The continuity shows us the shorts, but many times I guess, even if the wire has a resistance of up to 50 ohm, the continuity will beep to indicate a short.
Is it always 50 ohm, or what parameter should we look for in the manual?
\$\begingroup\$
\$\endgroup\$
7
-
1\$\begingroup\$ Are you trying to ask at what resistance does the continuity test beep for a short on a standard multimeter? It seems a very poorly worded question but this is what I got from it \$\endgroup\$– MCGCommented Jun 17, 2019 at 11:19
-
\$\begingroup\$ It should be in the specs, but I think it differs per brand. (Commonly, the cheaper the device, the less specs you get). E.g. the Fluke multimeters Models 175, 177 & 179 indicate it in their Specifications, Function = Continuity, page 12 \$\endgroup\$– HuismanCommented Jun 17, 2019 at 11:21
-
\$\begingroup\$ @MCG Yes that is the question. \$\endgroup\$– user1245Commented Jun 17, 2019 at 11:34
-
1\$\begingroup\$ I think it is important for beginners to realize that the continuity beeper indicates a fairly low resistance, and not necessarily a dead short. \$\endgroup\$– Peter BennettCommented Jun 17, 2019 at 16:40
-
1\$\begingroup\$ @atmnt I interpret that description as a latch; it will start beeping if the resistance drops below 25Ω, and stop only when it rises above 250Ω. So the behaviour in-between depends on the history. Such a mechanism would make sense to avoid intermittent beeps when a measured circuit is near the threshold. \$\endgroup\$– DaveCommented Jun 17, 2019 at 22:56
|
Show 2 more comments
4 Answers
\$\begingroup\$
\$\endgroup\$
6
It is not a standard value from meter to meter. Different models will beep at different resistances.
An example in the comments were the fluke models 175,177 and 179. In the datasheet for these, you can see on page 12 that the meter beeps at <25 ohms.
The multimeter I use is a Tenma 72-7732A, and in the datasheet for this model, on page 35 it is stated that the beeper will sound for conditions less than 50 ohms.
In the popular EEVBlog multimeter, page 25 of the Datasheet states the continuity threshold is between 30 and 480 ohms.
These few examples are enough to determine that there is no standard value. If the information cannot be found in the datasheet of your particular model, then getting a few resistors between 10 - 250 ohms and measuring them should be enough to tell you the threshold.
An easier way, as pointed out by @HarrySvensson in the comments is to turn a potentiometer/rheostat until the beeping stops/starts and measure the resistance.
-
3\$\begingroup\$ Or just use a potentiometer as a rheostat and rotate the winder until it beeps/stops beeping and then measure the resistance of the potentiometer. \$\endgroup\$ Commented Jun 17, 2019 at 11:49
-
\$\begingroup\$ @HarrySvensson ahh..... yes.... That is a much easier way and I almost did a full facepalm for not having that cross my mind while writing! \$\endgroup\$– MCGCommented Jun 17, 2019 at 11:51
-
\$\begingroup\$ "between 30 and 480 ohms" why would there be such a range on that? I get that to make the beeper fast you use analog circuitry with inherent tolerance limits, but an order of magnitude variance? \$\endgroup\$– HearthCommented Jun 17, 2019 at 14:06
-
\$\begingroup\$ @Hearth I'm not sure. I didn't actually look into the reasons why, it was just a good one to use as a further example. That is a very good question though! \$\endgroup\$– MCGCommented Jun 17, 2019 at 14:27
-
\$\begingroup\$ @MCG I went ahead and made it a question on here, since a cursory search wasn't able to turn up much (though it's difficult to figure out what keywords to search for for this). \$\endgroup\$– HearthCommented Jun 17, 2019 at 15:26
\$\begingroup\$
\$\endgroup\$
1
I'm not aware of any standard resistance values for the continuity mode of a multi-meter.
However, I would personally say, there is an unofficial industry expectation that the continuity beeper should not trigger on a pn-junction.
Assuming the meter is using an excitation current of 1 mA and choosing a conservative IV curve for a pn-junction (assume Id < 100 uA at say 200 mV - 300 mV). By Ohms law, that would give you an upper continuity threshold of 200 \$\Omega\$ to 300 \$\Omega\$.
-
2\$\begingroup\$ 121GW has two thresholds, 30 Ω and 300 Ω. \$\endgroup\$ Commented Jun 18, 2019 at 1:03
\$\begingroup\$
\$\endgroup\$
The continuity resistance threshold is not standard.
The exact value is certainly quoted in the datasheet/manual, if the instrument comes from a reputable manufacturer. If you have a cheap, unknown instrument then you can try a few resistors, and check which one beep, and which one do not beep.
Some expensive multimeters allow you to manually set the threshold, on my Keithley 2000 the threshold can be set anywhere between \$1\ \Omega\$ and \$1\ \text{k}\Omega\$.
answered Jun 17, 2019 at 11:36
\$\begingroup\$
\$\endgroup\$
I was curious as well. Just tested it on my Fluke 179 DMM with a relatively fine adjustable 500 Ω multiturn (8-turn) potentiometer.
simulate this circuit – Schematic created using CircuitLab
Data sheet says “Product beeps at <25 Ω, beeper turns off at >250 Ω; detects opens or shorts of 250 μs or longer.” See page 20 - Fluke 179 data sheet
My copy has the following behavior (quite different than specified in the data sheet):
It beeps as at <47 Ω, beeper turns off at >82 Ω
As a side note: I also tested the behavior of the pn-junctions on a BC547 NPN and BC557 PNP transistor with a continuity test. For all different junctions (forward direction) there was no beep, but a resistance of about 700 Ω is displayed on the LCD.
answered Dec 1, 2021 at 9:27