The new PSXX has all of the features of the original PSX. However, the PSXX has a new and unique feature that no other circuit breaker or fuse (to our knowledge) possesses. When you set the trip current, which can be set in integer increments from 1 to 10 amperes, you not only set the current at which the breaker will shut off, you also set the maximum amount of current that the breaker will allow to flow. The graph below illustrates the current output performance of the PSXX:
Along the bottom (X axis) is the current that would flow without a PSX or PSXX installed. Essentially the graph goes from open circuit to short circuit as you go left to right. The Y axis is the current that the PSXX will allow to flow with the same operating conditions. Note that at each current setting, it will allow the set current to flow without limiting or tripping. As the fault becomes more severe (lower resistance), the PSXX not only does not allow any more current to flow, it actually starts to fold back and allows less current than the set value to flow. Obviously, it will hold this current for a short time, but if the fault condition persists for a pre-set time, the PSXX will open the circuit. Lower current settings will hold the fault longer than higher current settings. If you have the PSXX set to AUTO reset, it will periodically try to turn on, but will not allow any more than the set current into the fault. This means that a short in one area of the layout will not draw all the booster current (essentially dropping the track voltage to zero) during the reset attempt as happens with other circuit breakers. This ensures isolation of the fault while retaining automatic reset.
There are several benefits to this design. As mentioned above, you can set the maximum current that any one block can draw, giving better management of the available booster current. Energy in a fault (which could be the truck of your favorite model!) is proportional to the square of the fault current times the time the fault current is allowed to flow. Past breaker designs essentially allow the maximum available booster current to flow until the output turns off. Some boosters will output 27 amperes or more during the time the output is held on. Even with the PSXX set to 10 amperes, this is over 7 times more energy for the same amount of hold time. At lower current settings, we can therefore allow the fault to stay connected without danger of damaging equipment and making performance more robust with incidental faults.
With today's sound decoders, there is a significant capacitive load on the track at power up. Indeed, for a period of time, it appears to be a short circuit. This can cause a breaker to trip, since it is hard to tell a real short from a temporary capacitive one. By limiting the current, we can charge any number of sound decoders since the "fault" current never exceeds the booster capability.
There are some boosters that will trip extremely quickly. We have had problems with several makes that reliably trip before the PSX if there is a solid short. By setting the PSXX to a value within the booster capability, the booster never sees the short, and the problem is solved.


The PSXX is compatible with PSX-AR and previous generation PSX-1,-2,-3,-4 circuit breakers. If you are using both PSXX circuit breakers and PSX-AR auto reversers on a layout, make sure the PSX-AR J1 input is taking power from the J2 DCC track output of the PSXX.
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