Tagged power
3 pieces
- · 8 min read
Turning a Sensor Off Does Not Turn It Off: Back-Powering Through Shared Bus Pull-Ups
Cut a sensor's supply rail and it can stay awake, drawing current through the pull-up resistors on a shared bus that never switches off. The current arrives through the part's own data pins via its protection diodes. Choosing a lower-power sensor changes nothing, because nothing about this is a property of the sensor. Negligible battery impact is a claim about the topology, not about the part.
- · 10 min read
Stopping The Signal Is Not Switching It Off
Detaching or stopping the control signal to a servo does not remove power from it. The internal controller keeps running off the supply rail, and under mechanical load the servo can still draw current and hum audibly. Real silence requires cutting the supply rail with a transistor. In a battery wearable, the average current is set by what stays powered between events, which is why duty-cycled power gating buys hours and part selection does not.
- · 10 min read
Your Multimeter Cannot See Your Sleep Current
Current consumption in a well-designed low-power device spans several orders of magnitude across a duty cycle, from microamps asleep to tens of milliamps transmitting. A bench multimeter integrates over its own measurement window and is effectively blind across most of that range. The useful quantity is charge per event, not current, and the rule that saves the most time is this: if the meter says zero and the battery disagrees, the battery is right.