Any continuity-Low Ohm Meter DIY projects around? - Page 1
Forum section: electronics / projects

author: caius
author_rank: Regular Contributor
timestamp: June 15, 2015, 03:22:47 pm
content: Hi all,quite often during my reparing job I came across dead shorted PCB.I know there are specific equipments to locate shorts like the "old" Toneohm or the latest "Shortsniffer" but they are expensive and I can't afford them.Is there any free project around that could fit in my case?I found this page:http://kripton2035.free.fr/continuity-repos.htmlThis one seems pretty simple:http://kripton2035.free.fr/Continuity%20Meters/continuity-milli.htmlbut it requires weird -12V and kelvin probes too as far I can see.Am I right?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 15, 2015, 06:40:13 pm
content: The kelvin probes are needed if you want to go to really low resistance - hard to get around that in any circuit. If requirements are not that stringent, simple probes will do as well, but some trimming towards 2 would help, to compensate for the probe resistance.The +-12 V is needed for the old OP. With a newer OP like TS912 a lower voltage like +-2 V or +-3 V (e.g. 4 AA Cells) would be enough, and one could likely even do without the transistors.

author: AG6QR
author_rank: Frequent Contributor
timestamp: June 15, 2015, 06:53:19 pm
content: If you don't need the thing to be in one single box, you can usually accomplish the task with a power supply in constant current mode, and a reasonable precision voltmeter with a good millivolt range scale. Power your shorted pcb with the power supply, using as much current as your board can safely handle (the more current, the more voltage available for your voltmeter to resolve, but the more damage you might do to your board). Hook one lead of the voltmeter to the point where your negative supply connects to the PCB, and probe around with the positive lead of your voltmeter. As you move the probe along the path where current is flowing, you'll see the voltage change. Voltage will be constant along paths where no current is flowing.No special probes required in this setup. There is essentially no current flowing through your voltmeter probes, since the meter is high impedance. This is using the same strategy as four-wire kelvin probing, but instead of feeding all four wires back to one single box, two of the wires are handled by the bench power supply, and two of the wires are handled by the voltmeter.

author: kripton2035
author_rank: Super Contributor
timestamp: June 15, 2015, 07:19:23 pm
content: I dont like the one pretty simple because it uses a 2W resistor and I think it's a waste of power ...I made this one :http://kripton2035.free.fr/Continuity%20Meters/continuity-an220.htmland it works pretty well.I'm now upgrading it to add another range for 10-100?, and add a 100KHz frequency measuring.and also add a speaker with modulation to find shorts like the simple one you're indicating.this meter works in circuit, the test voltage is very low below 100mV,I can test the esr of a capacitor with it, and also the internal resistance of a battery

author: caius
author_rank: Regular Contributor
timestamp: June 15, 2015, 07:22:06 pm
content: Quote from: AG6QR on June 15, 2015, 06:53:19 pmI tried a similar thing with a 6V lanterm battery but after few second a smell came from it.If I I use a PSU then chances are to blow out a lot of components (maybe the first to blow will be the shorted one..) not counting that most PSU won't start due a short protection.

author: caius
author_rank: Regular Contributor
timestamp: June 15, 2015, 07:56:55 pm
content: Quote from: kripton2035 on June 15, 2015, 07:19:23 pmGood design.It's CPU controlled as far as I can see but I unable to find board layout and BOM.P.S.I have a blue ESR meter but it's useless since it show same resistance on every IC.

author: AG6QR
author_rank: Frequent Contributor
timestamp: June 15, 2015, 08:59:36 pm
content: Quote from: caius on June 15, 2015, 07:22:06 pmA lantern battery would not be good for this purpose, because it's a fairly close approximation of a constant voltage source. If you put a short across it, it will draw large amounts of power. And as you noticed, things will get hot, and something may be damaged.What you need is a power supply that has a constant current mode. An ideal constant current power supply, when presented with an ideal short circuit, will dissipate zero power, and not heat anything up. A real-world constant current supply powering a board with an accidental short will be close enough to this ideal that it won't get hot and burn anything. Any typical laboratory bench supply that has an adjustable constant current limit can be used for this purpose.For example, the Mastech HY3003D would do the job. I didn't pick that model because it's a great power supply, but I only picked it because it's cheap and good enough.

