Is it safe to test for continuity with multimeter - Page 1
Forum section: electronics / projects

author: c64
author_rank: Frequent Contributor
timestamp: July 14, 2020, 03:22:35 am
content: Some multimeters have continuity function together with diode test. Is it actually safe to use it? My multimeter can light up an LED (2V at least) in this mode. I don't think it's safe to apply 2V between any two points in a circuit.

author: bob91343
author_rank: Super Contributor
timestamp: July 14, 2020, 04:10:12 am
content: You are right. A continuity test should be reserved for cables and connectors, and not used on circuitry. The diode test is on many meters just so they can say they have it but I don't think it's a good thing. Read the manual and see how much current it supplies and you can judge from that how to use it.

author: fourfathom
author_rank: Super Contributor
timestamp: July 14, 2020, 04:33:52 am
content: Yes, it lights an LED, but how brightly? My meters only put out a few mA in continuity mode and the LEDs glow, but fairly dimly. I have no qualms about using my meters for continuity and junction testing. To be sure, use another meter to measure the current sourced by the "continuity" meter. If it's reasonably low then it's not going to harm garden-variety semiconductor junctions.

author: Kashif
author_rank: Contributor
timestamp: July 14, 2020, 04:37:42 am
content: I always check continuity when power is removed from circuit and so far i haven’t have any issue.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 14, 2020, 08:19:21 am
content: Diode and continuity tests can destroy sensitive electronics easily. Keep in mind that the 2 or 3 volts some can generate is 2-3 times more than what high-speed digital circuits are rated for internally.Will you destroy a power supply with it? probably not. Something like a modern home media player or TV or computer? potentially, yes.

author: Ian.M
author_rank: Super Contributor
timestamp: July 14, 2020, 09:11:07 am
content: Another trap for the unwary is if your low voltage digital board has a memory backup battery or supercap, so part of its still powered. Inject a few mA with an unlucky polarity that takes a signal pin of a CMOS LSI chip outside the rails, and you could trigger latchup, and depending on the backup supply impedance, destroy the chip, even though the same pin of the same chip could have survived the injected current if it was totally unpowered.If you've got a ESR meter designed for in-circuit testing of electrolytics, with a peak excitation voltage under 0.7V, its far better than most DMMs for continuity testing tracks, vias etc. on populated boards as it won't significantly forward bias any silicon PN junctions so it will be safe for all except the most sensitive low voltage ICs.

author: EEVblog
author_rank: Unknown
timestamp: July 14, 2020, 09:49:55 am
content: Meters back in the old days had a "low ohms" function that reduce the test voltage to less than a diode drop (<0.6V). Fairly rare these days though.In continuity mode the test current is usually a few hundred microamps, a mA or two would be considered high. So most stuff in-circuit is pretty robust and won't be destroyed, but it's possible.

author: magic
author_rank: Super Contributor
timestamp: July 14, 2020, 10:24:02 am
content: I think some of the 3.5 digit no-names can output a milliamp or two.They also may not measure the voltage drop accurately by employing the 2000mV range, but use some approximative trickery which only works remotely right (say, 10% tolerance) for silicon junctions. Not even sure why they are doing it

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2020, 01:36:14 pm
content: Usually I would not worry about the diode/continuity test damaging circuits but there is a specific exception; the open circuit voltage may be high enough to cause reverse breakdown of a bipolar transistor's base-emitter junction. (1)How high this voltage is depends on the era when the multimeter was manufactured. During the tube era, 22 volts was common. My oldest digital multimeters from the 1970s are 10 volts. My handheld digital multimeters are all about 3 volts so not a problem. But more modern handheld digital multimeters tend to be 5 volts or even higher because they need to be able to test blue LEDs while my older meters only needed 2.2 volts or so for LEDs.So I avoid using my bench or modern handheld multimeters when testing bipolar signal transistors where base-emitter breakdown would be a problem, but my older handheld multimeters are fine.(1) Other sensitive parts include tunnel diodes, some small signal MOSFETs and some CMOS circuits, and oddly enough some specialty transformers which can have their magnetic core damaged by excessive flux.

author: Miti
author_rank: Super Contributor
timestamp: July 14, 2020, 04:34:37 pm
content: Fluke 177 open circuit is 7V. Some LEDs reverse voltage is lower than that.

