Member Since: July 15, 2011

Country: United States

  • For over ten years I have been using a variation on the four terminal Kelvin resistance measurement to find shorts on pc boards. I use a one or two amp current limited low voltage source. This could be a 5 volt supply with a four ohm power resistor or a lab supply set to 3 volts and 1 amp. I connect the current source to the two shorted traces at easy access points, usually by tack-soldering short scrap wires to components or connector pins on the problem foils. Then I set a DVM to read volts with resolution to millivolts and start measuring voltage along the foils. You may start out with 30 or 40 millivolts. Work your way along the traces to where the voltage goes to zero or a minimum and you are at the short. If you have a ten ohm tin whisker between two foils, pass the current thru one of the foils from one end to the other. Treat the foil with current as a voltage divider and the other foil as the wiper on a trimpot. At the place where the voltage on the current foil equals the other foil you have the short. I have used this to find whiskers that were barely visible with a microscope. A small scratch with an xacto blade cleared the short. Four terminal resistors and voltage dividers are handy tools!

  • I noticed you have light pipes taped to each PICKit3 module. Which LED are you monitoring? Do you have a write-up about gang programming with the PICKit3? Thanks.

  • 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.

  • 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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