3 Amp – DCC Track Booster – 2011 (LMD18200)
The DCC booster shown on this page uses the LMD18200 motor driver H-bridge IC. The booster’s designed output rating is 3 amps at 15 volts.
3 Amp Booster Circuit Features
- 3 Amp Design Capacity*.
- Overload Shutdown. (Manually reset)
- Double Acting – No Input Signal Shutdown. (Automatic reset)
- Shutdown Indicator LEDs.
- Optoisolated Input Signal.
- ‘Zero Stretching’ capable.
* – If this booster continuously carries more than 2.5 amps (83% of maximum), a larger booster should be considered or more power districts should be created.
3 Amp DCC Booster Schematic
The following diagram shows the schematic for the circuit board for the 3 Amp DCC Booster.
3 Amp DCC Booster Circuit Notes
The input signal failure detector will automatically reset when the DCC input signal is restored.
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 on the circuit board. If the RESET switch is held closed, the booster cannot shutdown.
- The overload trip setting is between 3.2 and 3.5 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.
- Two of four voltage comparators of an LM339 IC are used to detect the DCC input signal. The other two comparators are used to switch the H-bridge OFF if there is no DCC input signal or if there has been an overload shutdown.
Separate LEDs are used to indicate: Circuit power is ON (GREEN), an overload trip (RED) or if the LM18200 is hot (YELLOW).
- 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.
- The circuit board should be mounted on a vertical surface and in an area that has good air circulation.
NOTE: The variable resistor R10 shown on the schematic is an optional component and would only be installed if an overload current of less than 3 amps is needed. See the Optional Overload Setting Adjustment section below for further details.
The value of resistor R11 may need to be adjusted depending on the particular LMD18200 used in a given circuit. (390 ohms has been suitable in every case thus far.)
Power Supplies For This Booster
This circuit is designed to make use of surplus – 15 volt, 3 amp laptop power supplies.
Used laptop power supplies can be can be purchased inexpensively at many computer and electronics recycling facilities. (Have them tested before purchasing.) These supplies are also available through many other sources as well.
- 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 circuit board.
- 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 rapidly switch OFF and ON which could affect the commands being sent to decoders and cause locomotives to act unexpectedly.A power supply with a minimum output of 13 volts at 3 amps is recommended for use with this circuit.
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 terminals of the supply are 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 will likely not be suitable for use with DCC boosters as the negative terminal could be directly connected to the ground pin of the AC power plug. This will cause short circuits between other boosters in the system through common rail connections and reverse loop controllers.
Basic Circuit Board External Connections Diagram
With Increased Over current Delay
Adding a 33 to 100 microfarad capacitor at the external RESET terminals of the circuit board will slow the response of the overcurrent trip circuit and make the booster less sensitive to brief short circuits.
———————————————- 3 Amp DCC Booster Parts List
|CIRCUIT PART NUMBER||–||PART||–||Qty||–||Digi-Key Number|
|IC2A, B, C, D||–||LM339||–||1||–||LM339NFS-ND|
|D1, 3, 4, 6||–||1N4148||–||4||–||1N4148DICT|
|D2||–||3mm GREEN LED||–||1||–|
|D5||–||3mm RED LED||–||1||–|
|D7||–||3mm YELLOW LED||–||1||–|
|R1, 3, 5, 6, 8, 13, 14, 15, 16||–||10K||–||9||–||10KQBK-ND|
|R2, 4, 12||–||470 Ohm||–||3||–||470QBK-ND|
|R11||–||390 Ohm *||–||1||–||390QBK-ND|
|C1, 2, 5||–||2.2uF/35V||–||3||–||P5175-ND|
|Terminal Block||–||2 Position 5mm||–||2||–||ED1623-ND|
|Terminal Block||–||2 Position 3mm||–||2||–||ED2635-ND|
|HEAT SINK||–||For LMD18200||–||1||–||HS343-ND|
|S1||–||N.O. Push Button||–||1||–||SW400-ND|
* – The heat sink shown in the photo of the circuit board is not identical to the one in the parts list but they have the same capacity.
* – The value of resistor R11 might change, depending on the particular LMD18200 used in a given circuit.
3 Amp – DCC Booster PCB Parts Placement Diagram
Driving This Booster From The Output Of Another
It is possible to drive this booster from the output of an existing booster. There will be a very small shift in the phase of the second and subsequent boosters but this should not cause problems.
The input to the second booster should come directly from the output terminals of the existing booster to minimize any signal degradation or noise.
Driving This Booster From A Digitrax – Rail Sync Cable
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 R10 shown on the schematic would only be used if a lower overload current setting. Under normal operating conditions the position of R10 is short circuited by a path on the circuit board.
If a lower overload setting is desired, the shorting path can be cut, R10 added to the circuit board and adjusted as needed.
The formula for calculating the booster’s approximate overload setting is: 0.467V / (R11 + R12) / 377μA = Amps DC.
- For R11 plus R10min: 0.467V / (390Ω + 0) / 377μA = 3.17 Amps
- For R11 plus R10max: 0.467V / (390Ω + 1000Ω) / 377μA = 0.891 Amps
The value of 0.467V is the gate threshold voltage of the SCR formed by transistors Q1 and Q2. This value was determined by measurement.
The value of 377μA (microamps) is the ‘Current Sense Output’ (pin 8) output per amp of load current 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.
Circuit Credit: Rob Paisley