Hello,

I have a question with regards to measuring continuity using a DMM ohms setting on a non powered circuit board. I read the following here:

"Avoid: Any other kind of continuity tester for PCB work. Never use the Ohms, Diode or Beeper function on your DMM to measure continuity on a circuit board!" http://pdp-8.org/tools.html

The above site recommends to use a zero beeper such as this one:
http://www.jensentools.com/stanley-supply-services-zero-current-audible-continuity-tester/g/1214

The above linked tool indicates there is zero current through semiconductors and it is "CMOS SAFEST".

I also read here that "It is possible to damage semiconductors when using an ohmmeter to test them."

Am I correct to understand that if I use my DMM ohms setting to measure continuity on a circuit board (such as to check if a trace is broken), there is a possibility that components such as semiconductors (e.g. CMOS ICs) could get damaged? Would you consider it worth it to buy the above tool for measuring continuity on circuit boards (esp. those of 70s/80s vintage) instead of using a DMM?

Thank you

 

Am I correct to understand that if I use my DMM ohms setting to measure continuity on a circuit board (such as to check if a trace is broken), there is a possibility that components such as semiconductors (e.g. CMOS ICs) could get damaged?

Although theoretically possible it is most unlikely! -- Definitely avoid passive analog VOMs (high test current) and VTVMs (higher test EMFs) -- Moreover, remain ever vigilant as regards ESD!

Source: Decades of uneventful experience!

Best regards
HP

 

While it's theoretically possible it is most unlikely! -- Definitely avoid passive analog VOMs (high test current) and VTVMs (higher test EMFs) -- Moreover, remain ever vigilant as regards ESD!

Source: Decades of uneventful experience!

Same here. I've been doing continuity measurements with DMMs for as long as they've been commercially available and have never experienced component damage of any sort.

 

The quote is from a page entitled PDP-8!!
I actually worked on a few PDP-8's and do not remember taking any extra precautions of the type mentioned?
Max.

 

In my 40+ years of using analog and digital multimeters, I've never damaged a component with them. My old RCA analog used a battery around 30V for some of the resistance ranges.

 

Thank you everyone for the reassurance. I found it odd that several books also caution about damage to semiconductors by ohmmeters:

Introduction to Electronics, pg 75:
"First ensure that there are no components in the circuit that may be damaged by the current flow from the ohmmeter."
EDIT: the book goes on to say: "Unlike the VOM, the DMM uses very little current to test resistances. This allows the testing of semiconductor junctions."

Fundamentals of Medium-Heavy Duty Commercial Vehicle Systems, pg 312:
"Semiconductor circuits and sensors should not be checked with an ohmmeter, as the meter current may damage the device"

Linear IC Applications: A Designer's Handbook, 342:
"Diodes and small transistors can be damaged by ohmmeter currents."

Troubleshooting Electronic Equipment, pg 193:
"Do not use an ohmmeter scale that has a high internal current. High currents may damage the diodes under test"

===

I am guessing these statements are of more importance to those use analog meters as mentioned by HP above.

 

Thank you everyone for the reassurance. I found it odd that several books also caution about damage to semiconductors by ohmmeters:

If you're unsure of what you're doing, rely on the diode check function on your DVM for checking semiconductor junctions.

I've used a continuity checker consisting of 2 AA batteries and an incandescent bulb to check diodes for decades and have never damaged a transistor junction. I started doing this before DVM's had diode check functions and continue to use my continuity checker because I can see if the junction is normal, open, or shorted without looking away from the component I'm testing.

However, be careful when working on high frequency transistors. They're more expensive and more easily damaged.

 

While it is possible to damage some circuits with a typical ohmmeter, it is pretty unlikely. However, it is very possible to get bad readings because you are forward biasing diodes and transistors when you aren't expecting to.

 

I found the following similar discussion in another forum where someone who does in-circuit continuity tests to find shorts had a concern about their DMM destroying sensitive parts. Others in the discussion talked about using a low voltage continuity tester. The discussion evolved and linked a Q/A here which pointed to a risk of damage to components.