author: kripton2035
author_rank: Super Contributor
timestamp: June 15, 2015, 09:18:05 pm
content: I just saw this cheap device on ebay, but I dont know if it's any good or not ... did someone try it ?http://www.ebay.com/itm/351409147904

author: caius
author_rank: Regular Contributor
timestamp: June 15, 2015, 09:41:50 pm
content: Quote from: AG6QR on June 15, 2015, 08:59:36 pmAh, OK, I got it.I have already a similar lab bench supply (0-30V, 0-3A) but I don't have a good precision voltmeter.Only a question : since the boards I'm working on can draw up to 8A, how can handle this with a 0-3A PSU?How adjust the current limiter during this test?

author: ivan747
author_rank: Super Contributor
timestamp: June 16, 2015, 03:17:36 am
content: For close to that price you can find one of these:http://www.ebay.com/itm/Fluke-8810A-5-1-2-Digit-LED-Multimeter-/251992472348?pt=LH_DefaultDomain_0&hash=item3aabebfb1cAn old 5 1/2 digit multimeter. They used to cost a fortune when new. They are very useful and often very accurate.

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 16, 2015, 09:14:41 am
content: What OP seems to look for is what we call a "Buzzbox" continuity checker for ages. It's a small box that produces a high pitched sound (around 1 kHz) below 1 ohm and the pitch drops at higher resistance, say at 100 Hz at 10 ohm.I have one in my toolbox for over 30 years now and it is one of the very valuable basic tools. It runs from one AA cell which last for years and loads the DUT with less than 1 mA. It has also the nice habit to produce a low pitched sound with one diode drop, very handy looking for faults in PCB's. If there is interest I will see if I can find the schematic somewhere in my archives.

author: Fungus
author_rank: Super Contributor
timestamp: June 16, 2015, 11:13:34 am
content: Quote from: caius on June 15, 2015, 07:22:06 pmLantern batteries aren't constant current.

author: caius
author_rank: Regular Contributor
timestamp: June 16, 2015, 04:03:27 pm
content: Quote from: Pjotr on June 16, 2015, 09:14:41 amYes, it's what I'm looking for.Let me know if you manage to find schematics.Thanks.P.S.Found this:http://www.avrfreaks.net/projects/buzzbox-audible-ohm-meter

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 16, 2015, 05:41:24 pm
content: Yes, no electronics without a micro controller nowadaysI build it with only cheap bjt transistors and runs down to 0.9V. It was inspired by a commercial device called "Testofon", still available today (Google for it). But that thing cost you between 100 and 150 bucksIn the meantime here some pics. It has no on/off switch, has a volume control and the switch switches between buzzer and a Led.

author: kripton2035
author_rank: Super Contributor
timestamp: June 16, 2015, 06:37:01 pm
content: is it the plan for it ?

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 16, 2015, 06:49:54 pm
content: No, it is a bit more complex than that. But that circuit may work as well. Mine works on a single AA cell and has protection for accidental high voltages. Hope to find some time this week to dig up the original circuit.(edit)Here you'll find a complete original "Testofon" as a diy kit:http://taco-nauert.com/texte/prdktue_bs_en.htmand here the instruction manual with schematic (at the bottom of the page in German):http://taco-nauert.com/texte/download_en.htm

author: caius
author_rank: Regular Contributor
timestamp: June 17, 2015, 07:00:16 am
content: Quote from: Pjotr on June 16, 2015, 06:49:54 pmThe Testofon 4000 kit seems good but I don't if its range (0-1,5MOhm) can be suitable to locate shorts.It seems more a simple continuity beeper like the ones integrated in multimeters.What do you think about?

