Ham Radio Power Supply Sizing Calculator
Calculate the right power supply size for your amateur radio station based on transmitter power and accessories.
Sizing a power supply for amateur radio
Most HF and VHF amateur radio transceivers run on 13.8 VDC. The power supply must provide enough current (amps) for peak transmit load plus any accessories. An undersized power supply causes voltage sag during transmit, distorting your signal and potentially damaging the radio.
Basic formula:
Current draw (amps) = Transmit power (watts) ÷ (Supply voltage × Efficiency)
Typical transceiver efficiency during transmit is 40–60% depending on the mode and power class.
Total current = Radio current + Accessory current
Recommended PSU = Total current × 1.25 (25% headroom)
Typical current draw by radio power class:
| Radio Class | TX Power | Approx TX Current at 13.8V | Receive Current |
|---|---|---|---|
| QRP (5W) | 5 W | 2–3 A | 0.5–1 A |
| Mobile (50W) | 50 W | 10–12 A | 1–2 A |
| HF 100W | 100 W | 20–23 A | 2–3 A |
| HF 200W | 200 W | 35–40 A | 2–3 A |
Common accessories and their current draw:
| Accessory | Current Draw |
|---|---|
| Antenna tuner (automatic) | 0.5–1 A |
| External speaker | negligible |
| Digital interface (SignaLink, etc.) | 0.1–0.3 A |
| Cooling fan (external) | 0.5–1.5 A |
| LED desk lamp | 0.3–0.5 A |
| Rotor controller | 1–3 A (when moving) |
| Linear amplifier bias | varies (check manual) |
Worked example — 100W HF station:
Radio: Icom IC-7300 (100W HF, draws 23A peak on SSB transmit) Accessories: Auto-tuner (1A) + digital interface (0.3A) + cooling fan (1A)
Total peak current: 23 + 1 + 0.3 + 1 = 25.3 A With 25% headroom: 25.3 × 1.25 = 31.6 A
You need a 30A continuous-rated power supply (like the Astron RS-35A or Samlex SEC-1235).
Continuous vs. peak ratings:
Power supplies are rated for both continuous and peak (surge) current. Always size based on the continuous rating. A “30A” supply with 30A peak but 25A continuous is really a 25A supply for your purposes. SSB voice transmit creates repetitive peaks that are effectively continuous load during a conversation.
Linear vs. switching power supplies:
Linear supplies (like Astron RS-series) produce very clean DC with minimal RF noise, but are heavy and less efficient. Switching supplies (like Samlex, MFJ, Meanwell) are lighter and cheaper but can introduce RF noise on HF bands. If using a switching supply, look for models specifically designed for amateur radio with good RF filtering.
Wire gauge for DC connections:
| Distance (one way) | Up to 20A | Up to 35A | Up to 50A |
|---|---|---|---|
| Under 1m (3ft) | 12 AWG | 10 AWG | 8 AWG |
| 1–3m (3–10ft) | 10 AWG | 8 AWG | 6 AWG |
| 3–5m (10–16ft) | 8 AWG | 6 AWG | 4 AWG |