Semi Conductor Safe Continuity Tester? - Page 1
Forum section: electronics / projects

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 05, 2018, 10:29:54 am
content: looking atHow often do continuity testers damage boardsgot me thinking. i've built a poorman continuity tester a while ago with this in mind, powered by 9V battery. so i think 2V is the safe voltage value? and then 10Kohm impedance will make like only 200uA current when shorted? is this safe or just a brainfart? mainly i built this because the "latched" continuity tester of DMM is a real sucker, it cannot do "sweep" technique on IC pins to find connectivity at even a moderate rate, my tester worked and works a treat. and thinking, my circuit is configurable in term of applied voltage and resistance level that beeps. so if its not safe, maybe i can lower the value later in version 2. see attached, any comment?

author: IconicPCB
author_rank: Super Contributor
timestamp: August 05, 2018, 11:25:38 am
content: I am running with a buzzer which uses kelvin connections to the two probes.Excitation signal is 150mV notionally and a source impedance 1 ohm.Not t0o many junctions whch will turn on at 150mV

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2018, 12:37:20 pm
content: It depends on what you want from your continuity tester?Do you want it to show a forward biased junction as open or closed circuit? If the answer is a diode junction needs to read open circuit, then the test voltage needs to be considerably below 500mV, possibly down to 100mV if you want the same result for Schottky diodes.In any case, the test voltage should be below 5V, because this is the maximum voltage most BJTs can withstand, when reverse biased.Of course the current needs to be limited to under 1mA, to keep heating to a minimum.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2018, 03:50:58 pm
content: 10 K from a 2 V source should be reasonable safe for essentially all parts. The bigger problem would be more like ESD, when doing the first connection. If paranoid would could consider a 3rd wire for electrostatic discharge to be permanently connected via a large resistor.With a possible accuracy in the 1 mV range (after adjustment), the 200 µA test current would correspond to a limit accuracy of some 5 Ohms. The adjustment can only compensate one direction offset of the comparator. So the lower limit may be up to the 25 Ohms range.The circuit looks like a little odd in some points, like using quite some current to set the threshold.

author: not1xor1
author_rank: Frequent Contributor
timestamp: August 05, 2018, 04:02:48 pm
content: this might be better (BTW D3 is a green led)https://www.eevblog.com/forum/beginners/is-it-possible-to-use-a-continuity-tester-here/msg1168981/#msg1168981

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 05, 2018, 07:43:24 pm
content: Quote from: Hero999 on August 05, 2018, 12:37:20 pmno, i just want to check resistance tester, its configurable to beep up to 200 ohm iirc by looking back at the schematics (Ip and In), and it should beep fast or tell me something when i drag IC pins quickly. i will use DMM for diode tester as fast beep is not usually required.Quote from: Kleinstein on August 05, 2018, 03:50:58 pmbecause the noob design is easy to visualize by me, just compare 2 voltage points (Ip and In) and beep. the adjustable threshold will consume another 200uA, so total 0.4mA for probing on short circuit. plus another 100mA maybe for buzzer when beeps. i'll admit its not the best design, just right out of my noob brain. i havent changed the battery though since last year...edit: i just checked, it didnt beep on diode forward or reverse biased, so it cannot check normal diode function. recalculating it, the highest voltage continuity will beep is 39mV (2V * 200 / 10.2K), so its waaaay below any diode forward voltage.

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2018, 08:25:53 pm
content: Quote from: Mechatrommer on August 05, 2018, 07:43:24 pmI'm confused. Do you want it to beep, when connected to a forward biased diode or not? I thought you said you were happy to use your multimeter to test diodes.It should be easy to design a circuit which either treads a forward biased diode ss open or closed circuit. It's just a matter of changing the test and thresholds voltages.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 05, 2018, 09:21:54 pm
content: Quote from: Hero999 on August 05, 2018, 08:25:53 pmno, no need beep on diode for my diy circuit. yes, i'm happy with my DMM. i just check to make sure, not that important, you just raised diode functionality.

author: IconicPCB
author_rank: Super Contributor
timestamp: August 05, 2018, 10:16:30 pm
content: I am interested in locating a short on a board to within a few tens of millimeters.My buzzer does this admirably.

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2018, 07:47:27 am
content: Quote from: IconicPCB on August 05, 2018, 10:16:30 pmYes, why not just use a battery and buzzer?To make sure you don't fry anything, use a piezo buzzer, which doesn't produce and high voltage spikes and a battery with a voltage of under 5V, i.e. no more than three alkaline cells or a single lithium ion cell.It also has a the advantage of not drawing any power, when not in use.The main downside is there'll be no well defined resistance threshold when the buzzer sounds, which will depend on the battery voltage and ambient temperature. It will also have some hysteresis.

author: IconicPCB
author_rank: Super Contributor
timestamp: August 06, 2018, 11:06:35 am
content: The buzzer acts as a "tone ohm" kind of an instrument.Its pitch changes with length of copper track between ( kelvin ) probes to zero in on the actual short circuit fault.

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2018, 11:22:46 am
content: Some threads worth studying:https://www.eevblog.com/forum/eda/pcb-reverse-engineering/https://www.eevblog.com/forum/testgear/wavetek-sf10-short-finder-a-very-useful-tool-if-you-can-find-it/And another I just remembered:https://www.eevblog.com/forum/testgear/multimeter-with-low-continuity-test-voltage/