DEV Community: geoffery Bob

The RJ45 Selection Decision-Making Process

geoffery Bob — Thu, 13 Aug 2026 09:57:04 +0000

Now that we’ve looked at the design of the RJ45, let’s discuss the steps involved in selecting the right model.

Step 1: Determine the speed rating
The speed category determines the transmission bandwidth that an RJ45 connector must support, and directly affects the connector’s insertion loss and return loss specifications. The following selection recommendations apply:

10/100M → Select an RJ45 connector that supports a bandwidth of 100 MHz
1G (Gigabit) → Select an RJ45 connector that supports a bandwidth of 100 MHz and meets the return loss requirements of IEEE 802.3ab
2.5G/5G → Select an RJ45 connector that supports a bandwidth of 200 MHz/400 MHz
10G → Select an RJ45 connector that supports a bandwidth of 500 MHz

Step 2: Determine the PHY driver type

Refer to the PHY data sheet to confirm whether it is voltage-mode or current-mode.
Plan the centre-tap connection according to the drive type:
Current-mode → Connect the centre tap to VDD (3.3V/2.5V/1.8V).
Voltage-mode → Connect the centre tap to earth via a 100 nF capacitor.
If the PHY data sheet does not explicitly specify the drive type, you can make an informed judgement by measuring the DC voltage between the TXD± pins and ground: for voltage-mode, this is approximately VDD/2; for current-mode, it is close to 0 V.

Step 3: Determine PoE power requirements

No PoE required → Select a standard model (non-PoE).
PoE (15.4W) → Select a model supporting 350mA (compliant with IEEE 802.3af).
PoE+ (30W) → Select a model supporting 720mA (compliant with IEEE 802.3at; WoHu models with the suffix ‘DP’).
PoE++ (60W/90W) → Select a model supporting 1.2A/2A (compliant with IEEE 802.3bt; WoHu models with the suffix ‘PG’ or ‘PTG’).

Step 4: Determine the mounting method

DIP (Through-hole) → High mechanical strength; suitable for industrial equipment and vibration-prone environments; requires space on both sides of the PCB.
SMT (Surface Mount Technology) → Suitable for automated reflow soldering production; lower profile, suitable for high-density equipment; mechanical strength is lower than that of DIP.
Low-Profile → Part of the housing is recessed into a slot in the PCB; height can be as low as 8 mm; suitable for 1U/half-U slim-line equipment.
Wire-Bonded (Cable Type) → Wires are soldered directly; suitable for non-board-mounted applications.

Step 5: Determine the environmental rating

Commercial grade (0–70 °C): Offices, homes, controlled environments.
Industrial grade (-40–85 °C): Factories, outdoor environments, in-vehicle applications, rail transport.
Automotive grade (-40–105 °C): Extreme temperature environments such as engine compartments.

Step 6: Identify any special requirements

Waterproof and dustproof: Is an IP67/IP68 rating required (e.g. for outdoor base stations or marine equipment)?
Vibration-resistant locking: Is a metal spring, Cannon self-locking mechanism or threaded locking required (e.g. for rail transport or vehicle-mounted applications)?
LED indicators: Are specific colours (green/orange/yellow), brightness levels or common-cathode/common-anode configurations required?
Shielding requirements: Is a fully shielded enclosure required (for EMI-sensitive environments)?

How does a DIY alarm panel achieve single-wire PoE power supply and data transmission?

geoffery Bob — Tue, 11 Aug 2026 08:08:44 +0000

Today, a novice engineer approached me with a question: he wants to develop an alarm panel and is looking to use PoE to enable a single Ethernet cable to provide both power and data transmission. He hasn’t been able to find a PoE module and would like to know if this is feasible.

His key requirement is for the alarm panel to operate using just a single Ethernet cable, which is, of course, entirely possible; this is a standard application of PoE technology.

Returning to his question, this solution can be integrated onto the PCB using a PoE module, though I would recommend using an integrated RJ45 connector (a version with PoE functionality) in conjunction with an external PD chip, as this reduces PCB layout work and, once soldered, allows power and signals to be routed to the main control chip via the connector's pins.

