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Power supplies and regulators
Transformer, rectifier, filter, regulator; linear versus switching.
Transformer, rectifier, filter, regulator; linear versus switching.
A power supply turns the AC from the wall into the steady, low-voltage DC that radio equipment needs, most often 13.8 V for 12 V gear. How clean that DC is decides how much hum, noise and heat you get.
Many supplies add a bleeder resistor across the output so the big capacitors drain after switch-off, because their stored charge can hurt; see Electrical safety.
The capacitor sags between peaks. The rule of thumb for the size of that sag, ripple ≈ I ÷ (f × C), slightly overestimates it: 1 A, 4700 µF and 120 Hz gives about 1.8 V, where the simulation above gives about 1.5 V. Ripple falls with a bigger capacitor, a smaller load or a full-wave rectifier, which doubles the frequency. Unwanted ripple is heard as a 120 Hz hum on the radio's audio or on its own transmitted signal.
The regulator removes what is left, but only while its input stays above the output by a minimum dropout voltage; if the ripple valleys fall below that, the regulator can no longer hold the output and the hum comes through.
A linear regulator is a transistor in series with the load, adjusted to drop away whatever the input has in excess. All of that excess becomes heat. A supply feeding 20 A at 13.8 V from an 18 V input wastes (18 − 13.8) × 20 = 84 W, an efficiency of 13.8 ÷ 18 = 77%. It is simple and quiet but needs a heavy 60 Hz transformer and large heatsinks.
A switching supply instead chops the DC rapidly, typically tens to hundreds of kilohertz, and averages the result with an inductor and capacitor; the on-time fraction (duty cycle) sets the output. A transistor that is either fully on or fully off wastes little, and the transformer at that frequency is small, so the supply is light and runs cool.
| Linear | Switching | |
|---|---|---|
| Efficiency | falls as input exceeds output | typically high, 80% to 95% |
| Size and weight | heavy | compact |
| Heat | considerable | modest |
| RF noise | very low | can be a problem |
The price of switching is noise. The fast edges and their harmonics extend up through the HF bands and beyond, and radiate from the supply and its cables. They show up as birdies or a raised noise floor on the receiver, and sometimes as interference to nearby equipment. Mitigations are shielding, filtering and ferrites on the DC and AC leads, and shorter cables; a supply that is quiet on the bench can still be noisy on your bands, so judge by listening. See RFI and EMI overview and Power-line and switching noise.