author: Peabody
author_rank: Super Contributor
timestamp: July 14, 2020, 06:22:07 pm
content: Would this not also apply to any test for resistance between two points? Isn't continuity just a special case of an ohms measurement?Also, am I the only one that worries about lead polarity? My digital meter puts out positive voltage on the red (+) lead when measuring resistance, as you would expect, but my analog meter is the reverse.

author: c64
author_rank: Frequent Contributor
timestamp: July 15, 2020, 03:41:52 am
content: Quote from: Peabody on July 14, 2020, 06:22:07 pmI think ohms and diode test are different

author: bdunham7
author_rank: Super Contributor
timestamp: July 15, 2020, 04:37:52 am
content: Quote from: c64 on July 15, 2020, 03:41:52 amThey actually aren't--in some cases. On one of my meters the 2K Ohms and Diode check are the same thing. The meter puts out 1mA and reads the voltage on the 2V scale. Others may scale differently, but the mechanism is the same--both measure the voltage drop for a given current.

author: james_s
author_rank: Super Contributor
timestamp: July 15, 2020, 05:04:14 am
content: I won't say it's impossible, but certainly I've never managed to damage anything that way. The continuity mode on my Fluke 87 will make an LED glow dimly but I doubt it's more than a few tens of microamps.

author: EEVblog
author_rank: Unknown
timestamp: July 15, 2020, 08:01:32 am
content: Quote from: c64 on July 15, 2020, 03:41:52 amIIRC diode mode will switch in the low range ohms resistors and drive voltage. But DMM chip architectures do vary a lot.

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2020, 01:36:36 am
content: Continuity, diode test, and ohms all work the same way using the same circuits, (1) ohms converter plus voltage measurement, but with variations in test current, voltage compliance, and readout (beeper or not).On my Beckman RMS225, continuity and ohms mode are the same mode with a compliance of about 3 volts. Ohms mode has a compliance of about 0.6 volts.My Tektronix DMM916 has separate diode and continuity modes but with the same 3 volt compliance. Normal ohms mode is 0.9 volts and low voltage ohms mode is 0.25 volts.As I said earlier, more modern multimeters than mine have a higher diode test voltage because they need to be able to test higher voltage LEDs. Older multimeters kept the diode test voltage below the point that which would damage a bipolar transistor base-emitter junction. Older multimeters yet had much higher voltage compliance in ohms mode until it was realized that this might not be such a good idea.(1) I am sure there are exceptions, but multimeter designers tend to be parsimonious. (2)(2) Checks off $50 word for today.

author: ledtester
author_rank: Super Contributor
timestamp: July 16, 2020, 01:58:10 am
content: So far most of the discussion has focused on the safety of the circuit under test. But what about operator safety?Here's a design for a continuity tester that employs a TBU (transient blocking unit) to protect the tester in the presence of mains voltages:https://youtu.be.com/N2M-p-OGvPgDiscussion of the TBU starts around 9:00.

author: c64
author_rank: Frequent Contributor
timestamp: July 16, 2020, 02:21:33 am
content: I have two one-hung-low 9205a multimeters (different brands). I checked how much they output.First multimeter:about 2.9V in diode/continuity modein resistance mode 0.25V (except in 200MΩ range, it's 2.5V)Second multimeter:about 2.8V in diode/continuity modein resistance mode 0.5V (except in 200MΩ range, it's 2.6V)It's hard to say about current limit, current is actually jumping up and down, max I've seen was 1.2mA. Maybe better to check with scope.

author: fourfathom
author_rank: Super Contributor
timestamp: July 16, 2020, 03:06:46 am
content: I collected seven of my DVMs and measured the "continuity" and "diode" modes, short-circuit current and open-circuit voltage. The numbers were:Continuity: 0.17 mA -- 0.66 mA, 0.45 V -- 2.52VDiode: 0.47 Ma -- 0.86 mA, 1.49V -- 2.5VWith a couple of meters I also measured the currents through a small series resistor, and they changed very little if at all. The meters appear to use a constant-current source for these measurements (or at least not a pure resistive source)..

author: bdunham7
author_rank: Super Contributor
timestamp: July 16, 2020, 05:25:04 am
content: Quote from: c64 on July 16, 2020, 02:21:33 amSurely you mean the 200R range, not 200M! If so, my $0.00 @ Harbor Freight D830 is the same way, 0.25-sh on all resistance except 200 and diode check, which were 2.5-ish.

