Edmonton Society of Model Railroad Engineers - Other

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Other Layouts in the Area

There are a number of other layouts and clubs in the Edmonton area. A list of these is available through the Edmonton Area Model Railroad Pages

Miscellaneous Information

Short Circuit Protection Circuit

The Edmonton N Scalers' layout changed from homemade throttles to Aristocraft Train Engineer throttles in late 1996. A short circuit problem became apparent when several locomotives self-destructed. The failure caused the locomotive trucks to melt because of heating due to high current through the metal contacts on the trucks.

Investigation revealed that, in a short circuit condition, the current draw was insufficient to blow the 10A fuse in the Train Engineer unit and a 2A circuit breaker in the transformer box would not blow rapidly enough (circuit breakers normally need a great deal of overcurrent to fire quickly)

A number of solutions were possible:

changing to low value fuses

changing to a fast blowing circuit breaker

creating an alarm circuit

The fuse solution was rejected because the number of inadvertent shorts would require buying fuses in case-lots. Fast blowing circuit breakers were a possibility, but not easily obtainable. Both of these solutions also couldn't easily accommodate a widely varying load. This was neccesary because we may be running from one to 8 or 10 locomotives off one throttle at any given time.

Instead we designed an alarm circuit which has the advantages of virtually instantaneous response plus both audible and visual signals to warn the operator of the problem. Also, it will not reset until the short is fixed, ensuring that locomotives are protected. It handles the varying load through a Threshold control which is simply turned down until the alarm fires, then increased slightly. This gives maximum protection, even for high current situations when there may not be a hard short.

The full schematic is available here: (gif, 13K)

(gif, 13K)

It is free for non-commercial use.

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