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LM317 adjustable regulator

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

The LM317 keeps exactly 1.25 V between OUT and ADJ. R1 across that gap passes a fixed current, which also flows through R2, so V_out = 1.25 V × (1 + R2/R1) + I_adj · R2.

LM317 adjustable regulator: the circuit on the simulator's bench

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How it works

Vout = 1.25 × (1 + 1.5 kΩ / 240 Ω) + 50 µA × 1.5 kΩ = 9.137 V

Heat = (Vin − Vout) × I = (15 − 9.14) × 96.63 mA = 566 mW; it needs about 2 V of headroom.

Try it on the bench

  1. Turn the R2 pot (scroll over it): the output follows the formula from 1.25 V up to about 13 V.
  2. Set R_L to 10 Ω and take the LM317 off its heatsink: watch the junction heat up and the thermal shutdown cut in.
  3. Turn R2 up until V_out passes V_in − 2 V: it drops out of regulation.

Parts on the bench

In the course

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

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