Tortoise switch machine, power routing frog, DCC 5amps - Too Much???greenspun.com : LUSENET : Wiring for DCC : One Thread
I am installing Tortoise switch machines and am seeing a problem with loss of electrical continuity over the frog. I have been reading this forum and other web pages and know that I can modify the 'make or break' problem in the machine but my question is this, The tortoise machine says it can handle 1amp of current, our NCE system puts out 5amps. What do I do??? Can I use the switch machine to power the frog or will I burn it up???
-- Ken Braden (firstname.lastname@example.org), October 02, 2002
the 5 amps is only an issue if you derail on the switch and short circuit the frog rails. Otherwise the Tortoise contacts only carry the amount of current a loco (or lighted car, etc) draws while it is transiting the switch. Also, if your wiring is adequate and you can pass "the coin test" then the short circuit only lasts a very brief time. The culprit is heat generated by the high current. A brief high current pulse every second or so is not going to cause the destructive damage that a continuous high current flow will.
Personally, I think entirely too much is made of the "danger" of damage to a Tortoise from DCC installations.
However, there is a potential problem using a Tortoise to switch the frog power. I recently discovered that the contacts in the Tortoise don't transfer on the center of the travel, but are biased towward one side. This can cause a short circuit as the machine is in motion and is the reason for the suggestion about modifying the switch contacts. (Anyone who doubts this, checkout the installation paper that Circuitron provides with the Semaphore driver board; or check it yourself with an ohm meter.) If you are careful with your installation so that the points are centered when the toroise machine is centered then you likely will be OK.
I use unmodified Tortoise and have not experienced any problems. YMMV.
-- Dale Gloer (email@example.com), October 03, 2002.
Dale is correct in that the amp rating is not much of a problem. 1) If you are powering only the frog, not the wing or closure rails (Atlas), the short must occur at the frog for the amperes to flow through the tortoise switch. This is not tne usual case on a derailment. The switch contacts internal to the tortoise should have no problem if your controller/booster quickly shuts down on a short. But there are other issues...
2) If you also switch power to the point rails along with the frog (Peco, Walthers/Shinohara, Pilz, et al), WHEN/WHERE the electrical switch changes state in relation to the throwbar position is important. You must ensure that the point rail and frog polarity are changed while both points are open to avoid causing a short circuit as the switch is being thrown. This means adjusting the tortoise repeatedly under the table to get the proper result.
3) The internal pcb switches on older tortoise machines did not have a positive break-before-make action. This can cause a momentary short circuit between the wiper and both of the other feed contacts as wiper of the switch changed positions. This may have been OK for signaling and DC throttle circuits, but it definitely causes a spark and subsequent booster trip when used with DCC to select frog polarity. There is more than one web-site reference that shows how to modify the tortoise pcb to avoid this problem (but does void the warrantee). New Tortoise machines don't seem to have this problem.
-- Don Vollrath (firstname.lastname@example.org), October 07, 2002.