The PoE Power Supply Process

The PSE first transmits direct current via the network cable.
The PD device (alarm panel) feeds the power into the PoE power transformer at the rear via an integrated RJ45 connector.
The PoE power transformer, in conjunction with the controller, converts the 48V high voltage into the low voltage (5V/12V) required by the alarm panel, thereby supplying power to the main controller and sensors.

Given that the alarm panel does not have high speed requirements, I have compiled a list of a few components as examples for your reference, based on the requirements.

100 Mbps integrated RJ45: SYT111B032DB1A1DPQ

Depending on the device's voltage and power requirements, two PoE power transformers are recommended: (1) 5V/12W: ST-0880WHB (2) 12V/13W: WH13P-12L If your aim is to learn how to design a PoE circuit, VOOHU's reference design is an excellent starting point. If your aim is to quickly build a functional alarm panel, a PoE-enabled development board or a ready-made PoE splitter remains the more straightforward option.

Typical RJ45 Circuit Designs – PHY Driver Types and Centre-Tap Connections

geoffery Bob — Mon, 10 Aug 2026 10:10:11 +0000

Ethernet PHY chips are classified into voltage-mode and current-mode types based on the structure of their output stages. The two types have entirely different requirements regarding the connection of the network transformer’s centre tap; incorrect connection can result in reduced signal amplitude, waveform distortion or even failure to establish a link.

Voltage-Mode PHY

The output stage of a voltage-driven PHY employs a voltage-source configuration and incorporates an internal impedance-matching resistor (typically 50 Ω). Its output impedance is matched to the characteristic impedance of the transmission line (100 Ω differential), and the signal amplitude is determined by the internal voltage source.

Centre-tapped connection:
Must be earthed via a capacitor (typically 100 nF) to isolate the DC component.
**Principle: **The output of a voltage-driven PHY already has a defined DC offset point; the centre-tapped ground capacitor provides an AC coupling path, ensuring that the signal across the primary winding of the transformer is symmetrical with respect to ground.
**Typical chips: **Realtek RTL8211F, Intel LXT972, Jinglue JL2101C.

(2) Current-Mode PHY

The output stage of a current-mode PHY employs a current-source configuration, requiring an external resistor to convert the current into a voltage. The centre tap of the transformer provides a DC bias path, enabling the differential current to flow through the primary winding of the transformer to form a complete AC loop.

Centre-tapped connection: **This must be connected directly to the PHY’s supply voltage (VDD, typically 3.3 V, 2.5 V or 1.8 V), and a capacitor to ground (0.01 μF to 0.1 μF) must be used for high-frequency decoupling.
**Principle: **Connecting the centre tap to VDD provides a DC path for the differential current; the resistor (typically two 50Ω resistors in series or integrated within the chip) and the primary winding of the transformer together form a current-to-voltage conversion network.
**Typical chips: TI DP83848, Microchip LAN8720, Jinglue JL2201B/JL1111B.

The Function and Design of the Bob-Smith Circuit

The Bob-Smith circuit is an impedance-matching and common-mode rejection network widely used in the Ethernet physical layer. It consists of resistors and capacitors connected across a pair of differential lines near the transformer (each pair of lines is configured independently).

(1) Circuit Configuration

A standard Bob-Smith circuit consists of two resistors (typically 75 Ω) and a high-voltage capacitor (1000 pF/2 kV) connected in series between them, with its terminals connected to the differential lines (TX+ and TX-) respectively; common-mode surge energy is dissipated via the capacitor to the protective earth (PGND).

(2) Main Functions

Impedance matching: Provides a defined AC termination impedance for differential signals, reducing signal reflection.
Common-mode noise suppression: Provides a discharge path to ground for common-mode currents, reducing radiated emissions from the cable.
Surge energy absorption: External surge energy can be coupled via the capacitor to protective earth for dissipation.

