Continuity tester circuit - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: August 29, 2019, 08:15:49 pm
content: I want to build a continuity tester that is not fooled by ICs, so it should preferably use less than 0.3V on the probes. I tried taking apart my DMM to see if I can reverse engineer how it's doing it, but no luckI think I can make it work by using a Wheatstone bridge and comparator, to make a "resistance comparator" but I'm not sure how good this circuit is or if there are better ones:here is my circuitBy removing the 1\$\Omega\$ short between P+ and P- you can see the led turns off and the output voltage of the comparator goes to 0. this can be used to both visually detect the short and use the output of the comparator as a logic output to be used in some kind of automated test equipment.I tried looking online for some circuits but they just make something using a few transistors with no theory of operation / analysis of in what situations they might give a false reading. I think I can use a pot instead of the 500\$\Omega\$ resistor to have adjustable resistance threshold which would be nice.If there are better/simpler ways of doing this, please share the schematics, otherwise what do you think of my approach?

author: 2N3055
author_rank: Super Contributor
timestamp: August 29, 2019, 11:05:08 pm
content: http://elm-chan.org/works/cch/report_e.html

author: OM222O
author_rank: Frequent Contributor
timestamp: August 29, 2019, 11:15:19 pm
content: 500mV seems a bit muchthe circuit diagram is both more complicated and less flexible in terms of having thresholdsusing a micro controller for this task seems a bit of an overkill imho

author: IconicPCB
author_rank: Super Contributor
timestamp: August 29, 2019, 11:28:10 pm
content: would a toneohm like solution do You?Output variable pitch tone ( frequency goes up as You approach a short circuit) good for mm fault discriminationProbe voltage 100mVdoes not like capacitors since measurement signal is not DC

author: OM222O
author_rank: Frequent Contributor
timestamp: August 30, 2019, 12:15:47 am
content: that would also be acceptable since I can use a frequency counter using an MCU if it provides better accuracy than my method. unfortunately the capacitor issue is less than desirable since in circuit operation would be nice. what is the frequency range? if it's not too high, I think it can work.P.S: 100mV probe voltage is perfect!

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2019, 12:43:01 am
content: Quote from: OM222O on August 29, 2019, 11:15:19 pmWhen the microcontroller costs $0.57 (qty 1) perhaps it isn't overkill? Welcome to the new world!

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2019, 12:59:35 am
content: Multimeters use what is called an "ohms converter" which is simply a constant current source across the output which generates a voltage through the external resistance which is then measured in volts mode.

author: 2N3055
author_rank: Super Contributor
timestamp: August 30, 2019, 06:08:21 am
content: Quote from: David Hess on August 30, 2019, 12:59:35 amWhich is what el-chans version does. Processor measures voltage drop on Rx. It also changes pitch to indicate resistance. Resistor layout on input gives it quite nice protection from voltages. If needed a clamp could be added to input to make more robust.It's cheap and simple, and works well.

author: Gyro
author_rank: Super Contributor
timestamp: August 30, 2019, 08:38:59 am
content: I've use the attached one from an old EPE magazine for years. It's simple and reasonably robust against being accidentally applied to powered circuits (not mains!), and has a <0.3V test voltage. Two ranges - 1R (adjustable) and 1M.It's wheatstone bridge based, done well, with a very good circuit evolution description.

author: OM222O
author_rank: Frequent Contributor
timestamp: August 30, 2019, 01:00:48 pm
content: Quote from: Gyro on August 30, 2019, 08:38:59 amThis is pretty much exactly how I designed my versionjust didn't add the safety features such as diodes and resistors on the op amp / comparator inputs.also I don't need 2 ranges, just detecting resistances below the given threshold should be sufficient (for my particular design 500\$\Omega\$ is good enough, but if I want 1\$\Omega\$, it's just a simple math on the resistor values). the only difference is that my probes are between ground and one of the resistors, instead of VCC and the other resistor, which I don't think should make a differenceanyways, I think I'll go ahead with my design since it doesn't seem to have any major issues.Quote from: fourfathom on August 30, 2019, 12:43:01 amI know you can throw a micro controller at pretty much anything these days and call it quits, but it's a really bad practice that makes you lazy and prevents creativity / problem solving skills. all in all, it's bad engineering

