Choosing ceramic capacitors

You should compare the specifications of the dielectric. As a rule of thumb the working voltage should ba half the rated voltage. When the applied voltage comes close to the rated voltage, the capacitance decreases dramatically.

All but one are X5R and 6.3V is already twice my Vdd!

The one is X7R at 0.099 GBP.

Depends a lot on what you are doing. IMHO:

Stocking hobby bench parts - aim for a tradeoff of high voltage and low price (assuming you care about low price - if you care to believe that the most expensive part is best, then buy that) - pick your point on the curve, given that higher voltage parts will work at lower voltages, and the physical size is the same. Buy at least 100 for the price break - they don't take up much space.

If building for a particular project where cost is more the object and you won't be looking at different voltage applications, the math works differently, though if reliability is key then using 10V parts on 5V is generally better than using 6.3V parts on 5V.

If you can actually find country of origin information on the parts and you can avoid Chinese ones, that is worth doing.

Personal preference is to stop downsizing at 0805, but I'm doing hand assembly and rarely need the smallest possible board. Ease of hand assembly wins, and 0805 is enough easier than 0603 that I generally avoid smaller packages.

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Hello Andrew,

The X7R material is less voltage and temperature dependent if both materials are rated for the same voltage.

Best regards, Helmut

Ok, opposite question, whats the WORST dielectric with respect to applied voltage, A idea for a VCXO comes to mind.

Steve Roberts

I doubt that you want the worst. You only want a small tuning range. right? Dielectric voltage sensitivity can be a double edged sword... distortion products at large signal swing.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

You could make all sorts of fun stuff from nonlinear ceramic caps...

VCO Parametric amplifier Parametric oscillator Voltage-controlled phase shifter Voltage-controlled delay line Shock line Frequency multiplier Mixer Voltage-controlled gain element Tunable active or passive filters LED dimmer Active equalizer And probably more.

Most of these would be novelties, as there's usually a better way to do it.

I can't see any obvious way to make a non-pumped oscillator.

John

There are ferroelectric ceramics that have very high capacitance, but take a persistent stored charge if taken to high voltage.

There is hysteresis.

The temperature effects are horrid, too (a buddy once got a stack and tried to match them with a bridge; his warm hands caused nonrepeatable capacitance measurements).

The "C" in VCO stands for "controlled", you do NOT want to use those worst dielectrics. For power filters where hysteresis energy loss is a good thing, they're fine.

Looking at Digikey in-stock parts, I'd pick the Taiyo Yuden 1.0uF/10V X7R at $40 US for a reel of 4,000 (0.0066 GBP pp).

Good dielectric, widely useful voltage rating, reliable brand, good price, and lots in stock.

Best regards, Spehro Pefhany

These are pretty bad:

Z5U might be worse.

Wow, those are almost as good as hyperabrupt varactors, a decade's capacitance in just a few volts. At constant temperature. Or PTC/NTC thermistors, for measuring temperature (at constant voltage)...

Tim

Perfect, Thanks, Steve.

Wheeeeee ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

Beware of Murata's X7R material, it has a higher voltage dependence than most other manufacturers.

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Hello Mark,

Thanks for the tip. I will have a closer look next time when I choose a ceramic capacitor.

Best regards, Helmut

I've got a reel or two rated 6VDC. If you use the 10% to 20% of WVDC region you get ~50% change per volt!

I've been meaning to use them as varactors some day...

Cheers, James Arthur

Keep in mind that those things also make pretty good microphones, whether you want that feature or not. So your VCO might get the hives when uncle Leroy sneezes.

I'll second that. Taiyo Yuden is my favorite source when it comes to good caps at rock-bottom cost.

XOs can be very microphonic too. On one VME module we did, every time somebody yanked out an SMB test cable - snap! - we'd lose phase lock -

John