TL;DR — A magjack is an RJ45 jack (母座) with the Ethernet magnetics built inside the connector. One part = socket + isolation transformer + common-mode choke. VOOHU makes these as jacks; here's what matters.
What's inside:
- 1CT:1CT isolation transformer (turns ratio 1:1) — galvanic isolation + DC block
- Common-mode choke — EMI / common-mode rejection
- Shielded metal housing + integrated link/activity LEDs
- Phosphor-bronze C5210 contacts, gold on the contact area, PBT UL94 V-0 body
Why use it instead of discrete magnetics:
- Saves board area, shortens the PHY-to-connector trace
- Fewer placements on the line
- Same electrical job as a discrete module (IEEE 802.3 needs 1500 Vrms isolation)
How to spec it (read the part number):
- Data rate: 10/100M, 100/1000M, 2.5G/5G, 10GBASE-T
- Tab: up or down · Mount: through-hole or SMT
- PoE: non-PoE or 350 mA → 1.5 A per port (bias rides the transformer center taps, so it must not saturate)
- LED code + temperature grade (down to −40 ~ +85 °C)
Real numbers (VOOHU SYT511Q218AC1A8D057, Gigabit):
- 1:1 ±2% turns ratio · 350 µH OCL · ≤1.0 dB insertion loss
- Return loss ≥18 dB low band · crosstalk ≥40 dB · CMR ≥30 dB
- 2250 VDC isolation · ≥750 mating cycles (IEC 60603-7)
Gotchas:
- Don't drop a non-PoE magjack into a PoE port (saturation + heat)
- Match turns ratio / OCL to your PHY, not just the footprint
- Confirm panel cutout + PCB hole pattern (most are through-hole sink-type)
Full write-up + selection matrix: Integrated-Magnetics (Magjack) RJ45 Jacks Explained
— VOOHU Electronics · voohuele.com · olivia@voohuele.com
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