Hey all!
I built the ultra sensitive capacitor tester, the version before the dual display.
it went well and it works great, however,
in the calibration portion of the video Paul shows that by clamping the clips and the vr fully clockwise, it should begin to go into the red and light the leds in which you then adjust to calibrate.
in my case, if the VR is fully clockwise, it will not allow any leds to light other than the green of course, even if you short the leads.
so what I’ve done is clamped the leads with some pressure , and adjusted the Vr down until the leds begin to react and tune it from there. It works really good, and it’s extremely sensitive, but it’s not exactly calibrated as Paul’s build.
thoughts ?
Welcome, as usual with any project, my best advice is to really look it over. Double and triple check all components are located and oriented correctly, all values are correct, good connections with no solder blobs, etc. Generally it can be traced back to something simple.
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On my first build of the LV Cap Tester, I chose to do it on a generic through-hole perf board and hand wire everything. I had similar problems getting it to “tune up” properly, but even when it was less-than perfectly adjusted it still proved very capable of exposing leaky caps. Eventually however, after making a couple experimental transistor swaps to see if I could get better sensitivity, its behavior became increasingly sporadic and untunable. What I finally determined was that the perf board was becoming ever-so-slighly conductive due to all the solder rosin residue that started permeating the board itself. This very slight level of conductivity, on the order of a couple gigohms was enough to render the physical board unusable. No amount of cleaning or scraping between contacts would bring it back to a working state. I wound up starting over and drawing up a custom, through-hole PCB. (I’m not a huge fan of surface mount construction, and I have a huge supply of older through-hole surplus parts.)
Anyhow, the moral of this story is, watch out for conductivity on the PCB between components, especially at the front end, where the slightest bit of conductivity can throw the circuit off. It essentially is detecting a current leak! In this case it’s in the circuit itself rather than in a capacitor being tested.
@flannel-tuba That is the problem / issue most of the time when builders complain that they cannot either tune it or make it work as described at all. If you watch some videos regarding the newer dual LED versions of the same cap tester he actually stresses it out several times the importance of thoroughly cleaning out over and over again (using certain chemicals) any rosin residue keeping everything as pristine as possible, no touching with greasy or sweet hands etc. And also a key factor that he mentions in particular (at some point) is the need to use a “Low Leakage” fibre glass style PCB. He actually names the part number that he uses (in one of the videos) but I can no longer remember that now. Some other aspects when calibrating is the importance of doing it while the project is positioned in the final metal enclosure (not outside it) because any external factors such as human proximity as well as some noise generators (such as switching psu found all over the place these days) can strongly influence the calibration process. Another paramount aspect in achieving a successful calibration is using/keeping the same PCB design/artwork as he presented/provided because even the trace length and positioning is crucial ! Very important. So, by building it on a Vero-board you might have added some extra inductance and/or resistance that can negatively impact your final results. Also make 100% sure that the VRs you’ve using are new and of a proper high quality and that they have the stated value since even a minute movement or a slight vibration of the cursor can completely change the scenario giving you skewed results that cannot be trusted ! So…be very thorough with with everything and anything. My two cent.