user9253
Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan
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 Posted: Tue Aug 07, 2012 12:35 pm Post subject: Faulty troubleshooting |
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I had recently commented about erroneous troubleshooting techniques.
Then I made the same mistake myself. While working on an aviation project in my workshop, I had wires temporarily connected with dabs of solder or alligator clips. Then everything quit working. I tested the 5 VDC bus and it measured 12 volts! Oh no, I thought the 7805 voltage regulator shorted out and ruined my expensive 5-volt components with 12 volts. I removed the 7805 voltage regulator from the circuit board and tested it. There was nothing wrong with it. So I soldered it back in place and turned the power on. This time everything worked fine. Hmm, why weren't the 5-volt parts fried when 12 volts were applied? And where did that 12 volts come from? I checked the PC Board traces for solder bridges, but there were none.
Later I realized what must have happened. The negative power supply wire to the circuit board must have had a bad connection. So the board was not even powered up. No wonder nothing worked. So why did my digital volt meter measure 12 volts on the 5-volt bus? Because the meter has high input impedance, it was able to measure the minute voltage leaking through the 7805 voltage regulator. By the way, the voltmeter common lead was connected to the negative supply upstream of the bad connection. This situation is very similar to the diode test that Eric Jones described with a battery and diode and voltmeter all connected in series.
Joe
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Joe Gores |
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