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 17, 2015, 11:07:43 am
content: The disadvantage of multimeters is that many reacts too slow and have an auto power off at inconvenient moments. That is not handy if you want to check i.e. a complete wire wrap board or a hard wired digital proto board or PCB. The "Testofon" does a good job with that, reacts immediately and is rather simple to build. But if you also want to discriminate bad switches and connectors you need a resolution of 1 ohm. That is not what the Testofon is capable of. It is up to you. But if you build it, it will last probably for decades on your workbench/toolbox

author: caius
author_rank: Regular Contributor
timestamp: June 17, 2015, 11:19:59 am
content: Quote from: Pjotr on June 17, 2015, 11:07:43 amI would mainly use it to located shorted components on PCB and not for testing switches or connectors.Anyway I just built the circuit I early posted and it's doesn't work for me, I used a 8Ohm 0.5W speaker (no polarity is indicated on schematics) and I can't hear anything from it when I shortcircuit the probes.I used two UA741C and take + 12V and -12V from an ATX PSU.P.S:I can't measure any squarewave on the output first UA741C ('A1' on schematics) but only a 10V DC.

author: kripton2035
author_rank: Super Contributor
timestamp: June 17, 2015, 11:58:35 am
content: this one is also simple to build, and is made to detect pcb shortsI did not build it but intended to before making the locked milliohmmeter with a psoc.may be I will try it anyway.http://kripton2035.free.fr/Continuity%20Meters/continuity-elv.html

author: Fat
author_rank: Regular Contributor
timestamp: June 17, 2015, 02:18:13 pm
content: Quote from: Pjotr on June 16, 2015, 09:14:41 amI'm interested in getting a schematic for this.Thanks,Fat

author: kripton2035
author_rank: Super Contributor
timestamp: June 17, 2015, 03:03:33 pm
content: Quote from: Fat on June 17, 2015, 02:18:13 pmit's here at the top of this thread...https://www.eevblog.com/forum/projects/continuity-low-ohm-meter-diy-projects-around/msg694737/#msg694737

author: Fat
author_rank: Regular Contributor
timestamp: June 17, 2015, 10:35:36 pm
content: Thanks for the link!

author: eas
author_rank: Frequent Contributor
timestamp: June 18, 2015, 07:21:07 am
content: Quote from: kripton2035 on June 15, 2015, 09:18:05 pmTalk to Franky. I think may have one.

author: Gyro
author_rank: Super Contributor
timestamp: June 18, 2015, 09:41:33 am
content: You could always try the 'Ultimate Continuity Tester' that was published in Electronic Design. It will even tell you if your transformer secondary windings are in phase!http://electronicdesign.com/analog/ultimate-continuity-tester-hides-many-tricks-its-sleeve

author: Gyro
author_rank: Super Contributor
timestamp: June 18, 2015, 09:53:18 am
content: Or for actual low resistance measurement, the Maxim milli-ohm multimeter adapter App note 106, using whatever parts you have to hand:www.wentztech.com/radio/resources/Projects/AN106.pdf

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 18, 2015, 10:25:11 am
content: Quote from: Gyro on June 18, 2015, 09:41:33 amYes, that's the art of electronics

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 18, 2015, 01:46:15 pm
content: Finally found the schematic diagram of the Buzzbox I crafted 30 years ago. The original protection used a thyristor crowbar which is replaced by a more modern TVS diode. In contrast with I wrote before, it will not beep on forward biassed diodes, only with a resistance between 0 ohms (1 kHz) and 100 ohms (~ 10 Hz). The original had a high resistance option that did beep with diodes. But it was never used in practice, so I left it out. It consist of an exponential resistance to current converter which drives a current controlled a-stable mulitvibrator. Dependant on your sensitivity to pitch you can discriminate between 0 ohm and 1 ohm.