A booster's job is to take some form a common input power, DC or AC, and convert the power into DCC Power to run trains.
A booster, at a minimum, is specified by it's maximum track current that it can support when running trains without shutting down. This current can be 100% continuous or set by some long term duty cycle such as 10's of seconds (a period of full current operation followed by a shutdown period and then full current again) that might be based on some other limitations such as thermal (its getting to hot). There is no standard that governs this definition. But the intent or understanding is to say this is the maximum current you can get without it being considered a short circuit condition.
The other primary booster functions are:
1) Limiting the DCC Track Current. When a short circuit occurs, the current delivered into the short must be controlled or bounded by some means to prevent damage to the layout. For example, it has been determined by both experience and experimentally that 4A to 5A is about as high as you can go with most scales below O given the typical old layout wiring encountered and the electrical circuits inside rolling stock. Many times the layout wiring is subpar if the layout was old and built using DC power packs of 1 to 2 amps. If you had a 5A booster allowing 8 amps to that same layout wiring under a short circuit conditions, then damage can potentially occur to that layout wiring or the electrical systems of rolling stock. (See Note A) Hence a booster must REGULATE the track current to a consistent value that is higher than the rated current but not more than 20% or so beyond that rating. In other words limit the maximum track current to about 6Amps for a 5A rated booster. There is NO concern about what the track voltage is under these condition and in fact the track voltage will collapse to some low value that is determined by the total resistance involved in the current path.
2) Track Current Overload Protection. When a short circuit occurs, there is also the issue of how long it will it last. For example: Unattended operation that results in a derailment along with a short circuit. Even with 5A putting out 6A, the heat will start to build up in subpar wiring or the electrical systems of rolling stock quickly. Sustaining such a condition will lead to physical heat damage. Hence there is the need for the booster to eventually shutdown and remove track power. The question is when. You ask "Why not just shut down instantly when the current is being limited?" The problem with doing that is that there are two types of short circuits. The First type of short is the momentary short circuit(s) that occur naturally all the time due as trains running and dealing with imperfections of track, track switches or rolling stock electrical design which can include the maintenance and wheel gauging thereof. (See note B). This type of short must be FORGIVABLE for the user to experience the same electrical reliable operation they had under plain old DC power. The second type of short circuit is often associated with a much longer situation such as a train derailment that results in a steady short that is only corrected by manually removing/rerailing the rolling stock involved. The difference between the two types of shorts is the TIME duration. Hence a booster typically implement a "countdown timer" that start counting down to shutdown when current limiting becomes active in the booster. The timer resets the instant the booster exits the current limiting mode before it has completed the countdown. However, it the timer completes the countdown, the booster then shuts down. The duration of the shutdown can vary with the booster, but boosters will automatically try to restart themselves up again including in to the same short if still present. It the short is still present, then the shutdown cycle will repeat forever until the short is removed.
The optional or secondary features that some boosters may offer are: (Not in any order)
3) Regulating the DCC Track Voltage
Our engines are typically designed to work up to a maximum voltage for a given scale. Exceeding that voltage at a minimum can lead to overheating and burn out light bulbs faster or at worse, can potentially destroy electronics. For booster, track voltage regulation means one of two things. a) At a minimum it means it limiting or clamping the maximum voltage or the upper limit of the track voltage to prevent overheating or damage. b) A more sophisticated booster will actually go one step farther and regulate the track voltage BETWEEN an upper and lower limits independent of the load current. Full regulation means both a constant light brightness and consistent engine speeds for a given throttle speed setting under any normal operating condition.
4) High Efficiency Design
Some booster use an inexpensive but inefficient "linear voltage regulator" to regulate the track voltage. Inefficient means the regulator will generate a lot of heat that will require a fan and/or large heatsink to dissapate the heat into the air. A more sophisticated booster will use what is called a "Switching Regulator" to regulate the track voltage. This type of regulator is VERY efficient and means a cool running booster. Unfortunately, typically the booster manufacture will not advertise which type of regulator it is using. They booster manufacture might tell you if you call them, but seldom is this a criteria involved in booster selection. But if you find your booster case temperature running hot, you now have a good guess which type it is.
5) Thermal Overload Protection
If a booster starts to get to hot, there is the desire to shut it down before the internal heat can do damage to the booster itself. More specifically, when a specified temperature is reached, the booster will turn off and stay off until the temperature falls back below some lower temperature threshold and then restart itself. It may appear to look like a short circuit condition, but the temperature of the case will tell you otherwise.
6) Adjustable track voltage.
This simply means you allowed to adjust the track voltage to meet the limits of the scale you are using. Typically higher for larger scales and lower for smaller scale.
7) Autoreversing Support.
Until the release of "Autoreverser" devices that were separate from booster, the booster took on the job of dealing with reversing sections (reverse loops, wye's and turntables) need for track polarity/phase control of the track voltage. It should also be noted that Autoreversing is a luxuary and not a requirement. The same DPDT switches you use to address this problem in DC works just fine for DCC. Today many booster are dropping support for this function to lower the cost of the booster given not every layout has a reversing section or us happy using switches. So iF you want to automate a reversing section, go buy a autoreversing device for it.
8) Input UnderVotlage protection.
If the input voltage, DC or AC, is too low for the booster to operate properly, then the booster will typically indicate so and/or go one step further and take action by shutting down.
9) Input support for AC transformer operation.
Typically the least expensive purchase option to power a given booster is to use a simple AC step down transformer. Most DCC manufacture offer such an AC power supply. All booster will run from a DC power source, but many booster are designed to accept a AC input on the very same terminals. Many entry level low power DCC system will typically include a power supply and often it is DC wall adapter like you typically find with small appliances. Time and technology are changing and the cost issues are changing the power supply options available. Recently some DCC manufactures are now offering DC power supplies instead of an AC power supply due to lower cost.
10) Shutdown/Loss of signal from Command Station.
The booster is looking for the DCC commands from the command station to send to the track. The loss of this command signal results in the booster shutting down until the signal returns. Some command stations will intentionally shutdown all boosters when the programming track is active to protect the engines on the layout.
A) The picture on the left shows what happens when to much current is allowed to flow in rolling stock. The damage is to a HO locomotive on a layout using a 8 Amp booster involved in a short cirucit. Photo Credit: Rex Beistle. For more about choosing the right booster current rating, go here:
B) In fact there are articles on how to make certain brands or models of track guard against unnecessary short around the point rails and frogs that allow more reliable DCC operation. For more information go here: