1-Wire - What are Parasitic Power design constraints
Designing a reliable 1-Wire network requires careful consideration of the bus controller's power capacity and the instantaneous current draw of connected sensors.
This guide covers the power requirements for DS18B20 temperature sensors, the limitations of parasitic power, and how to manage large sensor arrays without destabilizing the bus voltage.
Managing Current Consumption on a 1-Wire Bus
When deploying multiple sensors, you must verify the maximum current output of your 1-Wire bus controller against the combined instantaneous current consumption of all devices.
- Initial Surge: When a DS18B20 capacitor is completely drained, it can draw close to 1.5 mA from the 1-Wire bus.
- Voltage Instability: If a 1-Wire controller is limited to 15 mA, the voltage will become unstable if more than 10 DS18B20 devices attempt to charge simultaneously.
- Device Limits: While you can have more than 10 sensors on a single bus, they must be polled or charged sequentially rather than all at once.
Increasing Available Current
For networks with high device counts, specialized circuits can be used to boost the available current on the 1-Wire bus. This ensures that even during peak charging cycles, the voltage remains within the operational threshold for all sensors.
Capacitance and Polling Frequency
The internal capacitor of a 1-Wire device holds a limited charge (typically 800 pF). Because devices rely on this stored energy during communication and temperature conversion, they require "rest periods" to recharge.
- No High-Frequency Polling: Constant, rapid polling of 1-Wire sensors is not possible because the capacitors will eventually drain, leading to communication failure.
- Refer to Datasheets: Always consult the specific device datasheet to determine the required recovery time between operations.
FAQ – 1-Wire Power Management
1) What happens if I exceed the current limit?
The bus voltage will drop below the minimum required level, causing devices to reset, fail to report data, or return "CRC Error" messages.
2) Can I use external power instead of parasitic power?
Yes. By providing a dedicated VDD line to each DS18B20, you eliminate the charging surge on the data line, allowing for more stable operation in large networks.
3) What is the maximum number of DS18B20 sensors on one bus?
While theoretically large, it is practically limited by the total bus capacitance and the power capacity of the controller. For most standard controllers, 20–30 sensors is the recommended limit without specialized hardware.
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