Power Implementation (48V Central Supply)

Overview

The Ma Bell Gateway system is powered by a single 48V/60W DC supply, with all required operating voltages generated by a combination of DC-DC converters and a ring generator circuit.

Key voltage rails and their uses:

  • -48V for the subscriber loop (on-hook voltage)

  • 12V for SLIC (HC3-5504B-5) chip primary power

  • 5V or 3.3V for the ESP32 microcontroller and logic

  • 90V AC, 20Hz for authentic telephone ringer operation

Power Architecture

#.. figure:: /_static/power-architecture.png # :alt: Power supply block diagram # :align: center

(Placeholder: insert PNG/SVG block diagram here)

   +----------------+
   | 48V DC Supply  |
   +-------+--------+
           |
   +-------+--------+-------------------+-------------------+
   |       |        |                   |                   |
   v       v        v                   v                   v
[12V Buck][5V Buck][Inverting DC-DC][Ring Generator]    (Future Option)
   |        |            |                  |
   v        v            v                  v
SLIC VCC  ESP32/Logic  SLIC Tip/Ring      Ringer

Step-by-Step Power Flow

  1. 48V Supply as Master Power

    • The central 48V/60W power brick is the main power source.

    • All subsequent rails are derived from this supply.

  2. -48V for On-Hook Loop

    • An inverting DC-DC converter generates a stable -48V rail referenced to ground.

    • Supplies the SLIC’s tip/ring pair for line voltage during on-hook state.

  3. 12V for SLIC Primary Power

    • A DC-DC buck converter steps 48V down to 12V.

    • Powers the VCC/VDD pins on the HC3-5504B-5 for analog functions.

  4. 3.3V for ESP32, Audio Codecs & Logic

    • Another buck converter steps down 48V to 3.3V regulated output.

    • Feeds the ESP32 dev board (~500mA), PCM5100 DAC (~10mA), PCM1808 ADC (~15mA), and digital logic.

    • Minimum current capacity: 600mA (typical load ~525mA + 15% margin).

  5. 90V AC, 20Hz for Ringer

    • The 48V supply feeds a ring generator module that creates a 90V AC, 20Hz waveform.

    • Used for authentic operation of electromechanical ringers.

Power Rail Table

Rail

Voltage

Typical Current

Power

Supplies

How Created

Vbat

+48V DC

250-350mA

12-17W

All conversions

Main supply (60W)

SLIC Loop

-48V DC

50-100mA

2.4-4.8W

SLIC (Tip/Ring)

Inverting DC-DC from 48V

SLIC Power

+12V DC

100mA

1.2W

SLIC (VCC)

Buck converter from 48V

Logic & Codecs

+3.3V DC

525mA (600mA min)

1.7W

ESP32, PCM5100, PCM1808

Buck from 48V

Ring AC

90V AC, 20Hz

20-40mA RMS

2-4W

Ringer circuit

Ring gen. from 48V

Note

Illuminated Dial Power: Separate 9V AC Supply

The 9V AC required for the phone’s illuminated dial is not derived from the main power system. Instead, a separate 9V AC wall transformer is used, with its output fed directly to the black and yellow wires of the phone. This mirrors the original Bell System approach, where illumination was powered independently from the telephone line circuitry.

Power Budget Analysis

The total system power requirement can be calculated from the individual rail loads shown in the Power Rail Table above.

Continuous Load (Idle State)

When the phone is on-hook and no ringing is occurring:

  • 3.3V rail (ESP32 + codecs): 1.7W

  • 12V rail (SLIC VCC): 1.2W

  • -48V rail (SLIC loop, idle): 2.4W

  • Ring generator: 0W (inactive)

Total idle power: ~5.3W from 48V supply = 110mA @ 48V

Peak Load (Active Call + Ringing)

When the phone is off-hook (active call) and ring generator is simultaneously active (worst case):

  • 3.3V rail (ESP32 + codecs): 1.7W

  • 12V rail (SLIC VCC): 1.2W

  • -48V rail (SLIC loop, off-hook): 4.8W

  • Ring generator (90V AC @ 20Hz): 4.0W (peak)

Total peak power: ~11.7W from 48V supply = 244mA @ 48V

Including Converter Efficiency

Buck and inverting DC-DC converters are not 100% efficient. Typical efficiency: 80-90%.

