What the heck kind of relays are these that would need a "one amp 'trigger' to activate"? What is it you are describing?rkcarguy wrote:BDD, the relays I have on hand require a 6 volt, one amp "trigger" to activate.
Track circuits are (should be) operated at very low voltages and the relays used in track circuits should be high sensitivity so they do not drag down the open circuit (unoccupied block) voltage. For example, in the MABS block occupancy detectors (BOD, illustrated below) made by my company, this hermetically sealed relay is used as a track relay.
In the above devices, the track circuit power source is 3.3 volts and when the drop in the track circuit ballast resistor is accounted for, the theoretical voltage across the track relay coil is 3.1 volts, which is also the theoretical voltage between the rails in an unoccupied block.
The reason for keeping the voltage very low is to minimize the effects of leakage through the ties and ballast, which is present even when conditions are completely dry. The power loss due leakage increases with the square of the unoccupied block voltage, so it is advantageous to operate the track circuit at the lowest practical voltage consistent with reliable relay operation. If leakage is excessive, the losses caused by parasitic current drawn through the ballast resistor will reduce the open circuit voltage below the track relay's pickup point, resulting in a false occupied state.
Relays that are suitable for use in track circuits are usually not compatible with sizable loads, especially incandescent lamps. In full-sized practice, track relays control other relays that are responsible for handling signal lamp loads. The above relay I linked is specified by the manufacturer for use in "signal" applications, which in electronic parlance means controlling low power devices, such as other relays or the inputs of solid state devices. This relay has gold-flashed, crossbar contacts that allow it to switch extremely light loads, as low as one milliamp. Such a design by nature cannot safely switch heavier loads and attempting to drive an incandescent lamp would likely result in contact welding.
Switch-mode power supplies of the type you describe will work but are gross overkill for the application, and contain a lot of electronics that can fail due to evinromental factors or line surges caused by nearby lightning strikes. Simple (and less failure-prone) linear supplies powered from readily-available transformers are more than adequate, less failure-prone, and can be made compact. Both of the BODs illustrated above have built-in regulated, linear power supplies and require only 12 volts AC input to operate. The non-directional BOD is about the size of the palm of my hand.It's my plan to use a couple 12 volt 6 amp power supplies that are used on several kinds of printers to power my system...
Delay-on-clear is also used in full-sized practice for the same reason. "Vital" (track) relays are gravity-released and often pneumatically damped so they are not affected by the variable contact resistance that occurs between the railhead and moving wheel. The damping prevents relay chatter when empties near the end of the train are about to exit the block, also the case if the last car is a caboose....and the detector circuits will contain a timed delay off module set for couple seconds to prevent flickering.
In hobby practice, where a pneumatically-damped, gravity release relay would be prohibitively expensive (and nearly impossible to obtain, not to mention it being a bulky relay), a solid state timer can be used, provide the timer circuit cannot start its sequence unless the track relay is truly actuated. Such a timer has to be fully resettable, which means each time the track circuit is cleared the timer must start from zero.
In an uninterrupted block, the delay-on-clear can be less than a second. More delay is required if the train must pass through dead sections, such as turnouts that are isolated from the track circuit. The above BODs have adjustable delay-on-clear that ranges from 0.8 seconds to 8 seconds. In uninterrupted blocks, we recommend setting them to the minimum to produce realistic operation.
The degree of leakage you will experience is highly variable and will be influenced by block length, track construction methods, tie material, tie size, the type of ballast used (fines are particularly problematic), track gradient, right-of-way drainage characteristics and even the presence of weeds on the right-of-way. I can tell you that the trackwork at ILS uses 260 treated wooden ties per 100 linear feet—the ties are made from two-by-sixes that are ripped to make a two-by-three tie, with the narrow dimension being the surface that supports the rails. Such construction, along with medium grade ballast, has a dry leakage rate of approximately 60 milliwatts per 100 linear feet of track in a 3 volt track circuit. At 6 volts, the leakage would theoretically be 240 milliwatts. Testing at the ILS immediately following a soaking rainfall indicated the leakage increased to approximately 180 milliwatts per 100 linear feet of track, again in a 3 volt track circuit, and safely below the point where false occupied states would occur. Elevating the track circuit voltage to 6 volts would increase that leakage to 720 milliwatts—a 100 foot block would not clear with that much leakage.I can of course, change relays if the above ones won't cut it. It's wet up here, so I don't know what the amount of leakage will be across wet ties and if it would be enough to false trigger the system or not.
I am in the process of evaluating a BOD design that would operate the track at 1.5 volts. Based upon theoretical computations and a certain amount of educated conjecture, I believe such BOD would successfully operate on a 1500 linear foot block in 7-1/2 inch gauge, assuming the construction characteristics described above (260 ties per 100 linear feet, etc.). That would be nearly 2.3 scale miles.