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12 V → 5 V regulators

Power, transformers and three-phase · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

Three ways to make 5 V from 12 V, each driving the same 10 Ω load. A linear 7805 burns the difference as heat, so its efficiency can never beat V_out / V_in. The switching bucks (LM2596, HW-688) stay above 80 %.

12 V → 5 V regulators: the circuit on the simulator's bench

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How it works

7805: Iin ≈ Iout = 5 V / 10 Ω = 500 mA, so Pheat = (Vin − 5 V) · I = 3.5 W and η ≈ 5/12 = 41.7 %

Junction temperature ≈ 25 °C + P · θJA = 25 + 3.5 W × 12 °C/W = 67 °C (shuts down at 150 °C). It needs about 2 V of headroom, so Vin ≥ 7 V.

Bucks: Pin = Pout / 0.86 + 60 mW of controller overhead, so the input current is well below the output current.

Try it on the bench

  1. Select the 7805 and take it off its heatsink. Watch the junction climb to 150 °C, shut down and cycle.
  2. Raise V_in to 24 V. The 7805 heat more than doubles, while the bucks barely notice.
  3. Lower V_in to 6 V. The 7805 drops out below 5 V, and the HW-688 stops under 7 V.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Diodes & Power Supplies”.

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