(3) Design Considerations

The resistance value is typically 75 Ω (for a differential impedance of 100 Ω, 75 Ω single-ended is a common choice for impedance matching).
A resistance tolerance of 1 per cent is recommended to ensure impedance matching accuracy.
The capacitance withstand voltage must be ≥2 kV to ensure compliance with the IEC 61000-4-5 surge test requirements (2 kV line-to-ground).

The RJ45 production process

geoffery Bob — Thu, 06 Aug 2026 10:12:44 +0000

The production process described here reflects only the procedures followed at our factory; it does not represent the manufacturing processes and procedures of other factories on the market. We simply wish to provide some insight into just how complex the production process behind a simple network port actually is.

(1) Automatic terminal insertion (see the diagram below for details)

Terminal insertion is the first step in the entire production process. It mainly involves arranging the eight gold-plated copper wires neatly and inserting them into the plastic body of the RJ45 connector, whilst ensuring the correct insertion angle.

(2) Terminal bending/shaping (see figure below)

Bend the inserted gold pins to the angle specified in the drawing (90° or 180°); this step creates the contact surface for the crystal connector.

(3) LED assembly (see figure below)

Insert the LEDs into the corresponding grooves in the plastic housing, sorting them by colour and polarity.

(4) Internal Pin Inspection (see figure below)

This step primarily involves checking whether the gold pins installed inside the housing have any faults, focusing on four key issues:

**1. Missing pins

Bent pins
Gold pins slipping out of the guide slots
Scratched gold pins**

(5) LED Moulding/Hot-Melting (see figure below)

Secure the LED leads and trim them to the same height as the leads

(6) External Pin Inspection (see figure below)

Check the external pins for defects, focusing on three main categories:

**1. Missing or bent pins

Deformation or scratches
For SMT-type RJ45s, check for coplanarity deviation **

(7) LED Testing (see figure below)

This step verifies whether the LED is functioning correctly, focusing on four key aspects:

On/off status
Forward voltage drop
Correct colour display and uniform brightness
Polarity matches the schematic

(8) Continuity and High-Voltage Testing (see figure below)

**Continuity test: **Primarily to confirm that each gold pin is connected to its corresponding lead
**High-voltage test: **Primarily to verify that the insulation withstand voltage meets the standard

Now that we have looked at the manufacturing process, let's discuss a few key quality factors that determine the quality of an RJ45 connector.

Firstly: the dimensional accuracy of the plastic body

The precision of the body's opening - that is, the socket - directly affects the tightness and stability of the connector's insertion. Generally speaking, manufacturers strictly adhere to the FCC Part 68 specifications. Our (VOOHU) solution is to use high-precision moulds and automated injection moulding machines to reduce tolerances to within 0.25 mm. Typically, issues such as loose connections, difficulty inserting or removing the connector, and poor contact are all caused by excessive dimensional tolerances at the interface.

Second: Co-planarity of gold pins

The solder pins on SMT-style RJ45 connectors must be perfectly horizontal; otherwise, this will result in poor soldering.

Third: Consistency of LED parameters

Whether the LED's brightness, forward voltage drop and polarity conform to the specification sheet is crucial to whether the panel's LED indicator lights up and whether the brightness is consistent.

Generally speaking, connector products undergo 100 per cent inspection prior to dispatch. The main test items for this comprehensive inspection are:

**1. Continuity/short-circuit testing

Contact resistance testing
Insulation resistance testing
High-voltage withstand testing
LED electrical parameter testing That concludes ** what I wanted to share in this instalment.

I'll leave it at that for now. If there are any errors or omissions in my explanation, please feel free to point them out; I shall be grateful for your feedback.

Are you designing an Ethernet solution? VOOHU utilises its own-brand industrial connectors, complementary magnetic components and chips to provide eight solutions for key industrial applications. Our services cover every stage from component selection and customised design through to mass production; every order undergoes an engineering review to verify the feasibility of the components and solutions.

