This is just a quick post to share something that I learnt only very recently, whilst chatting with a wise Sensei, about how the continuity mode of most multimeters actually works.

Continuity mode is perhaps one of the most used modes on a multimeter for many people. Simple, it lets you test for electrical continity between any two points. It’s super useful, in trying to reverse engineer a circuitboard you don’t have the schematics for; or for testing if joints like connectors or solder-joins are making a good connection.. If there’s a complete circuit between teh two points being probed, then a buzzer or beeper sounds; and if there isn’t then there’s silence.

Continuity mode is a special case of resistance measurement. Resistance mode on any kind of Ohm-meter simply works by creating a voltage divider circuit. The load is put in series with a high-precision resistor and then the voltage drop over the load is measured to enable the meter to calculate the resistance.

In this simple example circuit, if our voltage source was 5V, and we’re measuring the voltage over the “unknown” (in our case 560Ω) resistor then we’d expect our volt-meter will read 4.91V.

Of course we can easily turn this around to calculate the value of R_t instead…

And this calculation is basically what our Ohm-meter is doing any time we measure a resistance.

But the voltage that the Ohm-meter uses isn’t the same on every range. In fact for the lower resistance ranges, the voltage is much higher than for the higher ranges. And this is where things get intesting – and where I learnt something.

When the multimeter is in continuity mode, it has the highest potential difference between the two probes. So (at least theoretically) if you’re working on a board with some very voltage-sensitive components (for example modern 1.6V logic) on it – by using the continuity mode you might cause yourself problems.

Here are a few examples.

Here the hand-held UT61E+ meter (on the right) is set to resistance on a MΩ range, and the voltage is measured by the bench meter at 0.611V.

In comparison, here’s exactly the same setup when the range is manually switched to the lowest Ω range. The same voltage is also mesaured when the meter is in continuity mode.

As you can see this is very-significantly higher, and could cause damage to any low-voltage tolerant parts (despite the extremely low current of less than 1 µA).

For interest sake here’s the comparison made the other way around, measuring the bench UT802’s voltage in resistance mode with the UT61E+…

So there we are. If you’re using the continuity mode, then just be aware of the potential voltage that there might be between the two probes – and be aware of where in your circuit (and to which components – and in which polarity!) you are introducing that voltage – especially if you’re testing something that is voltage sensitive. For most things that is probably going to be okay (given that, at least for both of the meters I measured for this experiment, the voltage is less than 3.3V); but for any fragile older components (e.g. CMOS logic) that might not be safe to expose to a reverse-biased voltage, or for anything with a maximum voltage of less than 3V: you might want to consider using a manually selected higher range.

And, thank you again to my 先生 in this. Every day, really is a school day!

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Filed under: Electronics - @ August 8, 2022 15:41

Tags: Electronics, Multimeters