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In order to test some tracks on a mounted PCB, I would like to use a multimeter for continuity test.

However, then I measured the open-circuit voltage of two multimeters at hand, and the results were:

Both have a current limit of 1 mA, but when doing a measurement without connection, it applies the rather high open-circuit voltage.

I was pretty surprised to discover that these widely used high-end multimeters apply an open-circuit voltage for continuity test that is above the absolute maximum ratings of common chips like CMOS 7400-series 3.3-V logic with high impedance inputs, which is for example 6.5 V for TI SN74LVC00A.

Am I being overly concerned here, or is there actually a risk of damaging the components when doing continuity test on mounted PCBs with these multimeters?

Is there any suggestion of how to do continuity test with lower open-circuit voltage?

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2 Answers 2

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Can continuity test cause component damage?

Generally no. I have used ohmmeters to measure continuity on expensive aerospace electronics and was never concerned. Reliability engineers never warned us about risks.

For your example (LVTTL component), the part will never see a large voltage, the ESD protection circuit will limit the voltage. Connecting an ohmmeter is not the same as connecting a stiff voltage from a power supply. A stiff voltage probably would damage the part.

Some parts without ESD protection could be damaged by an ohmmeter if their absolute maximum input voltage was very low. These types of parts are not common.

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  • \$\begingroup\$ Thanks for your answer, and it is reassuring with the reference to measurement on aerospace electronics that was overlooked by reliability engineers. \$\endgroup\$
    – EquipDev
    Jun 6, 2020 at 16:09
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Note that your "open circuit" voltage test is probably applying a 10meg load (10M voltmeter) to auto-ranging ohmmeter. Perhaps the ohmmeter had switched into "20 megohm" range mode, which might apply a different voltage than a low-ohms range?

With this Fluke 189 here, with a 10meg voltmeter measuring the output, the 20meg range applies repeating pulses well over 2V, while the low ranges all apply 5.1V continuous.

So, try using the range button to force your meter into various range settings. See which ones give high-ish volts.

ALSO: try measuring a 1meg resistor, 1K, etc., then simultaneously check the voltage being applied. Even if your meter has no 'range' button, perhaps you can force it into a low-voltage ohmmeter mode. Set it to beeper continuity, then jam a 220 ohm resistor across the bananas? (Usually 220 ohms is far too high to trigger the "beep.")

I can't think of any modern circuitry which would suffer damage from an ohmmeter. However, I recall a famous story from WWII, where a new tech was testing some expensive experimental microwave-detector diodes. They all tested bad. In this story, his big black bakelite SIMPSON meter, set to the wrong ohms range, was applying well over 10mA to each one, destroying it.

Then there's the 'Darwin Award' story, where the military tech decided to find out his internal body-resistance by jamming the ohmmeter probes deep into his hands. This supposedly killed him, by triggering heart fibrillation. https://darwinawards.com/darwin/darwin1999-50.html I did something similar with three 9V batteries, salt water, and a soup spoon held in my mouth. No death; not even any pain, however I did see grey "flashes" in my whole visual field when I bumped the connections together. Nikola Tesla would be proud! Or maybe not, since he swore by healthful x-rays aimed at brain, rather than direct electrode connections.

Death's Ray (Mitchell & Webb): https://www.youtube.com/watch?v=8HgejSCHRi8

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  • \$\begingroup\$ Thanks for the answer and suggestion for using manual range. When selecting manual range, the Fluke 87 V has a voltage of 7.3 V for 600 Ohm, 6 K and 60 K, then 3.5 V for 600 K, 5.6 V for 6 M and 2.8 V for 60 M (where it measures the other DVM at 11 M); the Keysight 34465A do not allow manual range in continuity test mode. So it appears that using a low Ohm manual range can't provide a low open-circuit voltage. \$\endgroup\$
    – EquipDev
    Jun 6, 2020 at 16:46
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    \$\begingroup\$ @EquipDev also try wiring in a 200ohm (or smaller) resistor. That may allow continuity test at less than 0.6V diode turn-on voltage. Also, with "beep continuity" meters, one trick is to wire a 1K pot across the meter, then adjust it slightly below threshold of "beep." That way you can rapidly beep-test a large number of identical PCBs for the presence of quite large values of unwanted leakage paths. Even use a known resistor to calibrate your desired beep-noise threshold. \$\endgroup\$
    – wbeaty
    Jun 6, 2020 at 16:56
  • \$\begingroup\$ Applying a 500 Ohm resistor across will limit the "open-circuit" voltage to 0.5 V, due to the max current of 1 mA. So that is an option in order to get lower "open-circuit" voltage. Though still puzzling why high end multimeter developers like Fluke and Keysight has not reduced the open-circuit voltage in continuity mode, when for example Brymen BM257 has specified 0.5 V open-circuit voltage. \$\endgroup\$
    – EquipDev
    Jun 6, 2020 at 17:06

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