Before power MOSFETs and thyristors dominated, engineers built magnetic amplifiers ("mag-amps") — devices that used the nonlinear B-H curve of a saturable core to gate AC power with a small DC control current. They're still around today as post-regulators in multi-output switching supplies, where they trim a secondary rail without adding an entire switcher.

The principle: a toroidal core with square-loop material (like Metglas 2714A or square-loop permalloy) has an inductance that collapses from very high to nearly zero the instant flux reaches saturation (B_sat). Wind a power winding carrying AC and a DC control winding on the same core. When the control current biases the core closer to saturation, the AC winding saturates earlier in each half-cycle, and load current begins flowing sooner. The result is a phase-controlled AC output — but instead of a triac firing at a gate pulse, the core itself decides when to "open the gate."

Post-regulator example: A multi-output flyback produces +12 V (main, regulated) and +5 V (auxiliary, sloppy). Insert a saturable inductor in series with the +5 V rectifier. Every switching cycle, the inductor blocks current until it saturates — the delay is set by how close to saturation a small control winding has biased the core. A simple error amp comparing +5 V to a reference drives the control current. You get tightly regulated +5 V with ~95% efficiency, no extra switch.

Sizing rule of thumb: The volt-second product the core must block before saturating equals N · A_e · 2B_sat. For a Metglas 2714A core (B_sat ≈ 0.57 T) with A_e = 0.1 cm² and 20 turns:

• V·s capacity = 20 × 1×10⁻⁵ × 2 × 0.57 = 228 µV·s
• At a 100 kHz switcher (10 µs period, 50% duty), max blocking voltage per cycle ≈ 228 µV·s / 5 µs = ~45 V — plenty for a 5 V rail.

Design caveats:

• Square-loop cores are expensive — Metglas and nanocrystalline tapes cost 10× ordinary ferrite.
• Core loss climbs steeply near saturation; keep switching frequency below the material's rated limit (usually 100–500 kHz).
• Reset each cycle. Without an explicit reset winding or freewheel path, residual flux drifts and regulation collapses.
• Response is slow — mag-amp loops typically bandwidth-limited to a few kHz.

Historically, mag-amps ran everything from radar transmitters to naval fire-control computers. Today they're niche, but unbeatable when you need a robust, radiation-tolerant, silicon-free regulator on an auxiliary rail.