The Lionel Powermaster provides wireless remote control of the output of a fixed-voltage transformer from a TMCC CAB1 handheld controller. The voltage range from OFF to Full Throttle is broken into 32 steps.
Additional features include Whistle/Horn and Bell activation by means of an injected DC voltage component, variable starting points (STALL) on the 32-step control range, a variable rate of rise and fall of the voltage to simulate the momentum of a heavy train, overcurrent protection and TMCC address programming.
Originally introduced as PM-1 P/N 6-12867 with current limiting appropriate for a Lionel 135 watt Powerhouse transformer, the newer version, P/N 6-24130, includes selectable current limiting for either the 135 or 180 watt Powerhouses.
The power source is connected to a 3-pin Molex connector. Pin 1 of the connector is at the pointed end of the connector and Pin 3 is at the square end. The input power HOT lead is connected to Pin 2 and the COMMON lead is connected to Pin 3. Pin 1 is not connected to anything inside the Powermaster.
The output terminals for connecting to the track are labeled “A” and “U”. The U terminal is the track COMMON tied to the outer rail(s), and is a straight-through connection from Pin 3 of the power source connector.
Terminal A is the HOT lead to the center rail. This terminal is fed from Pin 2 of the power source connector after passing through a very low resistance current-sampling resistor (R28) and then a pair of FET (Field Effect Transistor) power control transistors that are connected in series back-to-back.
The FETs take turns controlling the positive (Q2) and negative (Q3) halves of the AC waveform, with the inactive FET acting as a simple diode in the reverse direction. (FETs have an advantage over TRIACs because FETs can be turned ON and OFF, while TRIACs can only be turned ON, but the input voltage must drop to zero before the TRIAC will turn itself OFF.)
The TMCC Receiver
Both the Powermaster and the Command Base have radio receivers tuned to 27 MHz for receiving commands from the CAB-1 handheld controller. The exact frequency is controlled by selecting a pair of crystals for the CAB-1 and Powermaster. The crystal in the CAB-1 is set to a frequency that is 455 KHz higher than the crystal in the Powermaster. The normal frequency pair is 27.255 MHz (Channel 6) in the CAB-1, and 26.800 MHz in the Powermaster and/or Command Base. The reason for the 455 KHz stagger will be described later. (Note that the plastic holders for the crystals both say “27.255 MHz” even though the receiver crystal is really 26.800 MHz. The actual frequency is printed on the shell of the crystal.)
The receiver’s antenna consists of 28” of 26 gauge wire wound in a rectangular spiral pattern. (Always check the antenna’s attachment wires after servicing to verify that the wires have not fractured where they enter the PC board due to twisting and turning of the PC board to replace components.)
The antenna feeds a one-transistor RF amplifier configured as a common base stage, with the collector load tuned to 27 MHz.
The boosted RF signal is fed to one port of the Mixer section of a MC3371 single-chip radio receiver. The Local Oscillator section of the chip provides a crystal-controlled 26.800 MHz signal that feeds the other side of the Mixer. When the two signals are mixed together, the resulting signal has frequency components at the sum of the incoming frequencies (27.255 + 26.800 = 54.055 MHz) and at the difference of the frequencies (27.255 – 26.800 = 455 KHz.) A ceramic resonant filter rejects the upper sum and passes the lower difference to the Limiter.
The incoming data is encoded as Frequency Shift Keying (FSK), which means that only two discrete frequencies are used. Since only the frequency information is important and not any amplitude changes, the Limiter greatly amplifies the incoming signal until the positive and negative tips of the waveform are flattened off to create a squarewave with no amplitude variation. The Demodulator uses the slope of a tuned circuit set near 455 KHz to change the frequency steps into voltage amplitude steps that represent the CAB-1’s encoded digital ones and zeros. The Audio Amplifier boosts this small voltage to about .5 volts P-P.
The boosted signal is fed to a lowpass filter and limiter circuit using one section of the quad comparator LM339 U2 (pins 10,11 input & 13 output). Test Point #3 displays the signal
before filtering and limiting, and Test Point #4 is the signal after filtering and limiting. The conditioned digital signal is fed to Pin 26 of the microcontroller (uC) for final decoding.
The Powermaster is controlled by an 18-pin PIC 16F84 uC. In addition to the FSK data from the receiver, the uC also receives information regarding current overloads and the timing of the zero crossings of the 60 Hz power sinewave. The PC board also has provisions for a squelch control from the radio chip, but a jumper connecting this signal to the uC is not installed in the circuit board.
The zero crossing detector applies the full 18 volts of the input power through a 100K resistor to a section of quad comparator U1 (pins 6,7 input & 1 output). A pair of parallel back-to-back silicon diodes connected to ground clips the input voltage to +/- .7V to avoid overdriving the comparator’s input. The comparator’s output produces a 5V squarewave synchronized to the AC line on Pin 12 of the uC.