 

If you use a meter that has a high terminal voltage, 3 or more volts, you can cause base-emitter breakdown and possibly a blown junction. I used a Simpson 260 and various Fluke DVMs for over 50 years without any blown transistors, but the possibility exists. Often you will breakdown the junction, but because the current is limited no instant result occurs, but repeated reverse breakdown will cause beta degradation.

 

If you use a meter that has a high terminal voltage, 3 or more volts, you can cause base-emitter breakdown and possibly a blown junction. I used a Simpson 260 and various Fluke DVMs for over 50 years without any blown transistors, but the possibility exists. Often you will breakdown the junction, but because the current is limited no instant result occurs, but repeated reverse breakdown will cause beta degradation.

Is the indication of damage analogous to ESD damage : For example they are those who say they do not follow proper ESD procedures and have never destroyed a component -- yet the component can become degraded but not destroyed.

Can it be said that those who use continuity mode on their DMM and never experienced a component being destroyed may have inadvertently degraded it?

 

I
Can it be said that those who use continuity mode on their DMM and never experienced a component being destroyed may have inadvertently degraded it?

I have never experienced or witnessed it personally.
Max.

 

Is the indication of damage analogous to ESD damage : For example they are those who say they do not follow proper ESD procedures and have never destroyed a component -- yet the component can become degraded but not destroyed.

@van53 -- A friendly bit of perspective...

While the potential for component damage secondary to examination with a DMM is, in certain rare instances, greater than zero - Such is far less likely than alteration/frank damage via exposure to ESD! --- IMO the monitions found in the service literature are upshot of the variety of test instruments available - it's simply 'cleaner' to specify a single instrument than leave the technician's selection to 'chance'. --- That said, inasmuch as it seems this matter is of extraordinary concern to you, investment in the manufacturer suggested instrument may make sense if only for the peace of mind it may provide!

FYI -- The described 'degradation' of, for instance, B-E junctions (Re: BJTs) is typically seen following chronic abuse (i.e. in improperly biased or over-driven units after hours of operation) -- Decreased gain (if any) owed to testing with a DMM would not be measurable - moreover such 'degradation' is non-progressive in the absence of continued abuse...

Best regards and good luck!
HP

 

Another thing to keep in mind is that someone working on their own equipment (or, in many instances, on company equipment) has the right to choose to follow practices that are "away from goodness", to borrow a phrase. But when you are working on someone else's equipment (including company inventory) you need to accept a greater responsibility -- morally, ethically, and possibly legally -- to adhere more closely to "industry best practices".

 

The only reason for a statement like this has to do with some DMM'S will use a 1.5 volt battery to send a signal from point A to point B and this can destroy some IC'S in a heart beat. I always go by the old saying a ounce of prevention is worth a ton of cure..
Bottom line is to make sure you are not going through a voltage sensitive device when using a DMM when checking continuity.
If you can remove the device than do so but make sure you wear a anti static wrist band when handling these devices. This way you can trace a path to the connector without having to worry about going through it.

 

The only reason for a statement like this has to do with some DMM'S will use a 1.5 volt battery to send a signal from point A to point B and this can destroy some IC'S in a heart beat.

In nearly 50 years of working as an electronic design engineer, I have NEVER encountered any DMM that applied a potentially damaging voltage or current during resistance measurement nor any IC (or any other component) that could conceivably be damaged in any way whatsoever by the minuscule voltages and currents being applied by these instruments, let alone be "destroyed in a heartbeat."

DMMs typically measure resistance by passing a calibrated constant current through the unknown resistance, measuring the voltage developed across that resistance, and applying the formula R = V/I to determine the displayed result. The test current is quite low, normally 1 mA or less; on one of my DMMs it is 0.5 mA on the lowest resistance range (500 Ω), going down to 0.045 μA on the highest (50 MΩ) range; on the other DMM, the highest test current is 0.8 mA and the lowest is 0.3 μA.

