent power interruptions going over turnouts, crossovers, etc.
Bottom line #2: Since the EMF from a coil or long length of wire/track is proportional to amount of current being interrupted, reducing current levels is a good thing. Optocouplers can be a key component in reducing the triggering current in an insulated rail circuit to a few mA rather than tens of mA...so at minimum a 10x reduction. Likewise, since it is the suddenness/abruptness of current interruption that directly affects the size of the EMF spike from an inductor, strategically placed capacitors can slow down interruption and drastically reduce the EMF.
