Look a little closer
See the mechanism
The reader supplies power through coupled coils
Read the diagram steps
Field present. The powered reader creates a changing magnetic field. The card coil receives an alternating voltage; its rectifier and regulator supply the chip.
Card replies. The card changes the electrical load on its antenna. Through magnetic coupling, the reader senses those changes as a response while continuing to supply energy.
Original functional schematic; dimensions and motion are exaggerated, not measured. Passive HF interface, not a payment transaction or security diagram. Circuit blocks are functional and have no component values. Rectifier basis: NXP NTAG213/215/216 sections 8.1–8.2.
Diagram basis: NXP AN11740: reader/card coupling and load modulation.
Sources: NXP Semiconductors · NXP Semiconductors
The reader creates the field
At a 13.56 MHz NFC interface, current in the reader antenna creates a changing magnetic field. Magnetic coupling transfers energy to the nearby card antenna. The card's rectifier and regulator turn the received electrical input into a usable chip supply. NXP documents that arrangement in its passive NTAG devices.
Sources: NXP Semiconductors · NXP Semiconductors
The card changes the load
The same coupled antennas carry communication. A passive card can answer by changing the electrical load on its antenna, a process called load modulation. The reader detects those changes in the coupled system. The card does not need to create an independent, battery-powered radio transmission.
Sources: NXP Semiconductors
Power depends on the coupling
The reader field supplies the tag during operation. Antenna geometry and position affect the coupling, so a card-shaped object being nearby is not enough by itself: its antenna and chip must receive a usable field. Power transfer is only the physical beginning of the exchange.
Sources: NXP Semiconductors · NXP Semiconductors
The thing to remember
The reader supplies energy; the card rectifies it and modulates the coupled circuit to reply.
What this example leaves out
This describes a passive HF contactless interface, using an NFC tag as a circuit example. It does not establish payment authorization, data security or a universal reading distance.
Sources & further reading
Source access dates are listed below. Manufacturer descriptions and patents support the specific constructions described, not every product.
- NTAG213/215/216 product data sheet, Rev. 3.2 ↗
NXP Semiconductors · Retrieved 2026-10-18 · 13.56 MHz passive interface; antenna coil transfers energy/data; rectifier/regulator and reader field supply.
Read pages 1–2 and 6–7, sections 8.1–8.2. NTAG is a Type 2 tag example, not a payment-card security architecture. - AN11740 — PN5180 Antenna design guide, Rev. 1.1 ↗
NXP Semiconductors · Retrieved 2026-10-18 · Reader/card magnetic coupling and card supply, load modulation responses, dependence on antenna geometry and spacing.
Read sections 2, 2.1 and 2.1.3, Figure 2 caption and section 3.1. Analog interface explanation only; no payment authorization or security conclusions. Independent review: original nxp.com URL returned404; same Rev1.1 primary PDF actually read on official nxp.com.cn host, sections2.1,3.1 and Figure11. Physical interface only.
