Whether an Ethernet transformer center tap should connect to power or ground depends on which side of the transformer it is on and the requirements of the PHY reference circuit. On the PHY side, the center tap may connect to a specified supply rail or to ground through a capacitor. On the cable side, the connection also depends on the termination network and Power over Ethernet (PoE) circuitry. The label “center tap” alone does not determine how the pin should be connected.
1. Distinguish the PHY Side from the Cable Side
A center tap is a connection brought out from the midpoint of a winding. The windings on the two sides of an Ethernet transformer have separate electrical connections. Even if both center taps are labeled CT, they are not the same electrical node.
The easiest way to identify the two sides is to trace the differential signal paths in the schematic. The side connected to the PHY transmit and receive pins is the PHY side; the side connected to the RJ45 cable contacts is the cable side. Do not rely solely on whether a winding appears on the left or right of the schematic, as transformer symbols may be mirrored.
The two sides serve different purposes. The PHY-side center-tap connections must be compatible with the PHY’s analog transceiver circuitry. Cable-side center taps may form part of a termination network or, in an applicable PoE implementation, connect to the power sourcing or receiving path. Do not copy a cable-side power connection onto the PHY side.
For a single-port 100 Mbps design that does not require power delivery over the Ethernet cable, the VOOHU WHD12401G can serve as a starting point for evaluation. First establish that PoE is not required, then verify the center-tap connections against the PHY reference schematic. This gives component selection and schematic design a common basis. If PoE is added later, select components to meet the new power requirements rather than simply adding a supply connection to the existing schematic.
2. Connecting to Ground Through a Capacitor Is Not the Same as Connecting Directly to Ground
Why do some schematics connect the center tap to a supply rail while others connect it to a capacitor? The difference comes from the PHY’s output-driver architecture and internal circuitry.
In a typical current-mode transmit-driver implementation, current flows from the specified supply through the center tap into the winding, with the PHY’s driver branches completing the transmit-current path. This type of implementation therefore requires the center tap to connect to the designated supply node with appropriate decoupling. Use the supply voltage specified in the documentation for the particular PHY; do not connect the tap to an arbitrary supply rail on the board.
The reference circuits for some voltage-mode PHYs connect each PHY-side center tap to ground through its own capacitor. The capacitor provides a path for AC components while blocking DC. Removing the capacitor changes the electrical connection.
These connections are not equivalent.
The rule of thumb “current-mode drivers use a supply connection; voltage-mode drivers use a capacitor to ground” can help explain common implementations, but it is not sufficient on its own for PCB design. Individual PHYs have specific requirements for whether center taps must remain separate, how they should be decoupled, and which external networks are needed. Follow the datasheet and reference design for the PHY you are actually using, rather than copying another board simply because it has a working Ethernet connection.
In particular, do not tie multiple center taps together without checking the requirements. Some reference circuits explicitly require each center tap to be decoupled independently. Joining them into a common node and using a single shared capacitor does not preserve the original circuit.
3. Document Every Center-Tap Connection During Schematic Review
Start with the PHY reference schematic and compare it with the Ethernet transformer’s internal circuit diagram. Do this before changing capacitor values or connections.
For each center tap, identify the winding and transformer side it belongs to, and check whether it is internally connected to any other nodes. Then verify its external connection: does it connect to a specified supply, to a specified ground through a capacitor, or to a cable-side network? Check the ground net names carefully as well. Signal ground and chassis ground must not be treated as the same node simply because they use similar ground symbols.
Repeat this review whenever a component is substituted. A similar package only indicates a similar physical form; it does not establish that the winding orientation, center-tap grouping, or pinout is identical. A useful review record includes “component pin — associated winding — external net — documentation page number.” This provides a clear reference for future changes to the PHY or Ethernet transformer.
The same review method applies when selecting VOOHU Ethernet transformers. Shortlist candidates based on the Ethernet port speed and power requirements, then compare the component datasheet with the PHY reference design. Considering component selection and center-tap connections together provides a clear basis for subsequent board revisions. At this stage, center-tap pin numbers, winding connections, and external component values should all be defined in the actual schematic.
4. Frequently Asked Questions
4.1 Can the Center-Tap Connections Be Copied Between Two 100 Mbps PHYs?
Not on the basis of data rate alone. The same speed does not imply the same analog driver architecture or internal termination. Check the new PHY’s reference schematic first, then verify the magnetics pinout and winding connections. If the PHY is replaced while the existing external circuitry is retained, recheck the center-tap supply, decoupling arrangement, and whether any center taps may be tied together.
4.2 Does Adding PoE Mean the PHY-Side Center Taps Must Connect to the PoE Supply Voltage?
No. In PoE implementations that carry power over the data pairs, the power sourcing or receiving connections are on the cable side of the Ethernet transformer. The PHY side must still follow the PHY’s requirements. Clearly distinguish the two sides during schematic review, then verify the magnetics’ PoE ratings. Do not change the PHY-side biasing simply because the port needs to carry power.
4.3 Does a Working Ethernet Link Prove That the Center Taps Are Connected Correctly?
A working link is an initial result, not a substitute for checking the connections. First confirm that the actual circuit meets the PHY’s reference-design requirements, then verify communication performance and electromagnetic compatibility (EMC) according to the system test plan. A successful test using one short cable does not establish that the product will meet its requirements with other cables or under different temperature and interference conditions.
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