The overload detector senses the voltage drop across a resistance wire in series with Pin 2 of the power input connector. Since Pin 2 connects to the Ground reference of the Powermaster, the voltage drop across the series resistance wire is a small AC voltage directly referenced to ground. This voltage is amplified by another section of quad comparator U1 (pins 4,5 input and 2 output). The comparator’s noninverting input is biased to .18V. Whenever the sensing voltage from an overload current of more than 7.5 Amps exceeds this .18V threshold, the comparator’s output toggles Pin 17 low on the uC, initiating an output shutdown.
Note that only overloads on the positive half of the sinewave are detected! This means that an overload starting on the negative half can cause large currents without any protection. It is only when the positive half of the cycle comes along that the uC will know that anything is wrong. The net result is that power output FET Q2 has not protection from the initial overload burst.
The .18V threshold was appropriate for the 135 watt Powerhouses originally used with the Powermasters. When the 180 watt PH version was introduced, Lionel added a slide switch to the Powermaster to optionally increase the threshold voltage of the sensing comparator by about 30% to allow 10 amp peaks. (I have modified older units to the higher current by shortening the loop of resistance wire. This drops less voltage across the sensing resistor, providing a slightly higher output voltage under heavy load. This modification would not be appropriate if the 135 watt bricks are used.)
The uC also receives status inputs from the Program/Run and Conventional/Command (Hi/Lo) control switches on pins 1 and 18, respectively.
The uC controls two LEDs to indicate proper operation. The green “Status OK” LED is illuminated by a pull-down on Pin 7, and the red “Incoming Data” LED is controlled by a pull-down on Pin 2.
The uC’s clock is generated by an 8 MHz ceramic resonator connected to Pins 15 & 16.
The “business end” of the uC is the power control output on Pin 8. When this pin goes High, Q4 and Q5 are turned ON, raising the gate drive circuit to +40V to turn ON the enhancement mode FETs. Since the FETs are connected in opposite polarities, only one FET will be in the active conduction mode, while the other FET is OFF, but its commutating diode will carry reverse current. The FETs swap roles for alternating halves of the AC waveform.
If the Emergency Stop button fails to kill the output voltage, one or both FETs have failed. The FETs usually fail in the shorted or ON state. If one FET fails, half the AC power will leak through. The red knob on the CAB-1 will be able to raise the power from half voltage to full voltage, but the voltage cannot be cut off completely. If both FETs short, the output will always be at full voltage.
The FETs can be tested with a digital ohmmeter. Connect the ohmmeter between Terminal “A” and the center pin of the 3-pin, and set the meter to the “Diode” symbol for testing semiconductor junctions. If the red meter lead is on Pin 2 and the meter reads about 550, Q2 is shorted. If the black meter lead is on Pin 2 and the meter reads 550, Q1 is shorted. If the meter reads near zero for both polarities, both FETs are shorted.
When replacing the FETs, use a small amount of heatsink grease to uniformly coat the back of the FET. Too much grease is not good. More isn’t better in this case!
I recommend adding a Transient Voltage Suppressor (TVS) near the output terminals in the position marked “VR2” on the schematic above and on some circuit boards. I have seen one Powermaster with a blown TVS in that position, but the TVS was much smaller than the 1500 watt version I use. The 1.5KE36CA TVS will fit in the space, but the leads are a bit too thick to fit the holes. I use a drill to enlarge the holes to .043” diameter (#57 drill) or to add a hole in the pad of R28 if one isn’t present.
For the following waveforms, the lower waveform is the incoming AC sinewave for reference purposes, and the upper trace is the controlled voltage feeding the track at Terminal “A”. The power settings advance from right to left on both halves of the waveform. The whistle/horn and bell advance one half and retard the other to get a DC component without changing the overall power.
The initial small section of slope after each zero crossing helps to activate the E-unit in conventional engines.
Setting a STALL voltage causes the waveform to jump to a value higher than the first step when the knob is initially turned clockwise to achieve the voltage necessary to start moving the locomotive.
Adding MOMENTUM causes the controller to slowly step through all the intermediate voltage steps whenever an abrupt throttle change is made.
OFF First throttle step
Horn/Whistle and Bell at first throttle step
Midway Throttle settings
Horn/Whistle at Full Throttle Bell at Full Throttle
The power supply consists of two parts – a simple half-wave rectifier feeding a 5V regulator, and a voltage doubler boosting circuit that provides approximately +40V to drive the gates of the power FETs. The “hot” side of the power supply is driven by Power Input pin #3, which ties directly to the “U” output terminal. The “Low” side of the AC input on pin 2 serves as the ground reference for the Powermaster’s circuits.
The 5V regulator circuit uses half-wave rectification through D7 into energy storage capacitor C22 to feed the input side of 3-terminal 5V regulator U4.
The boost circuit charges C26 through Zener diode D15 (10V diode) to about 14 volts on the negative half of the AC cycle. On the positive half, this 14 volts is transferred to C27 to be added to the half-wave supply’s 24V to yield about 40 volts.