Sometimes I use my multimeter solely for testing if there is a contact between two points. However, sometimes, when I measure between VCC and GND on some of my PCBs, I hear a short "Beep" and then it stops. Therefore I assume that there should be a short-timed connection between these two points (but it should not). But where?
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4\$\begingroup\$ I assume there are some capacitors, somewhere between the VCC and GND line. I guess there is a short period when the capacitors are not fully charged and the multimeter gives a beep then, and when the capacitors are charged up the beep stops. My theory could be wrong so please correct me if so. I would worry only if the beep would not stop. \$\endgroup\$– Bence KaulicsAug 6, 2015 at 9:12
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\$\begingroup\$ Swap the test leads around quickly and you'll hear it again, probably for longer. \$\endgroup\$– Spehro PefhanyAug 6, 2015 at 11:04
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3 Answers
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Sounds like capacitors to me. Since the multimeter is most likely verifying continuity with a DC supply, an uncharged capacitor will appear like a short for a brief moment.
Also found this at https://learn.sparkfun.com/tutorials/how-to-use-a-multimeter/continuity:
Continuity and large capacitors: During normal troubleshooting. you will be probing for continuity between ground and the VCC rail. This is a good sanity check before powering up a prototype to make sure there is not a short on the power system. But don’t be surprised if you hear a short ‘beep!’ when probing. This is because there is often significant amounts of capacitance on the power system. The multimeter is looking for very low resistance to see if two points are connected. Capacitors will act like a short for a split second until they fill up with energy, and then act like an open connection. Therefore, you will hear a short beep and then nothing. That’s OK, it’s just the caps charging up.
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\$\begingroup\$ To prove this, you can power on the circuit and remeasure across a capacitor. There should be no beep cause the cap will be already charged. \$\endgroup\$– ACDAug 7, 2015 at 12:26
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What you hear is most likely the capacitance between the VCC and GND which quickly gets charged as you measure the circuit.
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If the 'beep' is short, then it is probably the continuity tester charging up the power supply filter capacitors. However, if it is continuous, then this is not necessarily indicative of a short circuit. I have several FPGA boards (large Virtex 6) with Vccint supplies of 1.0 V, and and the Vccint supply rail on these read as shorted to ground by the continuity checker, even though they are not. My guess is that the transistors that make up the FPGA core are leaky enough that they let through the entire test current that the continuity checker applies with very little voltage drop. After all, these FPGAs can draw 10 to 20 amps on Vccint when in operation.
answered Aug 6, 2015 at 20:11