How 1-Wire Sensors Harvest Parasitic Power From Data Lines

This article explains how 1-Wire sensors operate without a dedicated external power supply by harvesting energy directly from the communication data line. We will explore the concept of parasitic power, the critical role of the internal capacitor and rectifying diode, how the sensor maintains operation during active communication lows, and the method used to supply extra current during power-intensive tasks like temperature conversion.

The Fundamentals of the 1-Wire Bus

Standard electronic sensors typically require at least three wires: ground, power (VDD), and data. However, the 1-Wire protocol, developed by Dallas Semiconductor (now part of Analog Devices), reduces this requirement to just two wires: a ground reference and a single combined data and power line (DQ).

To achieve this, the 1-Wire bus relies on an open-drain architecture. The master controller and the slave sensors can only pull the data line low to ground. To bring the line back to a high voltage state (typically 3.3V or 5V), a pull-up resistor is connected between the data line and the master's power supply. Because the idle state of the 1-Wire bus is high, there is a constant voltage present on the data line whenever data is not actively being transmitted as a logic low.

The Role of the Internal Capacitor and Diode

Inside every 1-Wire sensor capable of parasitic power is a specialized power-harvesting circuit consisting of an internal Schottky diode and an internal storage capacitor (often referred to as \(C_{PP}\)), which typically has a capacitance of around 800 picofarads (pF).

When the 1-Wire bus is in its idle high state, current flows from the master’s power supply, through the external pull-up resistor, and into the sensor's data pin. Inside the sensor, this current passes through the internal diode and charges the internal capacitor. This harvested charge becomes the sensor's private power supply, providing the necessary voltage to keep its internal logic, memory, and timing circuits awake.

Surviving the Logic Lows

Digital communication over the 1-Wire bus requires the line to be pulled to zero volts (logic low) for brief periods, known as time slots. When the data line drops to zero volts, the sensor can no longer draw power from the bus.

During these brief logic low intervals, the internal diode becomes reverse-biased. This is a critical function: it prevents the charge stored in the internal capacitor from draining back out into the grounded data line. Isolated from the bus, the internal capacitor slowly discharges into the sensor's internal circuitry, acting as a miniature battery.

Because 1-Wire communication protocols strictly limit the duration of logic low pulses (typically a maximum of 15 to 60 microseconds for data bits, and up to 480 microseconds for a reset pulse), the 800 pF capacitor holds just enough charge to bridge the gap. As soon as the line returns to the high state, the capacitor instantly recharges before the next low pulse occurs.

Handling High-Current Operations

While the tiny 800 pF capacitor and a standard 4.7 kΩ pull-up resistor are sufficient for basic communication and logic operations, they cannot supply enough current for high-power tasks. For example, when a DS18B20 temperature sensor performs an analog-to-digital temperature conversion or writes data to its internal EEPROM, its current draw spikes from a few microamps to roughly 1.5 milliamps.

If the sensor tried to draw 1.5 mA through the standard 4.7 kΩ pull-up resistor, the voltage drop across the resistor would be so severe that the bus voltage would collapse, starving the sensor and causing a power-on reset.

To solve this, the master device must employ a "strong pull-up" circuit. Immediately after issuing a command to start a temperature conversion, the master activates an external MOSFET to bypass the pull-up resistor, connecting the data line directly to the main power supply. This forces a hard voltage onto the data line for the duration of the conversion (up to 750 milliseconds), providing the sensor with all the current it needs while keeping the internal capacitor fully charged. Once the conversion is complete, the master disables the MOSFET, returning the bus to its standard resistor-pulled state to read the sensor's data.