Common electrical parameters of RJ45 connectors

geoffery Bob — Wed, 05 Aug 2026 09:12:17 +0000

The most common specification for an RJ45 connector is its data rate. Whilst the data rate is influenced by the product’s materials and structure, this applies only to integrated RJ45 connectors; in theory, discrete RJ45 connectors can support any data rate. The actual limitations on data rate are imposed by the network transformer and the PHY or switch downstream of the RJ45 connector. The figure below shows the standards and applicable scenarios for each data rate, which may serve as a reference.

Electrical performance refers to the reliable parameter standards that ensure the RJ45 meets transmission speed requirements. Let’s take a look at what these are:

1. Contact resistance

Put simply, this is a parameter that measures the quality of contact between the RJ45 connector and the contact terminals. Generally speaking, in industrial applications, the contact resistance must remain at ≤20 mΩ after 1,000 insertion and removal cycles.

2. Insulation resistance

This parameter primarily ensures the insulating properties of the connector’s insulating material

3. Dielectric Strength

Also known as a Hi-Pot test, this test is designed to ensure that the connector remains insulated at the specified voltage without breakdown.

Rated Current and Rated Voltage This is fairly straightforward to understand; it refers to the parameters for the maximum permissible current and voltage.

5. Operating Temperature

These are generally categorised as commercial grade, industrial grade, automotive grade, military grade and aerospace grade.
Commercial grade: 0°C to +70°C
Industrial grade: -40°C to +85°C
Automotive grade: -40°C to +125°C
Military grade: -55°C to +150°C
Aerospace grade: -55°C to +150°C

The Relationship Between Plating and Reliability

1. Gold Plating

Gold plating refers to the layer of gold applied to contact components. Why is gold plating used?
Gold plating is primarily used to ensure signal stability; the thickness of the gold plating determines the service life of the gold-plated pins during insertion and removal, as well as their environmental resistance. I have compiled typical parameters for different gold plating thicknesses, as shown in the figure below

2. Nickel Plating

Nickel plating usually refers to the nickel layer on the exterior of the shielding casing and on the pins. Nickel plating effectively resists environmental salt spray corrosion; when the nickel layer exceeds 3 μm, it effectively prevents ion diffusion between the copper and gold.

That concludes what I wanted to share in this instalment. I’ll leave it at that for now. If there are any errors or omissions in my explanation, please feel free to point them out; I shall be grateful for your feedback.
For more detailed information, please see this article I wrote previously: https://www.voohuele.com/news/2025-06-03-1/

Types of RJ45 Connectors

geoffery Bob — Tue, 04 Aug 2026 10:32:12 +0000

Types of RJ45 connectors
In the previous few articles, we provided a brief introduction to the history, standards and construction of the RJ45. In this instalment, let's take a look at the different types of RJ45 connectors.

1. Types of RJ45 Connectors

Over more than half a century of development, RJ45 connectors have evolved into a wide variety of types to suit different application scenarios, making classification rather difficult. We can categorise them based on various parameters, and the process of identifying the specific product we require by considering multiple parameters is known as 'product selection'. Below, I shall outline VOOHU's classification of RJ45 connectors as a reference for understanding this category:

1.1 Whether magnetic components are integrated

This is the most common classification method, comprising discrete and integrated types
** (1) Discrete RJ45 (Discrete Jack): **Consists solely of the RJ45 housing; the network transformer must be designed, laid out and soldered separately onto the PCB

**(2) Integrated RJ45 (Mag Jack): **This involves soldering the common-mode choke and transformer together onto a single PCB, which is then encapsulated with the RJ45 to form a single module; hence the name 'Mag Jack'.

1.2 Mounting Methods

** (1) DIP: **DIP-type RJ45 connectors have round pins that can be inserted into pre-drilled holes in the PCB and then soldered using wave soldering, resulting in high mechanical strength

(2) SMT: SMT-type RJ45 connectors are mounted directly onto the PCB; they typically feature flat pins that lie parallel to the PCB surface and are soldered using reflow soldering. As they are soldered to the surface of the PCB, their mechanical strength is slightly lower than that of DIP connectors

**(3) Low-Profile: **The port on this type of RJ45 is usually recessed below the pins; installation requires cutting a corresponding rectangular slot in the PCB into which it is embedded.

