Idiot-proof way to test Pico Farad caps  - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: October 13, 2013, 02:57:14 am
content: Hi all,is there an easy way to test caps such as 10 or 100 pico farads without a capacitor tester?I had been testing them on a scope using a signal gen square wave and scope, along with a 1k resistor but I'm not convinced the caps are working ok.Thanks in advance.

author: Stonent
author_rank: Super Contributor
timestamp: October 13, 2013, 03:13:28 am
content: What information has led you to you doubt it? We don't know what doubts you have.

author: Dave
author_rank: Super Contributor
timestamp: October 13, 2013, 03:23:02 am
content: You would need an active probe (FET input) to test a capacitance that low with an oscilloscope. Ordinary probes have an input capacitance that is way too big for what you are trying to measure (watchEEVblog episode #453).Active probes unfortunately cost a lot more than the average LCR meter.

author: johansen
author_rank: Super Contributor
timestamp: October 13, 2013, 03:29:07 am
content: LC oscillator.you can capacitive couple your signal generator to such, and only use the rising edge and falling edge of the relatively low frequency square wave to view the much higher harmonics exciting the LC oscillator on the scope.Then substitute known larger capacitors for the unknown and work the math out again, do this with several known capacitors across at least an order of magnitude of frequency.

author: Unknown
author_rank: Unknown
timestamp: October 13, 2013, 05:23:32 am
content: Quote from: Stonent on October 13, 2013, 03:13:28 amOk, these caps are in an Agilent 3312A Function Generator which is giving imbalanced waveforms, in that below 10MHz the waves lose their symmetry (examples in attachments).(Listen, I don't expect anybody to drop what they are doing and get massively distracted by this which is why I was searching for a solution that could be managed easily at this end).Some of the mica caps appeared to be somewhat scorched, so I pulled a few and compared the wave on the scope against known good (same rating) caps, yet all seemed ok, so I (perhaps mistakenly) assumed that all the scorched caps are still good. (photo 5)Following that, I tested all the transistors, without obvious problem, so am now back to suspecting a failed cap somewhere on the main board (as the modulation section seems to work fine). ~ see attachment.Another thing to mention is that the Trigger-Phase/Free-Run control knob has no effect whatsoever.There are no loose wires, nor connections and the only obvious places of excess heat are the Mica caps (which btw are non-existent in Rio de Janeiro).Above 10MHz the waveforms are pretty balanced (photo 6).

author: c4757p
author_rank: Super Contributor
timestamp: October 13, 2013, 05:50:40 am
content: That "scorched" appearance is common. I have brand "new" (OK, quite old, but never used) mica caps that look that way. They are fine.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 13, 2013, 06:44:10 am
content: Quote from: Dave on October 13, 2013, 03:23:02 amgo read Jim William app note (google also abundant with it), you can build FET probe from a few bucks.

author: kripton2035
author_rank: Super Contributor
timestamp: October 13, 2013, 08:44:21 am
content: use this method without the pic and the display ? claims 0.01pF resolution ...http://www.romanblack.com/onesec/CapMeter.htm

author: Unknown
author_rank: Unknown
timestamp: October 13, 2013, 02:30:46 pm
content: Quote from: kripton2035 on October 13, 2013, 08:44:21 amThat looks like a great little system. Shall give it a go for sure. Thanks.

author: edavid
author_rank: Super Contributor
timestamp: October 13, 2013, 03:12:38 pm
content: Quote from: Gazucha on October 13, 2013, 05:23:32 amWhy would you think a bad capacitor would cause that symptom? That's the last thing I would suspect. Start with the symmetry circuit, then compare the positive and negative current sources.

author: Unknown
author_rank: Unknown
timestamp: October 13, 2013, 06:40:55 pm
content: Quote from: edavid on October 13, 2013, 03:12:38 pmThanks for the pointers eDavid, a heads-up had been what I was hoping for.I also put my hand up that I have very limited experience in the field of frequency generation, but imagine that I thought of a bad capacitor because in the manual it states this...Section 4-6. The main generator of the 3312A uses a voltage to frequency conversion technique. A triangle waveform is generated by charging and discharging a capacitor from a constant current source. The time required to charge and discharge the capacitor determines the period of a cycle and, therefore the frequency.That and the fact that the only thing seemingly out of the ordinary in the circuitry were the discoloured capacitors.Manual here...http://circuitslab.case.edu/manuals/Hewlett_Packard_3312A_Function_Generator_Operating_and_Service_Manual.pdfGonna open her up again and have another poke around...