Assuming 85% average efficiency:

  • Idle: 5.3W / 0.85 = 6.2W from 48V supply

  • Peak: 11.7W / 0.85 = 13.8W from 48V supply

Why 60W Rating?

The 48V/60W supply provides a 4.3x safety margin over the calculated 13.8W peak load. This generous headroom offers several benefits:

  1. Future Expansion: Supports additional features (multiple lines, amplified ringers, status displays)

  2. Thermal Management: Operating at 23% of rated capacity keeps the supply cool and extends lifetime

  3. Inrush Current: Handles startup transients when all converters initialize simultaneously

  4. Component Selection: 60W supplies are readily available and cost-effective

  5. Reliability: Operating well below rated capacity improves long-term reliability

Alternative: A 48V/25W supply (providing 1.8x margin) would be adequate for single-line operation if cost or size is critical.

Component Notes

Buck Converters (3.3V & 12V)

Compact DC-DC buck modules are recommended for stepping down from 48V:

  • For 12V rail (SLIC power):

    • Recommended: DROK 48V to 12V Buck Module (~$10)

    • Input: 15-55V DC (covers 48V with margin)

    • Output: 12V DC, 1.5A capacity

    • Compact DIP module, easy breadboard integration

  • For 3.3V rail (ESP32, audio codecs):

    • Option A: Cascade from 12V using standard LM2596 3.3V module (~$3-5)

    • Option B: Use wide-input buck module (e.g., XL4015-based) directly from 48V

    • Output: 3.3V DC, minimum 1A current rating

    • Add 0.1µF + 10µF decoupling at output for audio codec power quality

Inverting DC-DC (-48V)

Note: This component selection is deferred pending further research for a cost-effective solution.

  • Requirement: -48V DC, 150mA minimum (7.2W) for SLIC loop voltage

  • Challenge: 48V-to-48V isolated modules at appropriate power levels are expensive (~$50-70)

  • Options under consideration:

    • Isolated DC-DC module with output referenced to create -48V

    • Dual bench supply approach for prototyping

    • Alternative inverting topologies

Ring Generator (90V AC @ 20Hz)

The ring generator uses the LT1684 IC from Analog Devices:

  • IC: LT1684 micropower ring tone generator

  • Input: PWM signal from ESP32

  • Output: 90V RMS @ 20Hz sine wave

  • Power: ~4-7W when ringing is active

  • Control: ESP32 GPIO provides PWM for frequency/amplitude control

For complete ring generator implementation details, see Ring Generator.

Design Considerations

  • The SLIC’s -48V is used only for line signaling on tip/ring, not for chip logic.

  • All low-voltage logic (ESP32, digital control) shares a common ground with the SLIC and power supply, ensuring safe logic-level interfacing.

  • The SLIC chip safely handles the interface between high-voltage line loop signals and your low-voltage control logic. Its output pins for status (such as off-hook detect or ring sense) are referenced to the same ground as your ESP32, so no additional optocoupler isolation is required between the SLIC and your microcontroller.

  • The ring generator and DC-DC converters must be sized to meet the maximum expected load (60W should be ample for most single-line systems).

  • For prototyping, use standard DC-DC modules; for production, design custom PCB layouts for compact integration.

Note

Why Avoid the Telco’s “Flip-the-Ground” -48V For Line Loop

Classic telephone systems often created -48V for the line loop by tying the positive terminal of the 48V supply to system ground, making the negative terminal act as “-48V.” While simple and authentic, this approach “floats” the project ground 48V above earth ground.

For this Ma Bell Gateway implementation, we use an inverting DC-DC converter to generate a true -48V relative to ground. This ensures all project circuits—including the SLIC, ESP32, and logic—reference true earth ground, keeping development and testing safe.

If you use the classic telco trick and connect any test gear (oscilloscope, logic analyzer, USB device) referenced to earth ground, you risk creating a dangerous short circuit and damaging equipment. The inverting DC-DC approach keeps all bench measurements and expansion safe and reliable.