author: c64
author_rank: Frequent Contributor
timestamp: July 16, 2020, 05:36:40 am
content: Quote from: bdunham7 on July 16, 2020, 05:25:04 amNope. I really mean 200M. In 200R range it's 0.25V / 0.5V

author: bdunham7
author_rank: Super Contributor
timestamp: July 16, 2020, 06:03:36 am
content: Quote from: c64 on July 16, 2020, 05:36:40 amWow! I just looked--that's fairly impressive for a $5 meter. I might just have to upgrade!

author: Ian.M
author_rank: Super Contributor
timestamp: July 16, 2020, 08:31:12 am
content: Ohms and continuity measurement has certainly come along way since the days of analog meters with a 22.5V dry cell battery for their high ohms ranges and a D cell for the low ohms range. IIRC the high voltage was needed to get enough current through high value resistors under test to deflect the fixed sensitivity movement adequately, as with no active electronics, the ohms scales had to be calculated based on the reciprocal function, (with an offset for a short-circuit current limiting resistor), as current sources were impractical, which meant they were always very cramped at the left hand end. IIRC ohms zeroing was handled by an adjustable shunt across the movement, which 'threw away' sensitivity to match the full scale (zero) deflection to the current battery voltage. The analog meter polarity reversal on ohms ranges was because it was usual to basically use the current ranges shunts and switching, and to maintain the same direction of current flow in the external circuit driven by an internal voltage source, the polarity had to be reversed.If anyone's got an Avometer or similar old pro grade analog multimeter handy with good batteries in, perhaps you could post the open circuit voltage and short circuit (zeroed) current on its highest and lowest Ohms ranges, together with the nominal range and the meter make and model.

author: EEVblog
author_rank: Unknown
timestamp: July 17, 2020, 09:35:06 am
content: FYI, beware measuring ohms output voltage in the high resistance ranges, as then the typical 10M input impedance of the test meter comes into play.

author: mikerj
author_rank: Super Contributor
timestamp: July 17, 2020, 09:45:03 am
content: Quote from: Peabody on July 14, 2020, 06:22:07 pmThe vast majority of analog meters have polarity reversed w.r.t test lead colours in ohms mode, something you quickly learned if you grew up using them. This makes sense when you think about the design, ohms mode is effectively a current mode with a voltage source connected in series. Correcting the polarity would require more complex switching.

author: kripton2035
author_rank: Super Contributor
timestamp: July 17, 2020, 09:56:53 am
content: make yourself a simple continuity meter, like the "shorty" device.http://kripton2035.free.fr/continuity-repos.html

author: c64
author_rank: Frequent Contributor
timestamp: July 18, 2020, 03:25:14 am
content: Quote from: bdunham7 on July 16, 2020, 06:03:36 amYes it's quite good. But yours is still better value for money, because is costs infinitely less ($5/$0=∞)

author: H713
author_rank: Regular Contributor
timestamp: July 18, 2020, 09:56:35 am
content: Quote from: bob91343 on July 14, 2020, 04:10:12 amIf you're working on audio power amps or anything with lots of discrete transistors, you will use the diode testconstantly. If an amp is blowing fuses (or making a lab supply current limit), one of the first things to check for a collector-emitter short on the output devices. Extremely common failure, especially in some amps. Diode test doesn't tell for sure that a transistor is good, but it will tell you if it is completely toasted.You should also keep in mind that the capacitance in your scope input can be dangerous too, especially on circuits with high voltage rails. An obvious example would be a modern high voltage power supply. You may have 600V for the power section (not all that rare to find a power supply that can output 600V), but also some sensitive 3.3V logic. It's very easy to blow things up by charging that capacitance up to a high voltage, then probing a logic circuit or other low voltage solid-state part.Mind you, it's also easy to blow things up by slipping with a probe.The point I'm trying to make is that you always need to be careful when taking measurements to ensure that you're 1) not interfering with the circuit you're trying to measure, thus affecting your measurements, 2) causing damage to your DUT, and 3) not causing damage to the test equipment and 4) not causing damage to yourself. If you're only dealing with digital logic circuits, #3 and #4 are probably not all that likely, but if you're dealing with a big power amplifier (be it audio or RF), you may need to carefully consider all four.You should never just assume that your test equipment is invisible to the circuit you are testing.