quick way to measure C-V curve

I connected a 47 uF (that's MICROfarad) ceramic cap to a DVM and a 50 ohm function generator.

Remove the cap and set the fungen to some reasonable frequency, sine wave. Set the DVM to AC and adjust the gen to 1 volt RMS.

Now connect the cap. Set the DC offset of the fungen to some desired value and confirm on the DVM in DC mode.

Now switch the DVM to AC and tune the frequency until the AC voltage is 0.5 RMS. The capacitance in uF is 5500/f.

You can hit a lot of voltage:capacitance points quickly.

Make sure the DVM is reasonably accurate over the frequency range; some are terrible.

I used the 1/2 voltage point because that's easy to eyeball on an oscilloscope; 0.707 would work, with the constant 3180.

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John Larkin         Highland Technology, Inc 
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John Larkin
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47 nF, you say :-)

At least that's what it looks like on the Post-it note.

Reply to
Ralph Barone

From a few larger parts I've checked I've gotten the impression they're deliberately making them on the lower edge of the tolerance band, even at 0V. If I read the part number correctly ;-) that's a 10% part, so it should be at least 42uF at 0V. John's number is lower.

--sp

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Best regards,  
Spehro Pefhany 
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Reply to
Spehro Pefhany

In everything I ever read about designing power supplies, nobody ever said you have to make the cap 3x as large when filtering 15v. Were they holding back?

Reply to
Tom Del Rosso

The capacitance spec is probably given for other frequencies than those resulting from John's test.

Pere

Reply to
o pere o

So, some electronics is built on LIES like that?

Reply to
Robert Baer

Well, if this isn't a real diamond ring, I just got cheated out of $15. ;)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Obamatronics, like Obamacare? 8-)

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Another method would be to hang it in a Schmitt inverter oscillator in series with a really large film cap that is guaranteed not to have any C/V derating. Vary the voltage and log frequency or period. Then you only need one instrument since modern function gerenators can create DC levels (within reason) and also have a frequency/period counter built in.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

How about a little jig? X-Y plot dV/dT against V?

-+- | .---[R]---. | | | | [i] V |\ | |\ | | | \ | | \ | +------------------+--|x1>----||-+-|- \ | | | | / | >---+-> dV/dT V | |/ .--|+ / V ||--' | | / | ||

Reply to
dagmargoodboat

Ceramic caps sure are.

The other Big Lie is mosfet power dissipation. IR started the practise of grossly inflating the power dissipation and current capability of their fets, and most everybody else went along to be competitive.

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John Larkin         Highland Technology, Inc 

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John Larkin

I just used what was in front of me on the bench.

It's surprising how often that the tool that you need is directly in reach. As long as you don't mind using, say, a dial caliper as a hammer.

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John Larkin         Highland Technology, Inc 

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John Larkin

And always have a Swiss army knife in pocket. In case something needs cutting, scraping, filing, prying, punching, stripping, poking, honing, loosening, or a bottle has to be opened.

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Regards, Joerg 

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Reply to
Joerg

Yes!! I added the necessary voltage output.

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 Thanks, 
    - Win
Reply to
Winfield Hill

I've done that! Well, not in real life.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

Samsung ceramic capacitors are notoriously bad. We always use Murata. They measure as advertised on the data sheet

Cheers

Klaus

Reply to
Klaus Kragelund

Thanks Win, I added an oscilloscope!

(Sorry for the low-res screen shot.)

Cheers, James Arthur

Reply to
dagmargoodboat

Thanks, I added a printer output.

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 Thanks, 
    - Win
Reply to
Winfield Hill

Hah! what do you guys think it's Friday? (And anyways James has the voltage going into the external trigger input and not channel 2. :^)

George H.

Reply to
George Herold

And a Toblerone chocolate bar, which every bench needs.

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John Larkin         Highland Technology, Inc 
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