I'm powering my Pi 3 with Weidmuller 5V/5A power supply and the Pi is running Raspbian Jessie. I have connected only a wireless mouse with wireless keyboard and USB web camera. I still get the yellow lightning icon. Any ideas what could be wrong? It's messing with my computer vision application.
Well, "undervoltage" isn't really ambiguous. You can check it directly on the 5V and ground pins with a multimeter, but if the icon only flashes intermittently, it may be because the supply cannot deliver the required instantaneous current, causing the Pi's voltage to fluctuate, and I'm guessing the average multimeter won't help with catching that.
A reason for this problem could be:
That the supply is primarily intended for charging USB devices, as opposed to actively powering electronics, and so deals with instantaneous draws in a way suited to the former but not the latter.
That the supply is a USB charger that might be okay, but the microUSB cable used is sub-par. Try another cable.
That the supply is low quality or defective.
That the Pi is defective.
To differentiate the last case from the previous three, you would have to test using another, high quality supply intended for power, not just re-charging batteries. If you get the low voltage indicator no matter what you do, then you should contact the distributor for a warranty claim.
The voltage monitor chip (APX803) (or similar) used in virtually every computer made in the last 30 years. It is unlikely to be due to a faulty Pi.
If the undervoltage indicator comes up the voltage is below 4.63±0.07V. See Raspberry Pi Power Limitations. Even if the power LED is lit the voltage could still be below spec 5.0±0.25V.
The ONLY way to check that the supply is adequate is to measure the voltage on pins 2,6.
Poor quality cables (the majority) can exacerbate the problem — again the only way to confirm is to measure voltage (or monitor on a CRO to detect transients).
Changing software will NOT help! The voltage is detected in hardware, the firmware does stretch the indicator to 3 seconds, to make it easier to detect transient undervoltage.
I have had similar problems with undervoltage. The solution was to use a better USB cable for the power. The cable voltage drop can be excessive if you use a long cable and/or a thin gauge wire. More current draw from continuous heavy CPU time (like vision processing) will make it even worse. Use as short a cable as possible and make sure it is 20AWG. I highly recommend a USB power monitor. They are inexpensive and can help diagnose these issues.