Thursday, August 27, 2015

Current optimization of wireless measurement sensors - part 1

I've been developing wireless temperature and humidity sensors as commercially available ones are not good enough or too expensive, often both.

I'll post on the sensors perhaps later, but right now I'm working on reducing their power consumption. They have an approximately 1 uA sleep current, which I currently don't know how to reduce further.

Initially, when measuring every 5 minutes and transmitting 3 measurements at a time over the radio, I got an average current consumption of around 2 uA. Initial current consumption over time is show in figure 1. This was measured using the setup described in the previous post and using the data port of my UT61E.
Figure 1. Initial current usage.
The smaller spikes in figure 1 are due to measurement only while the larger spikes are due to both measurement and communication over radio.

The measurement time used by the sensor (Sensirion SHT20) is significantly reduced when the measurement resolution is lowered. According to the data sheet of SHT20, the measurement with full resolution takes a maximum of 350 uA for 114 ms, which translates to about 40 uC of charge. The measurement with the lowest resolution however takes a maximum of 350 uA for 15 ms, which translates to about 5 uC of charge. This was the first course of action to reduce current consumption. The result of the first try of this is shown in figure 2.
Figure 2. Current consumption with what I thought was reduced resolution measurements.
Surprisingly the measurement resolution didn't seem to have any impact on the current usage. I suspected some error in configuring the sensor, and sure enough, after a long time debugging, I found the problem. I had assumed the datasheet listed the configuration bits in order and didn't pay attention to the actual indexes... Figure 3 shows the current consumption when actually using reduced resolution measurements.
Figure 3. Current consumption with reduced resolution measurements.
As can be seen, the reduction while measuring is quite significant, bringing the average current down by about 20%. However, integrating over a single measurement spike gives around 20 uC (this is computed ignoring the background current of ~1 uA). This is quite large compared to the maximum of 5 uC used by the sensor, and suggests that the majority of the current is consumed elsewhere. This will be the course of investigation in the next part.

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