Electytic capacitor ..always with less than 10% of its rated voltage across it..

[treez]

Hello,
C8, in the following schematic , is a 10uF electrolytic capacitor. It is rated 16V.
As you can see, it is across the Base-emitter of a BJT, thus it never gets any more than 0.7V across it.

Do you believe that this should be a ceramic capacitor instead?

 

[Kral]

Your pdf file appears to be damaged/corrupted. Can you re-attach it?

 

[jpanhalt]

Works for me, for what its worth.

John

 

[treez]

yes I just tried it, and it was ok,but here is again

 

[kam1787]

why ceramic? easier to find electytic

 

[treez]

thanks all...by the way...quoting page 11 of the book "IC555 projects" by E.A.Parr (babani publishers) "An electrolytic capacitor does not become a capacitor until about 0.1 of its voltage rating....."
book first published in 1978
book revised and reprinted n 1981
Book printed last in 2006.

is this book wrong?

 

[jpanhalt]

So what are E.A.Parr's credentials for such a statement in that unobtainable book? Can you propose a reasonable mechanism to support that statement? If it is not a capacitor until 10% of the voltage rating, then what is it before that? How do you reach, for example, 9% of the voltage rating?

In fact, according to Vishay, you and Parr have it backward. Capacitance for tantalums decreases with voltage: https://www.vishay.com/docs/40144/capchange.pdf

Is it worth checking other electrolytic types?

John

 

[The Electrician]

In fact, according to Vishay, you and Parr have it backward. Capacitance for tantalums decreases with voltage: https://www.vishay.com/docs/40144/capchange.pdf
John

The Vishay graph shows tantalums as having quite constant capacitance with voltage. It's the MLCC caps whose capacitance is decreasing with voltage.

 

[FvM]

Capacitance for tantalums decreases with voltage

The curves are in fact said to describe MLCC (multi layer ceramic capacitors). I'm not aware of a voltage dependent capacitance with tantalum (or aluminium) electrolytic capacitors.

There's however a problem with leakage currents of aluminium electrolytic capacitors. This may be background of the confused sounding book quote. (From an EPCOS application note)

3.7.5 Reforming
To IEC 60384-4, aluminum electrolytic capacitors are to be subjected to a reforming process before acceptance testing. The purpose of this preconditioning is to ensure that the same initial conditions are maintained when comparing and assessing different products.

For this purpose, the rated voltage is applied to the capacitors via a series resistance of approximately 100 W for VR £100 V DC, or 1000 W for VR >100 V DC, for a period of one hour.

Subsequently, the capacitors are stored under no-voltage conditions for 12 to 48 hours at a temperature between 15 and 35 °C. The leakage current must then be measured, at the latest after 48 hours.

If the capacitors meet the leakage current requirements without preconditioning, this procedure can be omitted.

3.7.6 Leakage current behavior with no voltage applied (voltage-free storage)
The oxide layer may deteriorate when aluminum electrolytic capacitors are stored without an externally applied voltage, especially at higher temperatures. Since there is no leakage current to transport oxygen ions to the anode in this case, the oxide layer is not regenerated. The result is that a higher than normal leakage current will flow when a voltage is applied after prolonged storage. As the oxide layer is regenerated in use, however, the leakage current will gradually decrease to its normal level.

Aluminum electrolytic capacitors can be stored voltage-free for at least 2 years, and capacitors of the SIKOREL series for as long as 15 years without any loss of reliability. Provided that these storage periods have not been exceeded, the capacitors can be operated at rated voltage directly after being taken out of storage. In this case, reforming as described under chapter "General technical information, 3.7.5 Reforming" is not required.

When designing application circuits, attention must be paid to the fact that the leakage current may be up to 100 times higher than normal during the first minutes following the application of power.

When the capacitors have been stored for more than two years, it is decisive whether the circuit will tolerate high initial leakage currents. A circuit that has been stored for more than two years with the capacitors incorporated, should be operated trouble-free for one hour. This will usually regenerate the capacitors so far that storage can be continued.

 

[The Electrician]

thanks all...by the way...quoting page 11 of the book "IC555 projects" by E.A.Parr (babani publishers) "An electrolytic capacitor does not become a capacitor until about 0.1 of its voltage rating....."
book first published in 1978
book revised and reprinted n 1981
Book printed last in 2006.

is this book wrong?

This is totally wrong. All one has to do to verify is to measure an electrolytic cap with an LCR meter. Most LCR meters apply less than a volt AC to the cap, and the DC bias is zero (unless a DC bias is turned on). Under these circumstances, the cap will measure its rated capacitance (assuming it isn't defective).

However, with respect to your question in post #1, there is controversy over this question. Some believe that an electrolytic will "un-form" if substantially less than its rated voltage is applied.

But even if it does, it will probably take years to happen. You can convince yourself of this by considering that electrolytics that have been unused (zero applied DC voltage, in other words) are still ok, and generally measure near their rated capacitance, after years of just sitting in a drawer.

If you're expecting this circuit to perform for years, maybe you should use a non-electrolytic to be safe.

 

[treez]

thanks all...
The Electrician

However, with respect to your question in post #1, there is controversy over this question. Some believe that an electrolytic will "un-form" if substantially less than its rated voltage is applied.

thanks do you have a quote for this

When designing application circuits, attention must be paid to the fact that the leakage current may be up to 100 times higher than normal during the first minutes following the application of power.

..I wish the above quote would state under what conditions such high leakage currents may flow...presumably after x years of storage?..but what is x?

 

[FvM]

..I wish the above quote would state under what conditions such high leakage currents may flow...presumably after x years of storage?..but what is x?

They do state it: "at least 2 years". It's however a conservative estimation, which is right way to approach the problem if you want avoid instrument failure. Please consider also, that a manufacture will only assure product properties that can be verified with reasonable effort.

On the other hand, I observe that most capacitors which have been stored for 20 years and more work flawlessly from the start. A few have large initial leakage currents. It's known that power supply filter capacitors sometimes fail with short circuit when voltage is applied without current limiting after several 10 years of storage. Likewise, timing circuits can show irregular behaviour and coupling capacitor leakage cause wrong bias points.

I'm not absolutely sure about the long time behaviour of capacitors operated without or with very low DC bias. I don't see this point discussed as a problem in application literature. Personally I expect that they still work as capacitors within the capacitance and loss factor specification. I'm sure that there's no "percentage of rated voltage" relation, otherwise the electrolytic capacitor portfolio would have dedicated low voltage/low µF series which isn't the case.