author: 2N3055
author_rank: Super Contributor
timestamp: August 30, 2019, 01:59:38 pm
content: Quote from: OM222O on August 30, 2019, 01:00:48 pmThanks for reminding me that most of the time people doesn't want help, but confirmation how they know best...It 's not bad or lazy engineering, it's different engineering and works very well. I didn't recommend to use whole Arduino board, or RPi.ELM-chan's schematic has less and cheaper parts, has variable pitch to indicate resistance, is protected from voltages in circuit, and uses very little current. Your definition of good engineering is different form mine..All you needed to say that you want (for whatever reason) fully analog solution.No need to insult people for using microcontroller.Especially if it is a very good solution.Have a good one.

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2019, 04:18:14 pm
content: Quote from: OM222O on August 30, 2019, 01:00:48 pmThis is thread drift, but you started it! Anyway, I'm here for the philosophy as much as the tech discussion:Building creativity and problem-solving skills isveryimportant, but do that on your own time if necessary. If the problem you are tasked with can be solved simply and obviously and well and cheaply, then that's how you should solve it.Now that I'm retired, I get to define my own problems. Sometimes it just "get the job done". Other times it's "I wonder if I can make this hair-brained scheme actually work?" I'm a true believer in elegance as an engineering principle, but that's not the only principle to be considered. And to a large extent elegance is in the eye of the beholder. I consider that 50-cent micro design to be elegant. A three-transistor design with a resistor bridge might also be elegant. Neither one would be "bad engineering", and which was more suitable would depend on the customer requirements.

author: kripton2035
author_rank: Super Contributor
timestamp: August 30, 2019, 04:44:34 pm
content: here I collected lots of schematics around the web about continuity testers. enjoy.http://kripton2035.free.fr/continuity-repos.html

author: IconicPCB
author_rank: Super Contributor
timestamp: August 30, 2019, 09:35:48 pm
content: Krypton,Thank You.Quite a few years ago i came across the 1995 EDN article and built the buzzer.At the time I was interested in locating copper shorts on PCBs.This buzzer is a Phenomenal Performer. It will zero in on the copper short along a track to within tens of millimeters.

author: OM222O
author_rank: Frequent Contributor
timestamp: September 02, 2019, 05:08:32 pm
content: As a really barebones version I tried it with an MCP6002 and it seems to be fairly reliable, so I bought a couple of LM393As just because the local RS had them available and I wanted to make the prototypes quicklyNow I'm not sure if the device already contains ESD protection diodes or if I should add some (I even searched for "diode" in the datasheet and it didn't find anything). The output is open collector which is nice, since if I want to interface it with an MCU for automated testing, I can just use the pullups on the MCU and have an active low signal, regardless of the MCU voltage. In an AnalogDevices app note I read adding extra diodes can throw off the accuracy (i.e: input offset voltage) by quite a lot (almost 25mV!) so I want to avoid that if the device is already protected (although absolute accuracy is not that important, since in the final application, I will use an analog mux rather than building the same circuit 32 times for automated probing! analog muxes have quite a large tolerance on the contact resistance).