(4) Cable-Type: Cable-type RJ45 connectors have no pins; wires must be soldered directly to them or crimped onto the cable.

1.3 Port Angle

The port angle refers to the angle between the port and the PCB, and is primarily categorised as 90°, 180° or 45°.
(1) 90° (Right-Angle): See the figure below.

(2) 180° (Vertical): See the figure below.

**(3) 45° (Angled): **See the figure below.

1.4 Number of Ports
(1) Single-port: See the figure below.

**(2) Multi-port: **See the figure below

**(3) Stacked: **See the figure below

There are many other classification methods, such as the presence or absence of a shielding housing or LED indicators, which we will not list here. If you are interested in learning more, please visit this website (WWW.VOOHUELE.COM);

experiencing the selection process first-hand will help you better understand the information above Well, the content in this instalment is relatively basic. In the next instalment, we'll take a look at the electrical characteristics and manufacturing processes of VOOHU's RJ45 connectors.

Key components of the RJ45 connector

geoffery Bob — Mon, 03 Aug 2026 10:27:17 +0000

In my previous article, I discussed the history and structure of the RJ45 connector. In this instalment, let’s take a look at the key features of the RJ45 connector’s design and the scenarios in which it is suitable. Before we begin, let me introduce myself again: I’m a hardware engineer at VOOHU, and I’m always keen to learn. I’d love to discuss any experiences or queries you may have regarding the Ethernet field with you all. I’m passionate about all aspects of hardware knowledge in the Ethernet field and am always happy to share and learn. Without further ado, let’s get started with this instalment’s content.

We discussed how integrated RJ45 connectors consist of a plastic body, contact terminals, board-side terminals, a metal housing, an LED indicator, a network transformer and a PCBA substrate. Let’s take a look at the specifications of these core components.

Key Specifications
1. Types of contact terminals
The eight metal pins inside the RJ45 port are crucial for signal transmission via the network cable; they make contact with the metal contacts in the cable plug. Depending on the manufacturing process, these pins are classified as round or flat.
Round pins: These are fully gold-plated, which makes them more expensive; however, due to the higher physical strength of their circular structure, they offer superior shock resistance and durability against repeated plugging and unplugging compared to flat pins, making them suitable for industrial applications.
Flat pins: These utilise a partial gold-plating process, with gold plating applied only to the contact points that engage with the network port. Consequently, their cost is significantly lower than that of round pins. However, as the structural strength of flat pins is slightly inferior to that of round pins, they are not suitable for high-intensity applications and are generally used in consumer products.

2. Shielding structure

A ‘shielded’ RJ45 connector refers to one that has a metal shield around the connector; if it has one, it is known as a shielded RJ45 connector. If it is made of black or any other colour of plastic, it is an unshielded RJ45 connector.
Shielded RJ45: Features a shielded housing; EMI is suppressed by grounding the pins of the shielded housing or connecting them to the chassis. Taking the part SYT111B465AB2A1DP as an example, grounding is achieved via the grounding pins of the shielded housing.

**Unshielded RJ45: **The housing is typically made of black plastic. Taking this product as an example, SYT111Q066JB2B1D, it is commonly used in consumer electronics, such as home routers.

3. Port anti-detachment mechanism
This generally refers to spring tabs; in specialised RJ45 connectors, these are further categorised into Cannon self-locking and threaded locking types.
**Metal spring tabs: **Commonly used in industrial environments where there are high requirements for insertion/removal force and cycle life.

**Cannon self-locking and threaded locking: **Commonly used in environments subject to severe vibration and where high reliability is required.

Are you designing an Ethernet solution? VOOHU utilises its own-brand industrial connectors, complementary magnetic components and chips from brands it represents to deliver eight solutions for key industrial applications. Our services cover every stage from component selection and customised design to mass production—every order undergoes an engineering review to verify the feasibility of the components and the solution.

