July 15, 2011
News - According to Pete: Curren…
about a year ago
Current mirrors or constant current sources are great for making a linear voltage ramp when charging a capacitor. Some of the earliest analog oscilloscopes used constant current source charging a capacitor to make the horizontal sweep time base.
News - The Moment of Truth
about 2 years ago
I have about ten plus years of basement manufacturing experience plus several honest jobs as an electrical engineer. I have found power to ground shorts to be a challenge for the ohmmeter. I have developed a technique that can zero in on the short location. Connect a low voltage power supply of a voltage that is safe for your problem board (don’t use 12V on a 5V or 3.3V board) to a current limiting power resistor that limits short circuit current to a safe value (1 amp for heavy traces, .25 amp for smaller traces). Connect the low voltage, current limited power to the power and ground terminals of the problem board. Use a digital voltmeter set to read voltage in the millivolts range. Connect the voltmeter minus to the ground connection and take a reference reading at the plus power terminal. You will see a few tens of millivolts. Then start probing around on the power and ground traces. The place where the power millivolts equals the ground millivolts should be very close to the short. This technique has helped me find shorts that were from invisible solder under a chip.
The same technique can be used to find where foils are shorted together under solder mask. Connect the current limited test power supply to the two ends of a shorted trace and the voltmeter set to millivolts connected to one end. Measure the total millivolts across the trace and compare to the millivolts on the other shorted trace. The point on the test trace that has the same millivolts as the shorted trace will give you the location of the trace. In this case the test current does not pass thru the short but the short becomes a “wiper” on the voltage divider formed by the trace with the current passing thru it.
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