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2019, 06:29:21 pm
content: I don't believe the LM393 contains ESD protection diodes. Bipolar ICs generally don't have ESD protection didoes because they're generally less sensitive to ESD, than MOS devices.ESD diodes connect the input to both supply rails, so they're reverse biased during normal operation: anode to the -V and cathode to +V. It's easy to test for them using a multemeter's diode function.Do you have a link to the application node you're talking about? I can't see how ESD protection diodes will cause errors, unless the impedance is high enough for leakage to be an issue.https://www.electronics-notes.com/articles/constructional_techniques/electrostatic-discharge/esd-circuit-design-guidelines.php

author: OM222O
author_rank: Frequent Contributor
timestamp: September 02, 2019, 07:02:38 pm
content: https://www.analog.com/en/analog-dialogue/articles/op-amp-input-overvoltage-protection.htmlI got the figure wrong, it was 15mV not 25 and it was at a high temprature, but still really significant! the normal offset of the comparator is 2mV max, so even diodes match that, it's a 100% error added, just by using protection diodes! this is due to the reverse leakage current (which schottky diodes that are usually used, are a lot worse compared to regular silicon diodes) and the input resistor. Now there are things that I can do to minimize this, like use 1K resistors instead of 5K, use lower leakage diodes etc, but I think I need to do some error calculations before finalizing the design. I also need to somehow clamp the diodes to VCC-2V which seems like a really odd requirement, but it's specified in the LM393A datasheetI was wondering if I can use regular or even low leakage diodes instead of schottky to reduce the error, but AFAIK the reason they aren't used is because of slower acting and higher voltage drop (0.7 to 1V instead of 0.3).Do you think bipolar ICs don't need as good of a protection compared to MOS counter parts? they seem to match the ESD requirements without including any diodes which seems odd to me.

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2019, 08:12:06 pm
content: You didn't say Schottky diodes, which of course have a huge leakage and contribute to errors, especially if the input impedance is high. The ESD diodes integrated into an IC have a very low leakage current and are seldom an issue. The bias current specification of a MOS input op-amp will account for the ESD protection diodes, so there's no need to consider them separately, unless you've added external ones.You seem to be confusing the common mode range, with the maximum ratings. The LM393's common mode range only extends to +V-2V, but its inputs can withstand 36V, irrespective of the supply voltage. Refer to page 4 of the data sheet.http://www.ti.com/lit/ds/symlink/lm2903-n.pdfBipolar parts don't need as good ESD protection, compared to MOS devices because they're inherently more resistant to ESD. This is because the gate of a MOSFET is an insulating layer of metal oxide, which will be damaged if the voltage across it exceeds its breakdown voltage. Bipolar devices have a PN diode junction at their inputs which will non-destructively avalanche over, when subject to high voltages. Of course there is a limit: a BJT can be damaged by ESD, but it's much less likely to fail, than a MOSFET when subject to the same abuse.

author: OM222O
author_rank: Frequent Contributor
timestamp: September 02, 2019, 08:23:55 pm
content: yes, you are correct! I misread the common mode voltage and the maximum input voltageI'm not sure what kind of diodes they use integrated in the IC, but pretty much everyone uses shcottky for external diodes.I can see how the PN junction of the BJTs would be more immune to ESD, but that also means if I use normal diodes with about 0.7V forward voltage, they would be completely pointless, since the BJTs base starts to conduct around the same voltage, so it's a lucky choice between the base and the external diode conducting(whichever that has a lower forward voltage). is my understanding correct and I should just ignore using external diodes, and just rely on the input resistor?

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2019, 09:41:57 pm
content: The internal ESD protections diodes will be ordinary silicon, not Schottky and the error specifications on the data sheet will obviously take them into account. An external silicon diode will have a larger junction, so will still pass the lion's share of the current, especially if there's another resistor in series with the input.The BJT input is only one diode. The LM393 has PNP inputs, refer to page 1 of the data sheet, so will conduct if the input voltage is negative, but not positive. Positive voltages which exceed the reverse voltage rating of the BJT's PN junction, (much greater than 36V in this case as it's the maximum rating) will result in avalanche breakdown. If the current/energy is low, then no damage will be done, but a large pulse will damage it. If you're worried about damage, then a diode between each input and the positive supply will provide some extra protection. A low leakage silicon diode will not pass enough current to cause an error, as the input bias current to the LM393 will be much higher than that.http://www.ti.com/lit/ds/symlink/lm2903-n.pdf