The History, Standards, and Structural Basics of RJ45 Connectors

geoffery Bob — Fri, 31 Jul 2026 09:27:51 +0000

1.The History of the RJ45 Connector

In the 1970s, AT&T Bell Labs developed a series of modular connectors, including 4P4C, 6P6C, and 8P8C, which correspond to telephone handset cords, telephone lines, and data cables, respectively.
By the 1980s, the 8P8C connector had been formally incorporated into the ANSI/TIA-568 cabling standard and became the dedicated interface for 10BASE-T Ethernet (IEEE 802.3i). It was during this period that the name "RJ45" came into official use; "RJ" stands for "Registered Jack," while "45" represents the connector's registration number in the FCC system.
By the 1990s, with the emergence of the 100-megabit Fast Ethernet and Gigabit Ethernet standards, the electrical performance requirements for RJ45 connectors increased significantly, and the bandwidth requirement rose from the earlier 10 MHz to 100 MHz.
Today, with the successive release of standards such as 2.5GBASE-T, 5GBASE-T (IEEE 802.3bz), and 10GBASE-T (IEEE 802.3an), the transmission bandwidth of RJ45 connectors has been pushed beyond 500 MHz, while placing higher demands on the interface's isolation voltage and PoE load capacity. Based on my current experience, the RJ45 connector with the highest PoE load I have encountered is an Ethernet port that supports PoE++ (90W) power delivery, with a data rate of up to 10 Gbps.

2.The RJ45 Design Standards System

Unlike connectors such as RJ45, USB, and HDMI, which have dedicated industry associations, the standardization system for RJ45 connectors is relatively fragmented and is primarily based on the following commonly followed protocols:

1. FCC Part 68 - Mechanical Dimensions and Interface Standards: This standard specifies physical parameters for RJ45 receptacles, including window dimensions, contact spacing, and latch positions.
2. IEEE 802.3 - Electrical Performance and Signal Specifications. This standard defines the electrical characteristics of the Ethernet physical layer, including signal levels, transmission rates, impedance matching, and isolation requirements, specifically for 10BASE-T (802.3i), 100BASE-TX (802.3u), 1000BASE-T (802.3ab), 2.5GBASE-T/5GBASE-T (802.3bz), and 10GBASE-T (802.3an)
3. IEC 60603–7 / IEC 61076–3–106 - Connector Reliability Standards: The IEC 60603–7 standard specifies general technical requirements for 8P8C shielded and unshielded connectors, with key parameters including mating cycles, contact resistance, and other metrics,and insulation resistance. The IEC 61076–3–106 standard, on the other hand, sets more stringent requirements for rectangular connectors used in harsh environments, including higher mating cycles (≥2,000), a wider operating temperature range (-40 to 85°C), and greater vibration resistance.

3.Components of an RJ45 Connector

An integrated RJ45 connector typically consists of the following components:

1. Plastic body, usually made of materials such as PBT or PA9T, which primarily serves to provide structural support, insulation, and high-temperature resistance
2. Terminals/gold fingers, typically made of phosphor bronze. Depending on the plating method, they are classified as flat pins or round pins; flat pins feature partial gold plating, while round pins are fully gold-plated. Their functions are divided into two parts: those inside the port are called gold fingers or contact terminals and provide electrical connection, while those on the exterior of the housing are called solder terminals and are used for soldering to the PCB
3. Metal Housing: Typically made of nickel-plated copper alloy, it is primarily used for EMI shielding and grounding.
4. LED Indicator: Colors can be customized based on the application and function; it consists mainly of epoxy resin and a light-emitting diode.
5. Network Transformer: Composed of a magnetic core and enameled wire, it is primarily used for signal isolation, impedance matching, and common-mode noise suppression.
6. PCB: Made of fiberglass-reinforced laminate, it primarily secures components such as the network transformer and connects them to the interfaces.

The LAN Transformer: PoE, Speed, and Selection

geoffery Bob — Thu, 30 Jul 2026 08:43:45 +0000

In Part 1 of From Zero to PoE, we covered the LAN transformer's basic job: signal coupling, DC blocking, common‑mode rejection, and isolation. Now we add two extra dimensions that turn a simple transformer into a critical design decision — PoE current and data rate.

