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DCC Circuit Breakers & Auto Reverser


PSX vs. OnGuard - What's Best for You?

by Don Fiehmann

Don Fiehmann compares the features of the two popular DCC power regulation products.

How to choose the appropriate circuit breakers and auto reversers for your layout? Here's the difference between the new PSX series and OnGuard series of circuit breakers and auto reversers.

Layout Blocking

With DC you split up a layout into blocks so engines could be controlled by separate power packs. When a short occurred, only the power pack connected to the block with the short was affected. With DCC, power is supplied by the DCC system. Without some type of blocking a single short will shutdown the whole layout. This is why power districts are needed in DCC.

Districts and Subdistricts

The area of a layout that is powered by a single DCC booster is called a district.

When a district is divided into smaller blocks, these are called subdistricts. Each subdistrict requires some type of circuit protection, like a circuit breaker. The circuit breaker acts like a fuse when there is a short and prevents the booster from shutting down. Fuses are not a good choice as each short would require them to be replaced. There are two types of circuit breaker and reversers available. They are the relay based and the solid-state types.

Relay based circuit breakers are slow to react and cause sparks. In addition to causing sparks due to their slow speed they also allow the short circuit to persist "longer" allowing the locomotive headlights and lights in passenger cars to "clearly blink" giving the train a more toy train look. Un-realistic! A "black eye" for the relay solutions! The best choice is a solid-state high speed electronic circuit breaker that will trip before the booster trips and will automatically reset when the short is cleared. When relay type reversers are used they can causes a hesitation as the reverser trips. (Thanks to Mark Gurries for pointing out this relay based problem.)

Available Solid-State Circuit Breakers

Most of the DCC systems and circuit breakers were designed to handle steady state loads.

Sound decoders increased the value of the capacitors used to stabilize the power in the decoders. The added capacitance resulted in virtual overload when a circuit breaker attempts to reset. The surge of power needed to charge these higher value capacitors after a short confuses the circuit breaker. The breaker could not tell the difference between the capacitor's inrush loads and a real short. A few sound decoders in a subdistrict may be OK. But as more and more sound decoders were added, the reset problem became unacceptable.

Another item that has caused a similar problem has been passenger cars with incandescent lamps. Lamps that are off are a much lower resistance than when on. More current is needed to turn on the lamps, this can also confuse a circuit breaker as an inrush load.

The solution to the breaker overload problem required a new approach to the design of DCC circuit breakers. What was needed was a breaker that could intelligently analyze the reason for the circuit breaker tripping, inrush load vs real load. The PSX series was designed to solve this problem using intelligent overload sensing and the PSXs can distinguish between an inrush load or a true short overload. The OnGuard series does not have the intelligent features and is best suited for N scale or low density HO scale.

PSX or OnGuard

The PSX series is a premium line of breakers and breaker/ reversers. There are many advantages to using the PSX over the OnGuard series. Do you need the extra performance offered by the PSX series? The answer depends on the subdistrict to be protected. A small switching yard (or a turntable) off the main line needs protection and my not have more than a couple of engines in the subdistrict at one time. This would be a candidate for the OnGuards. Larger yards and passenger terminals, where sound engines congregate, would best be served by the PSX breaker.

Low Power DCC Systems

The fixed trip setting of the OnGuard breakers and reversers are higher than the power output of some of the low power systems like the Zephyr and PowerCab. When a short occurs the system goes into overload before the OnGuard breaker reaches its trip setting. In this case, the PSX series with the adjustable power setting of down to 1.27 Amps can be used. The PSX also has a low current boost for low power systems.

Current Range
Screw Terminals
Onboard LED Indicators
Number of Sound Decoders*
Breaker Type
4 A (fixed)
None (2 remote)
4 - 5
Solid State
1.27 - 17 A
Solid State

*Number of HO sound decoders before the breaker failed to reset after it was tripped.

Small Scales

The adjustable lower current setting of the PSX series is important when used with smaller scales with small rail sizes. With the high setting of the OnGuard there can be enough resistance in the small rails to look like a heavy load and not a short circuit. This can lead to damaging heat in delicate parts.

Large Scales

The PSX series is designed to also be more usable in larger scales with 8 and 10 Amp or larger boosters. The settings of up to 17 Amps can make this very useful in cases where there are high startup currents with older motors.

Wire Size

All wire has some resistance. Using small wire and high current can lead to a situation where the resistance of the wire prevents the circuit breaker from tripping because it sees the resistance as a load and not a short.

Length in Feet for 1/2 Volt Drop

Wire Size
1 A
2 A
5 A
10 A

Note that this 1/2 volt chart shows a voltage drop for one way. If wired out and back (two ways) the chart shows a 1 volt drop.

Part Part Number
OnGuard breaker OG-CB
OnGuard Breaker/Reverser OG-AR
PSX Single Breaker PSX-1
PSX Double Breaker PSX-2
PSX with 3 Breakers PSX-3
PSX with 4 Breakers PSX-4
PSX Breaker/Reverser PSX-AR

Tech Info
Broadway Ltd
CMX Products
DCC Specialties
Generic / Other
NCE Corporation
Pricom Design
TDP Associates
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