Based on a Dead Rail Society Facebook post by Rich Steenwyk, I investigated the suitability of using the Pololu TB9051FTG ($11.95, 2.6A continuous, 1″ x 1″) and the Pololu TB67H420FTG ($19.95, 3.4A continuous, 1″ x 1.2″) as a DCC amplifier in conjunction with the ProMiniAir Receiver for Dead-Rail operation. This post investigates their feasibility and shows connection details.
Feasibility and Connections
The TB9051FTG truth table below shows the proper bipolar operation on highlighted rows. Note the pin values for EN and ENB. Based on this truth table, this device should be capable of delivering bipolar DCC to the decoder.
Connections for the Pololu TB9051FTG are shown below. When connected to a “large” decoder such as the LokSound 5 XL shown here, a current-limiting resistor is required to prevent the TB9051FTG from shutting down. The Negative Temperature Coefficient (NTC) resistor is initially at 1 Ohm, but with increased current demand, it heats up, reducing its resistance and voltage drop to a low value. This solution is superior to a high-wattage constant 1 Ohm resistor CVP recommends for its smaller Airwire CONVRTR receivers.
A close-up of the connections for the Pololu TB9051FTG is shown below. Bipolar 3.3V DCC outputs and +5V from the ProMiniAir Receiver are required for proper operation. Note the jumpers that set EN to VCC (High), ENB to GND (Low), and OCC to VCC (High, retry after shutdown).
Example DCC output for the decoder provided by the ProMiniAir Rx/TB9051FTG combination is shown below.
A demo of the Pololu TB9051FTG in dead-rail operation, which will NOT operate with the LokSound 5 XL (5A max) without a current-limiting resistor, is shown below.
Now let’s turn to the larger, more expensive TB67H420FTG.
The TB67H420FTG truth table below shows the proper bipolar operation on highlighted rows. Note the value of PWMx must be High. Based on this truth table, this device should be capable of delivering DCC to a decoder.
Connections for the Pololu TB67H420FTG are shown below. When connected to a “large” decoder such as a LokSound 5 XL, a current-limiting resistor is NOT required to prevent the TB67H420FTG from shutting down. However, the TB67H420FTG is more expensive and larger than the TB9051FTG.
A close-up of the connections for the Pololu TB9051FTG is shown below. As with the TB9051FTG, bipolar 3.3V DCC inputs from the ProMiniAir Receiver are required for proper operation. Note the single jumper connecting VCC (High) to HBMODE that turns on the single-output function. Jumpers between A+/A- and B+/B- deliver the maximum single DCC output of 3.7A.
Testing with the TB9051FTG when interfaced with a ProMiniAir Receiver was successful.
Conclusion
Both the Pololu TB9051FTG ($11.95, 2.6A, 1″ x 1″) and TB67H420FTG ($19.95, 3.4A, 1″ x 1.2″) can be configured to deliver full-power DCC to a decoder when used in conjunction with a Dead-Rail receiver such as the ProMiniAir Receiver. They require five connections to the ProMiniAir receiver, including 3.3V or 5 V bipolar DCC outputs. The lower power TB9051FTG does require a current-limiting resistor for some decoders that produce large “in-rush” currents during power on, and the TB9051FTG does not.
The Adafruit DRV8871 amplifier is perhaps a better choice because:
- Cheaper: $7.50+s&h
- Comparable output: 3.6A
- Smaller: 1″ x 0.8″
- Requires only four connections to the receiver: GND, VPOWER, DCC+, and DCC-
The disadvantage of the Adafruit DRV8871 and the Pololu TB9051FTG is they require a current-limiting resistor for some larger decoders. The Pololu TB67H420FTG does not need this current limiter, but it’s slightly larger and more expensive. You decide!
