Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers four power supply designs for amateur radio stations: a 13.8V linear regulated supply (0–30A), a switching SMPS supply (13.8V, 20A, compact), a LiFePO4 field portable pack (12.8V, see also TM-GEAR-004 for BMS details), and a metering and protection module (voltage, current, RF, over-current protection).

Chapter 2 — Theory of Operation

2-1 Linear Regulated Supply

A transformer (120V/18V, 40VA) followed by a full-wave bridge rectifier and large filter capacitor (10,000–20,000 µF) provides unregulated DC. A series-pass transistor (multiple 2N3055 or IRF540 in parallel) regulated by an op-amp error amplifier maintains constant output voltage. Linear supplies have extremely low ripple and noise (<1 mV RMS) at the cost of lower efficiency (∼50%) and heat dissipation.

2-2 Switching SMPS

An SMPS converts AC to DC at high frequency (50–500 kHz), allowing much smaller transformers. Efficiency: 85–92%. The PWM controller (UC3842 or SG3525) regulates output voltage by adjusting duty cycle. EMI filtering is critical for SMPS; they generate conducted and radiated interference that can degrade receiver performance.

2-3 Metering and Protection

A metering module uses a series current shunt (100 A / 75 mV shunt) and a voltage divider to display voltage and current on a CYD display. Over-current protection uses a comparator circuit to trip a power relay if current exceeds the set limit (adjustable 0–30 A via a trimpot).

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 Linear Supply Heatsink Sizing

The series-pass transistors dissipate: P = (V_in − V_out) × I_out. At worst case (13.8V output from 18V rectified): P = (18 − 13.8) × 30 = 126W. With four 2N3055s, each dissipates 31.5W. 2N3055 thermal resistance junction-to-case: RΘjc = 1.5°C/W. Required heatsink resistance: RΘsa ≤ (T_j_max − T_a) / P − RΘjc = (150 − 50) / 31.5 − 1.5 = 1.7°C/W per transistor.

4-2 SMPS EMI Filtering

1. Install an IEC inlet filter (common-mode choke + X/Y capacitors) on the AC input before the SMPS. This keeps SMPS switching noise off the mains.
2. Add a ferrite bead choke (2 turns of DC output cable through #31 toroids) on the DC output to suppress conducted emissions from reaching the transceiver.
3. For receivers: use the linear supply; the SMPS may interfere with sensitive RX even with filtering.

Chapter 5 — Operating Procedures

1. Linear supply: allow 5 minutes warm-up before setting critical output voltage. Adjust the voltage trimmer to 13.8V (verified with DMM). Set current limit to 5A above the transceiver’s peak draw.
2. SMPS: verify output voltage before connecting transceiver. Do not exceed rated current; SMPS enter fold-back current limiting and may drop voltage abruptly under severe overload.
3. LiFePO4 portable: check pack voltage before operation. Below 12.4V (SOC ∼20%), plan for recharging soon. Do not discharge below 10.0V.

Chapter 6 — Calibration

1. Calibrate voltage: set output with a precision voltmeter (4.5 digits or better). Target: 13.800 ± 0.050V.
2. Calibrate current meter: apply a known resistive load and measure load current with an external shunt and DMM. Adjust current meter shunt gain in firmware until displayed value matches.
3. Calibrate over-current trip: increase load until the trip circuit activates. Trip current must be within 0.5A of the set value.

Chapter 7 — Verification and Acceptance

1. Output voltage: 13.8 ± 0.1V at no load and full rated load.
2. Ripple: <10 mV peak-to-peak (linear); <100 mV pk-pk (SMPS) measured with oscilloscope at full load.
3. Current limit: trips within 0.5A of set value on all supplies.
4. SMPS EMI: with transceiver in receive mode, key the supply at full load. Noise floor must not increase by more than 1 S-unit at any HF frequency.
5. Log: date, supply type, measured voltage at no load and full load, ripple measurement, current limit trip point, operator.

Appendix A — Voltage Drop at Load

For a linear supply, the voltage drop across series wiring:
  V_drop = I × R_wire
  For #14 AWG, R = 8.4 mΩ/ft (2.76 mΩ/m)
  At 30A over 3m of #14 AWG (6m round trip):
  V_drop = 30 × 0.00276 × 6 = 0.50V
  This means the bench at the end of 3m of #14 AWG gets 13.8 − 0.5 = 13.3V.
  Use #10 AWG for long runs or supply >20A.

Appendix B — Fuse Sizing