March - 2011
The circuit board for the 3 Amp Booster on this page has been replaced by a newer version. The new circuit will have fewer parts and use a larger, one-piece heat sink.
2011 - 3 Amp DCC Booster
All other functions and features remain the same.
2009 - 3 Amp - DCC Track Booster (LMD18200)
The DCC booster shown on this page uses a LMD18200 motor driver H-bridge IC. The booster's designed output rating is 3 amps at 15 volts.
This booster was developed from the one shown on the My MiniDCC© System page at this site.
3 Amp DCC Booster Circuit Features
3 Amp DCC Booster Circuit Notes
The input signal failure detector will reset when the DCC input signal returns.
Double Acting - No Input Signal Shutdown means that the booster will shut down if the input signal fails in either a HIGH or a LOW state.
The overload detector is manually RESET by momentarily closing switch S1. An external reset switch can be connect via a terminal block mounted on the circuitboard.
The overload trip setting would normally be fixed at just over 3 amps but can be lowered if needed. For more information, see the Optional Overload Setting Adjustment section below.
The overload section of this booster uses transistors Q1 and Q2 as the equivalent of a silicon controlled rectifier. (Similar circuit)
An HCPL-2200 Logic Gate Optocoupler was used because its output can operate at 15 volts, eliminating the need for a 5 volt regulator in the circuit.
Two voltage comparators of an LM339 IC are used to detect the DCC input signal. The remaining two comparators of the LM339 are used to switch the H-bridge OFF if there is no DCC input signal or if there is an overload condition.
Separate LEDs are used to indicate: Circuit power, an overload trip or if the LM18200 is too hot.
A power supply with a minimum output capability of 13 volts at 3 amps should be used with this booster. For more information, see the Power Supplies For This Booster section below.
3 Amp DCC Booster Schematic
The following diagram shows the schematic for the circuitboard for the 3 Amp DCC Booster.
NOTE: The variable resistor R12 shown on the schematic is an optional component and would only be installed if an overload current of less than 3 amps was needed. See the Optional Overload Setting Adjustment section below for further details.
The value of resistor R11 may change depending on the particular LMD18200 used in a given circuit.
Power Supplies For This Booster
The DCC Booster circuit will be damaged if the polarity of the power supply is reversed. Care must be taken when connecting a power supply to the circuitboard.
An adequate power supply must be used with this booster as the LMD18200 has an internal voltage sensor that will shut the bridge down if the supply voltage dips below 11 volts. This will cause the output of the bridge to switch OFF and ON at high loads which could affect the commands send to the decoders.
A power supply with a minimum output of 13 volts at 3 amps should be used with this booster.
This booster can operate with supply voltage of up to 18 volts.
Power supplies with isolated outputs should be used with this booster. Isolated means that the output of the supply is not electrically connected to the ground and/or neutral wires of the 110 volt AC supply circuit.
If a power supply has a grounded, 3 prong plug, it might not be suitable for use with DCC boosters as the negative terminal will likely be directly connected to the ground pin of the AC power plug. This could cause short circuits between other boosters in the system through common rail connections and reverse loop controllers.
Keep the power supply wires to the booster as short as possible to minimize losses.
A suitable power supply for this circuit from Skycraft Parts & Surplus - 15 VDC 4.5 Amp, Switching Type.
Optional Overload Setting Adjustment
This booster circuit is designed to have a fixed, overload current setting of between 3.2 and 3.5 amps.
The variable resistor R12 shown on the schematic would only be used if a lower overload current setting. Under normal operating conditions the position of R12 is short circuited by a path on the circuitboard.
If a lower overload setting is desired, the shorting path can be cut and R12 adjusted as needed.
The formula for calculating the boosters overload setting is: 0.467V / (R11 + R12) / 377μA = Amps DC.
For R11 plus R12min: 0.467V / (390Ω + 0) / 377μA = 3.17 Amps
For R11 plus R12max: 0.467V / (390Ω + 1000Ω) / 377μA = 0.891 Amps
The value 0.467V (volts) is the gate threshold voltage of the SCR formed by transistors Q1 and Q2. This value was determined by experimentation.
The value 377μA (microamps) is the 'Current Sense Output' (pin 8) output per amp of load current in amps as given in the LM18200's datasheet. ( 377μA = 377 X 10-6 Amps )
NOTE: The actual output from the Current Sense Output pin 8 may be higher or lower than that specified in the LMD18200's datasheet. Therefore, resistor R11 may need to be tailored to the actual conditions of the circuit.
A DC ammeter with a 3 amp or greater capacity connected between the POSITIVE output of the power supply and the PLUS input of the booster circuit can be used to set the trip point while the booster is under load.
* - The value of resistor R11 may change depending on the particular LMD18200 used in a given circuit.
Please Read Before Using These Circuit Ideas
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.