[ §4 · calculators · self-oscillation ]
Self-oscillation frequency
calculators270 pF fitted + 10 pF stray = 280 pFFrequency ≈ 496 kHzWith ±15 % spread: 422–571 kHz Measure at comparator pin 7 with the bridge unpowered.
For a modified UD2.x with R7 removed and 50 kΩ added. Leave the target frequency empty to go forwards from a capacitor value, or fill it to go backwards.
formula
τ = C · (R26 + R5‖R6) = C · 790 Ω half period = τ · ln(A/Δ) ≈ 4.57·τ f [kHz] ≈ 139 000 / C [pF] C including the stray of the layout
limits
Accurate to ±10–15 %. The 4.57 is ln(A/Δ) and depends on an offset Δ ≈ 21 mV that was estimated, not measured. At Δ = 32 mV the constant would be 152 000 rather than 139 000.
Checked against hardware: Mathieu’s 300 pF measured 450 kHz and the formula with 10 pF of stray gives 448; Rafft’s 200–330 pF gave 446–470 kHz.
Treat it as the estimate for the first fit and trim on the scope. The capacitor must be C0G/NP0. X5R and X7R drift.