The maximum open-circuit voltage these instruments apply to a circuit, in the case of infinite resistance, is likewise limited. For the first DMM cited above, the OC voltage is 250 mV; for the second, it is 300 mV. These voltages are so low they can't even forward-bias a silicon PN junction, and that's why they're so low: to allow you to poke around in a circuit measuring resistances without the semiconductors drawing current and giving you "funny" readings.

Sorry, but I find the idea that ANY modern digital multimeter can destroy ANY integrated circuit to be absolute nonsense.

 

@van53 That said, inasmuch as it seems this matter is of extraordinary concern to you, investment in the manufacturer suggested instrument may make sense if only for the peace of mind it may provide!

Are the other benefits which the instrument can provide a justification for purchase?

For example, Wayne mentioned about the high possibility of forward biasing diodes and transistors. Though OBW0549 mentioned in his post how his meters would not do so, the link to the EEVBlog thread indicated how the Fluke 87V put out 7.30V and 1.00mA in continuity mode (interestingly these values also happen to exceed the maximum of some IC as shown in table 2 here).

I would take it that some DMMs would activate semiconductor junctions and the product I made reference to says that it "eliminates all the false readings caused by measuring through a component".

If only for this reason (prevent activation of semiconductors), would the aforementioned tester be a good choice for measuring continuity in a populated circuit board?

 

The normal resistance range on a modern meter does not provide the current to forward bias a diode/transistor junction, this is why they usually have diode test scale which is capable of providing sufficient forward bias current and indicates a volt drop on the meter.
I have always used this to test transistors without any ill effects, generally out of circuit though, as other components can affect the reading.
Max.

 

Are the other benefits which the instrument can provide a justification for purchase?

For example, Wayne mentioned about the high possibility of forward biasing diodes and transistors. Though OBW0549 mentioned in his post how his meters would not do so, the link to the EEVBlog thread indicated how the Fluke 87V put out 7.30V and 1.00mA in continuity mode (interestingly these values also happen to exceed the maximum of some IC as shown in table 2 here).

I would take it that some DMMs would activate semiconductor junctions and the product I made reference to says that it "eliminates all the false readings caused by measuring through a component".

If only for this reason (prevent activation of semiconductors), would the aforementioned tester be a good choice for measuring continuity in a populated circuit board?

I concur with @OBW0549 and @MaxHeadRoom --- As an aside, if you wish to perform in-circuit resistance/continuity testing sans interference from semiconductor junctions - please use the resistance ranges (as opposed to 'Junction Test'). With every DMM I've encountered, the resistance range{s} apply insufficient EMF to bias 'standard' Si and Ge junctions by design. --- Note that 'junction test' does not indicate a resistance - but, rather, the probe-to-probe EMF (so as to indicate forward drop across junctions)...

The specifications/operation of 'Continuity Mode', on the other hand, varies from instrument to instrument - while quality instruments will not likely supply damaging currents - said mode may yield misleading results owing to forward biasing of junctions in the circuit under test -- For these reasons I personally avoid the dedicated 'Continuity Test' feature (as applied to in-circuit examination) -- Should you wish to use said mode, please consult your specific instrument's documentation as regards said mode's maximum OC EMF...

Bear in mind that certain semiconductors exhibit, in essence, a 0V forward bias characteristic -- and, hence, will 'interfere' with readings despite mode (which is not to say said devices are subject to damage by DMMs).

Hope this answers some of your questions?

Very best regards
HP

 

The specifications/operation of 'Continuity Mode', on the other hand, varies from instrument to instrument - while quality instruments will not likely supply damaging currents - said mode may yield misleading results owing to forward biasing of junctions in the circuit under test -- For these reasons I personally avoid the dedicated 'Continuity Test' feature (as applied to in-circuit examination) -- Should you wish to use said mode, please consult your specific instrument's documentation as regards said mode's maximum OC EMF...

For my Extech 430 the manual mentions a test current of less than 0.7mA for continuity mode. The open voltage in continuity mode was measured at .434V. While in continuity mode I tried to forward bias a 1N5817 Schottky diode. I was thinking it would forward bias it and the meter would beep however it was read as open. Why is this the case? Isn't .434V enough to forward bias this diode in continuity mode?