---
title: "W5500 Implementation With External Transformer"
url: "https://maker.wiznet.io/forum/16096"
markdown_url: "https://maker.wiznet.io/forum/16096/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "mlobomx"
created: "2026-06-16T15:09:32+09:00"
last_activity: "2026-06-16T15:09:32+09:00"
language: "en"
tags: ["W5500"]
views: 5
replies: 0
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# W5500 Implementation With External Transformer

## Question

Asked by mlobomx on 2026-06-16 in Ethernet Chips.

I have some questions regarding the integration of the W5500 with an external transformer (without PoE).

I've seen the WIZnet reference designs for the W5500 with both an integrated transformer in the MagJack and with an external transformer. However, I've noticed that the resistors for the differential pair termination impedance of the MDI Rx cable change from 49.9 Ohms (in the MagJack) to 82 Ohms (with an external transformer).

I understand that the characteristic impedance of the differential pair for Ethernet should be 100 Ohms, so two 49.9 Ohm resistors would meet this requirement. However, in the case of the 82 Ohm resistors, this changes to 164 Ohms, which seems odd. Furthermore, on the RJ45 side after the transformer, the same differential pair Rx now also has something resembling a termination resistor, consisting of two 150-ohm resistors. Its center connection is branched off to the same 1nF, 2kV capacitor used for the Bob Smith termination.

I don't understand the reason for these two changes and why the circuit of an internal transformer would be different from an external one when, in both cases, the transformer's ratio and inductance values, as well as the Bob Smith termination, are the same.

Has anyone tested this circuit and had it work?

Reference Schematic for Integrated Magnetics: https://docs.wiznet.io/pdf-viewer?file=%2Fassets%2Ffiles%2Fw5500_sch_v110_use_mag_-dfff8bbb103d13be4f25cd2b2f8b950d.pdf
Reference Schematic for External Magnetics: https://docs.wiznet.io/pdf-viewer?file=%2Fassets%2Ffiles%2Fw5500_sch_v110_use_trans_-18b3297138bdf20fafd0f95e4c19358d.pdf

---

Source: https://maker.wiznet.io/forum/16096