How does PoE get added? If your system uses PoE, the transformer takes on an extra role.

The core engineering challenge in PoE is: how do you send data and power over the same pair of wires without them interfering?

The answer is simple — frequency multiplexing. Data is high-frequency AC, power is DC. They occupy different frequency bands, so they can coexist on the same physical medium. The question is how to inject the DC power without messing up the differential signal.

That injection point is the center tap.

In a PoE system, the PSE (power-sourcing equipment) side applies 48V DC through the center tap. The DC current flows into the center tap, splits into two paths through each half of the winding, and recombines on the secondary side to go out to the cable.

The key is: the DC currents in the two halves of the winding flow in opposite directions (assuming ideal symmetry), so the DC fluxes they create in the core cancel out — no net DC flux. That way, the DC power superimposes on the signal path without disturbing the differential signal or biasing the core.

On the PD (powered device) side, the transformer at that end does the reverse: it extracts the DC power from the center tap and feeds it to the downstream DC-DC converter.

That's the underlying principle of PoE — the center tap provides a DC path, the differential signal uses the winding ends, and the two coexist on the same core without interfering.
**
But there's a catch: **the transformer must be designed to handle that DC current. Not every transformer can do that.

5. What's the difference between a "PoE" transformer and a regular one?

A lot of engineers ask this. The answer is straightforward: it's about structural parameters, not a functional category.

When PoE current flows through the center tap, it creates a constant DC bias field in the core. That bias pushes the core's operating point toward saturation.

Every core has a physical limit — saturation flux density (Bsat). As you approach that limit, permeability drops sharply, and inductance plummets. When inductance drops, the differential signal can't couple properly, return loss worsens, and BER goes up. In severe cases, the link simply fails.

So PoE capability boils down to one question: under a given DC bias, can the transformer still maintain enough inductance?

That depends on three design parameters:

Wire gauge — current capacity; too thin and it overheats
Core cross-sectional area — larger area means more saturation margin
Core material — high Bs materials can handle higher bias In the datasheet, the difference between non-PoE and 720mA is not a software switch — it's a physical difference. A non-PoE transformer isn't "unsupported" — it's simply "not optimised for DC bias." Forcing PoE current through it might work temporarily, but in the long run — saturation, heat, signal collapse — it's only a matter of time.

The current ratings you see on transformer models — 350mA, 720mA, 900mA, 1200mA, 2000mA — correspond to these standard levels.

6. Why is data rate also a hard constraint?

Same core structure, but why can't you use a transformer rated for one speed with another?

Because core materials and winding structures have frequency responses.

10/100BASE-TX: bandwidth ~20 MHz, low-frequency losses are minor, more turns can be used
1000BASE-T: four pairs full‑duplex, requires ≥100 MHz bandwidth
2.5/5G BASE-T: needs ≥200 MHz
10G BASE-T: **needs ≥500 MHz Higher rates tighten the margins for **insertion loss and return loss. A transformer optimised for 100M might have excessive loss or reflections at high frequencies — the Gigabit link may fail to negotiate or may constantly downshift.

Conversely, using a Gigabit transformer on a 100M link usually works fine — bandwidth margin exists. But the reverse is not true.

High-frequency signals in a low‑frequency‑optimised winding will hit distributed capacitance and leakage inductance, causing signal integrity issues. This isn't "slightly worse performance" — it's "will it work at all."

7. Back to that black box

The LAN transformer is not just an "accessory" between the PHY and the RJ45.

Physically, it does one thing: find an engineering compromise between electrical isolation and signal integrity.

No electrical connection between primary and secondary, but magnetic coupling — signal gets through, ground doesn't
The center tap is the DC injection point — power is added, signal isn't disturbed
The B-H curve of the core determines how much DC bias it can take — PoE capability is not a feature, it's physics
The core's frequency response dictates the maximum data rate — speed grade is not a configuration, it's a structure
Once you understand what's happening inside that black box, selection becomes a matter of physics:

How much DC current do you need to carry? How fast does the signal have to go? What isolation voltage does your safety requirement call for?

