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PSX Reset

Posted by on Apr 2, 2015 in Tony's Tips |

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I am the Secretary for the Lake County Model Railroad Club and technical advisor for the club’s DCC Committee. Our club layout has been in operation for 40 plus years and uses Digitrax Command Control. The layout is divided up into 5 Power Districts with its own Digitrax Booster. Each Power District has its own Sub Districts with their own PSX or PSX AR board. 

How The PSX Senses a Short

We have purchased and installed 20+ PSX boards on our club layout and have a few issue preventing us to continue with the installation.

  1. Placing a locomotive on the layout causes the other locomotives on the same PSX to reset.
  2. PSX board on the main line resets when exiting the reverse loop protected by a PSXAR.
  3. PSX resets when moving from a PSX siding to a PSX mainline.
  4. Cars with LED’s reset PSX boards when crossing from one PSX protected Power District to another.
  5. Any small or nominal (less than specified) short protected PSX track trips and resets boards. 

In short (no pun intended), these boards trip instantaneously when a fault is detected, even though there has been no over current, or even any detectable current at all drawn.  This makes operation impossible, since even a led turning on causes a trip and a timeout restoration.  This whole phenomena is similar to ground fault trips on house wiring. This is unacceptable for our operation, and not something we can live with. 

These issues are magnified while running during our open house or during operating sessions.
Please provide us feedback or suggestions to resolve. Thanks,  Jim S.


First question: how old are your PSX units? I ask because the solution for these issues was released in January. You can tell the revision of the software by moving the program jumper to the program position and turning the unit on. You will see the program LED flash. For the PSX-AR you should see short long short short. For the breaker short short short short. If so, you have the latest software. If you get something else, let me know. If you have an earlier version of the software, we can work out an update for you. The changes made in these versions of software allow you to enable and adjust a delay between the detection of the high current condition and the operation of the reverser or the breaker turn off. The delay is enabled by setting CV55 to 1 and then setting CV65 for the delay. CV65 defaults to 8 (about 1 ms), but can be set to any value up to 255 (about 30 ms). To answer your questions:

1) Most locomotives today have relatively large capacitors in the decoder power supply to keep the decoder working during power loss caused by dirt, switch frogs, etc. These capacitors actually are a short circuit for a short time until they charge up. Dropping a locomotive on the track is equivalent to dropping a screw driver on the track (which then bounces off). The best answer is not to do this, but you can insert a delay in the breaker sequence (as discussed above) that will prevent the breaker from tripping while the capacitor is charging. While the exact delay depends on the booster, your wiring, the decoder, etc., a value around 40 (5ms) should work. The smallest value that works is the best choice.

2) Layout wiring, particularly on a large layout, can cause delays in the current detection. Hence, the PSX-AR is apparently a little behind the PSX breaker causing the trip (basically a race conditiion). Again, adding a little delay to the breaker will fix this problem.

3) The fact that the PSX is tripping at a power boundary indicates that the two boosters are operating at different voltages. As you move from one district to the next the locomotive will actually connect the two booster outputs together. If they are different by more than a couple of tens of millivolts, then the higher voltage booster will try to raise the voltage of the lower voltage booster causing a very large current to flow; essentially a short circuit. In this case, the PSX is preventing one booster from damaging the other. Assuming you are using a DCS100 (if not, check your manual), on page 28 there is a paragraph “DCS100 track voltage fine tuning”. Take a meter (or a 12V light bulb, but a meter is more sensitive) and connect it from one power district to the next ON THE SAME RAIL. The voltage you see is the difference voltage between the districts. It will probably not be large. On one of the boosters, fine tune the track voltage until this difference is as close to zero as you can get it. Once you have tuned all of the boundaries, the PSX should not trip.

4) This is the same as 3). The LEDs are not causing a problem, but the cars have pickups in the wheels to power the LEDs. The wheels and pickups can bridge one booster to the other (as noted above) and cause a short. Once you have fine tuned the voltages, this should not be a problem.

5) While 3) and 4) above could also be fixed by using the delay feature, I do not recommend it because each time a train changes districts, you are pounding the booster. Particularly a club layout, which is used a lot, should try to avoid doing this. In the remaining cases, it appears that the addition of a delay should solve the problem.

As I mentioned above, let me know the LED flash pattern when you place the PSX in program. If you don’t have the code version that implements the delay, we will work with you to get everything upgraded with as little impact as possible. Since a number of the issues appear due to booster voltage mis-match, I would correct that first, then see which points still cause problems and need the delay. There may be just a few units that actually need the update.  Larry-