[sdiy] Using non-linear passives

[Tom Wiltshire]

It looks to me there's something we're not understanding (or misunderstanding) here. There's no way a capacitor that is spec'd to 35V loses 90% of its capacitance by 35V. That simply wouldn't be *useful*.

What does this specification and that graph *really* mean?

T.


On 5 Apr 2017, at 18:20, cheater00 cheater00 <cheater00 at gmail.com> wrote:

> The effect is much more dramatic than you say, and it seems to be easily achievable. Here's a post by evilterrance at hotmail.com on the Electronics 101 mailing list:
> 
> > how about losing 80% of their capacitance by the time you are only halfway to their voltage rating?  Look at the graph in the upper right:
> > 
> > https://product.tdk.com/info/en/documents/chara_sheet/C1608X5R1V225K080AC.pdf
> >
> > That +-10% tolerance?  Only counts to ONE volt.  To be fair, that is an X5R...but look up your favorite X7R cap and check that DC bias curve.  Disappointing.
> 
> 
> On Wed, 5 Apr 2017 17:25 Gordonjcp, <gordonjcp at gjcp.net> wrote:
> On Wed, Apr 05, 2017 at 03:04:19PM +0000, cheater00 cheater00 wrote:
> > Hi guys,
> > I've recently been reminded that ceramic caps (eg x5r) will only adhere to
> > their rating up to 1V, and will then start losing capacitance as you go
> > beyond. So effectively a DC bias could create a voltage controlled filter
> > just like that, with no semiconductors. I wonder, has anyone tried this?
> 
> Not significantly.  I guess if you biased them up to a few kV you could get a couple of femtofarads of a swing out of it.
> 
> Bear in mind that varicap diodes actually have a practical method of operation where the gap between semiconductor layers varies with bias voltage, and need about 30V to get a couple of pF out of them.
> 
> --
> Gordonjcp MM0YEQ
> 
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