OCL in Network Transformers: The Spec That Determines Everything

One spec. Two numbers. More design failures than any other parameter. Here's everything about OCL you need to know.
What OCL is
Open Circuit Inductance (OCL):
The inductance of one transformer winding,
measured with the other winding left OPEN (disconnected).

Standard test condition: 100kHz, 0.1V RMS
Unit: µH (microhenries)
Why it matters: the low-frequency cutoff
The transformer winding inductance creates a high-pass filter with the circuit impedance:
f_cutoff ≈ R / (2π × L)

For 100Ω circuit with 350µH OCL:
f_cutoff = 100 / (2π × 0.00035) ≈ 45 kHz ← below Ethernet signal range ✓

For 100Ω circuit with 350µH in a Gigabit design (needs 1000µH):
Required OCL by speed — the numbers that matter

⚠️ 350µH transformer in Gigabit design = 2.65× below minimum OCL
Symptoms: link fallback to 100M, packet loss under load,
intermittent disconnects, range reduction
Root cause: rarely diagnosed immediately → wastes days
How to verify OCL with an LCR meter
Setup:

1. LCR meter → Inductance (L) mode
2. Frequency: 100kHz
3. Signal level: 0.1V RMS (if selectable)
4. Probe: across one winding terminals
5. Other winding: OPEN (nothing connected)

Pass criteria:
✓ 10/100 component: reading ≥ 350µH
✓ Gigabit component: reading ≥ 1000µH
✗ Below minimum: reject lot, contact supplier
Temperature effect on OCL

Ferrite permeability decreases with temperature

→ OCL decreases as temperature rises

Example (typical behavior):

temp_vs_ocl = {
-40: 420, # µH — higher at low temp
25: 380, # µH — room temperature (datasheet "typical")
70: 345, # µH — near minimum at commercial max temp ← check this!
85: 320, # µH — below minimum! Industrial parts must spec this range
}

Key: always verify minimum OCL at maximum operating temperature,

not just room temperature. A "typical" room-temp number is insufficient.

Sourcing
Voohu Technology (voohuele.com) stocks verified 10/100 (≥350µH) and Gigabit (≥1000µH) network transformers. OCL test reports available. 50pcs min, DHL 3–5 days to Japan/Korea/SEA.