Older schematics omit the calibration trimpot entirely. Without RV1, the tester is unreliable for different inductance ranges. Our schematic includes a in series with R5. Calibration procedure:
: Because different transformers have different native Q factors, it is often best to compare readings against a known good component . blue ring tester schematic diagram exclusive
A timer circuit (often using a U1 timer or discrete transistors) creates the fast voltage pulse required to "kick" the inductor into resonance. Older schematics omit the calibration trimpot entirely