Interesting phenom capacitor as a double Zener - Page 1
Forum section: electronics / projects

author: ebclr
author_rank: Super Contributor
timestamp: November 11, 2017, 09:37:20 am
content: I was testing my AVR transistor tester, with some high precision capacitor, and the results are impressively accurate.Later I tried a 10-year old electrolyte capacitor never used, and for my surprise resulted in a diode with fv of 3.87V on one side and fv of 4.14V on the reverse side.Then, l connect the capacitor to a power supply with 10V to check if is shorted and show some signal of capacitor action, it's perfect and holding energy, I discharge the capacitor and reconnect to the avr testers now the result is simply a regular capacitor with near the right capacitance value.Apparently, the capacitor must be "activated" before measure in the right way, I guess is related to some chemistry inside the capacitor to form the dielectric, any idea and what is the exact phenom?

author: jeroen79
author_rank: Frequent Contributor
timestamp: November 11, 2017, 11:04:23 am
content: Can you reproduce this with your other 10 year old capacitors?

author: ebclr
author_rank: Super Contributor
timestamp: November 11, 2017, 11:06:07 am
content: Yes is I did with 3 pcs including one, that was used long time ago, same kind

author: Damianos
author_rank: Frequent Contributor
timestamp: November 11, 2017, 07:25:37 pm
content: The phenomenon is due to a chemical reaction and I am not qualified to explain it! So, you can search for technical notes on manufacturers ' websites.From electrical behaviour, an electrolytic capacitor needs application of voltage in order to create or preserve the dielectric. If left unused for many years the dielectric degrades, resulting in increased leakage current, in both polarities. This is the reason for the confusion of the instrument.It can be back to normal by applying voltage to reform the dielectric...The complete equivalent circuit of an electrolytic capacitor includes, among others, two Zener diodes connected in series and in reverse, between the edges of the capacitor. If the reverse voltage, of any of them, reaches the knee, the current increases rapidly and the consumed power boils the capacitor...

author: Circlotron
author_rank: Super Contributor
timestamp: November 11, 2017, 10:14:57 pm
content: https://en.m.wikipedia.org/wiki/Rectifier#Electrolytic

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2017, 10:53:03 pm
content: Yes, whilst the capacitor was left unused, the oxide layer gradually broke down over time and applying a voltage restored the oxide layer. Fortunately nothing bad happened this time, but there is a risk of overheating and explosion, if the current isn't limited. In future add a resistor to limit the current to a safe level, whist reforming an old capacitor.All electrolytic capacitors act as diodes and will conduct if the voltage is reversed, which will cause breakdown of the oxide layer, electrolysis of the electrolyte, releasing hydrogen gas, overheating and an explosion, if it happens for too long. However, if the reverse current is brief or the voltage is kept low, hardly any current will flow, so no damage will occur, which is why a non-polarised capacitor can be made by connecting two electrolytic capacitors back-to-back.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2017, 01:57:44 am
content: Such is the problem with automated tests. You can probably fool it with a small capacitor in parallel with a large one, with a series resistor to the large one.Consider how the tester must perform its check:- Apply a small current to pin 1. Ground every other pin.- Repeat for pin 2, and so on.- For each pin-pair test, measure the voltage drop twice, some time apart.- If a diode is detected on a common pin pair (a BJT is a dual diode), check for transistor behavior by applying reverse voltage to one diode and forward current to the other. Check both directions.- If a capacitor and diode is detected, it may be a MOSFET. Do another special routine.The double voltage measurement is the required minimum. Why? If the load is a resistor or diode, both measurements will be the same. If a capacitor, the second measurement will be higher; if inductor, lower.But how can you tell if anything is ever a resistor or diode? Generally speaking, no two measurements ever match.It may well be that this device uses a 10 or 12 bit ADC, where repeatable measurements are fairly easy -- but, NOT guaranteed, so some allowance must be made.So let us say this: if the absolute change was less than a threshold, it is probably a resistor or diode, and otherwise, an inductor or capacitor.And all this has to take place within a certain time frame, because we can't have the user sitting there for minutes or hours at a time, just to determine if she's testing a LiIon cell or a supercap!So I think it will be very easily fooled by something that looks like a resistor or diode for short time scales (hundreds of milliseconds?), but is actually a capacitor. And I think that's what you have here: an electrolytic that accidentally has a lot of leakage or ESR, which fooled it into the diode-test path.The moral of the story:ALWAYS DOUBT YOUR INSTRUMENTS.Especially if they're telling you something as high-level as what they think a component is! All the instrument knows is what it measures at those terminals, mixed with its internal noise and errors, and filtered through a state machine. It is not smart -- do not let it be an excuse to forfeit your smarts.Tim