Ceramic cap hoarding

I use your GPIB tools pretty routinely, so I owe you several beers for spectrum analyzer plots. It's just hard to resist when somebody leads with his chin that way. ;)

Cheers

Phil Hobbs

That's right, step aside, a REAL programmer is coming through. And he's got a screwdriver! Duck and cover!

-- john, KE5FX

;)

Cheers

Phil Hobbs

Pry bar, paint stirrer, small hammer, back scratcher,....

Cheers

Phil Hobbs

It's impressive how often a usable tool is within reach.

My default is all 0805s. With 60 mil reference designators. My standard bypass cap is 1U 50V.

Apparently the physically bigger the cap, the better the C-V curve, generally.

Even this 1206 ain't impressive:

Want some distortion?

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as significant capacitance drop at 12 volts. I want to say it was still be low 25% of rated value. That was true of both the X7R and X5R formulations . Since the board has been working for years I figured it just didn't matt er. Decoupling is not an exact science unless a lot of measurements are ta ken of the device current surges.

You speak from too little knowledge. I don't know why you felt the need to make this post. I was talking about the CV curve which impacts every use of a capacitor. I never said CV effects were unimportant as your post woul d seem to imply. What was your point?

I do design analog circuits as well.

Rick C.

That would very much depend on the circuit, no? Even in a decoupling app t he impedance-frequency curve has two pieces, the part defined by the capaci tance and the part defined by the parasitic inductance with a null at the r esonance point. The goal is to get the impedance below a given level over a range of frequencies since PDS (Power Distribution System) noise tends to be broad band. Of course every design is different, but often a range of frequencies just below the null are not addressed by the bulk capacitance. So if your smaller value caps are a tenth of their rated value you may not be designing the circuit you think you are.

Rick C.

And do you use a lot of MLCCs in your "circuit" design where you care about the actual values? That is what we are discussing, no?

Rick C.

Sure. In the present instance, the MLCCs are part of a multi-stage RC filter preventing SMPS junk out of a system with >30 ultrasensitive optical front ends (avalanche photodiode systems for biomed). When you have 3 filter stages, and have to worry about somebody opening the system up and destroying the APDs due to overcurrent from the bypass caps, you care a lot about the C(V) curve. A 3:1 capacitance variation makes a 30 dB difference to the ripple rejection. Not even slightly subtle.

Cheers

Phil Hobbs

Maybe, but according to what you wrote above, a 4:1 variation didn't make any difference. Not exactly a precision application.

My point was that your saying that C(V) didn't matter in your circuit demonstrated that you and I design very different sorts of circuits. Which is fine. ;)

I imagine so, but if you think that a 4:1 capacitance uncertainty is OK in general, your circuits are a lot more forgiving than mine. Which is also fine. ;)

Cheers

Phil Hobbs

People have made nonlinear transmission lines, shock lines, out of inductors and ceramic caps.

The amount of bloody-minded industry required to make that work boggles the mind. If I did that, I'd save up for 100 reels of each kind. ;)

Cheers

Phil Hobbs

They're different between two capacitors in the same size.

Anyone using them above 10v does.

Which are even bigger and more costly.

Daaang, feelin' snarky tonight? ;)

Thinking about having an Appleton Estates rum on the rocks, myself...

Tim

Incidentally, have you noticed how film-like aluminum polymers are? Practically the same energy density, ESR, etc., scaled to a different voltage and capacitance of course. (The low ESR ones anyway. While they cluster towards low ESRs, the full spectrum is available, just as with tants, so you can use them for supply damping without adding a resistor.)

JL's apparently tested some "self healing" polymers, but I don't think he ever said which ones were tested.

I've tested some WE brand ones, which slowly fail shorted at about 120% of rating.

Probably the tempco, and certainly the aging, isn't as good as PP, but you might find other useful commonalities.

Tim

If only it were that simple; there's LOTS of CV curves, the material is nonlinear (it doesn't trace the same curve all the time, it takes on a history, there's hysteresis...).

If you want a well-characterized capacitance value, don't try to save money and/or space with X7R material: it won't work. Knowing a curve is not going to save you, either.