Answer those three questions, and that little black box won't be a black box anymore.

📌 Next up in From Zero to PoE: The transformer already has some common-mode rejection, so why do we need a dedicated common-mode choke? Part 3 breaks down their respective roles — the transformer handles isolation and coupling, the choke handles noise suppression — and explains how they complement each other.

The LAN Transformer: Signal Path and Isolation

geoffery Bob — Wed, 29 Jul 2026 10:12:18 +0000

A few weeks ago, I was looking for a clear, beginner-friendly explainer on how Power over Ethernet actually works — not a glossy marketing brochure, not a 100-page IEEE spec, just a straightforward technical walkthrough that starts from the components and builds up.

I couldn't find one.

So I decided to write it myself. And along the way, I figured I'd bring you with me.

This is the first entry in a series where we'll start from the very basics — the discrete components that make up an Ethernet port — and work our way up to complete systems. I'm not here to pretend I know everything. I'm learning too. But I've been designing hardware long enough to know where the gaps are, and I want to fill them — for you, and for myself.

We'll start with the LAN transformer — that little black box sitting between your PHY and the RJ45. You've probably seen it a hundred times. Let's finally figure out what it actually does.
If you've ever drawn an Ethernet port schematic, you know the part I'm talking about.

You have the PHY chip's differential outputs — TD+, TD−, RD+, RD− — and they go through a transformer before hitting the RJ45 connector. On the schematic, it's usually just a box labeled "T1" or "LAN Transformer," with a few windings and a center tap drawn next to it.

That little black box is the LAN transformer.

Most hardware engineers think of it as "for isolation." But pop the hood and you'll see it does a lot more than that — how the signal gets across, how DC is blocked, and how PoE power gets injected — all happen inside that same magnetic core.

This first part covers the fundamental role of the transformer in the signal path. In Part 2, we'll dive into PoE, speed, and selection.

1. Where does it sit?

First, its place in the circuit. It sits right between the PHY and the RJ45.

Signals come out of the PHY's differential pair, go into the transformer's primary winding, couple magnetically to the secondary, then through the RJ45 and out to the twisted pair.

PHY diff pair → primary winding → magnetic coupling → secondary winding → RJ45 → twisted pair

The primary side connects to the PHY ground. The secondary side connects to the chassis ground. There's no wire connecting them — the signal travels through magnetic coupling alone.

The grounds are separate.

That's the first reason it exists: the signal gets through, but the ground doesn't.

2. What's inside?

Open up a typical LAN transformer and you'll find:

A ferrite core – the physical medium for signal coupling and energy transfer
Primary winding – connected to the PHY side, usually with a center tap
Secondary winding – connected to the cable side, may also have a center tap
Insulation between windings – this handles the isolation voltage
In essence, it's an isolation transformer with a center tap.

The primary and secondary windings share the same core, but they're physically insulated from each other. That insulation has to withstand a 1500Vrms hipot test without breaking down. The signal from the primary is "replicated" to the secondary through changing magnetic flux. There's no DC path — only magnetic coupling.

The center tap is a pin brought out from the middle of the winding. It's critical for PoE, so remember where it is.

3. How does the signal get across?

The transformer carries a differential-mode signal.

The PHY's differential output stage drives TD+ and TD− with equal and opposite voltage swings. That differential voltage appears across the primary winding, generating changing flux in the core. The secondary winding picks up the corresponding differential voltage and sends it to the RJ45 and the cable.

Two key points here:

First, only changing signals pass. Magnetic coupling relies on "changing flux induces EMF." DC components don't produce changing flux, so they're blocked at the transformer. That's why it's also called a "DC-blocking transformer."

Second, common-mode signals are naturally rejected. If common-mode noise appears on the cable — both wires moving together in the same direction — the fluxes in the windings add up in the same direction and cancel each other out, so nothing transfers to the other side. In other words, the transformer itself has built-in common-mode rejection. It naturally "recognises" differential signals and "ignores" common-mode ones. That's why in some designs, the transformer can do part of the work that a common-mode choke would do.