Author Topic: Strange Readings (HIGH ESR) from Ceramic Caps  (Read 1046 times)

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Offline phantomEcho

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Strange Readings (HIGH ESR) from Ceramic Caps
« on: December 21, 2016, 08:00:02 pm »
Hello All,

This is my first post to the EEV Blog Forum. 
I've been watching the You-tube channels for a couple years, but never been on the forum until now!

     I have a large batch of purchased random 'grab bag' Ceramic caps.  Values range from a few pF to ~0.1uF.  Some are newer and some older, but there is a large (over 1000) assortment.  Most of the Capacitance values are right in the hunt for the type/class of the Cap I test (ExTech LCR 380193).  If I put a standard Electrolytic on the meter the ESR reading @1kHz is just about right... same with Film caps, etc...  The problem is that the ESR of *EVERY* Ceramic cap I measure has a reading between 10s of Ohms, and low kOhms!  I've looked on google read a number of forum posts in various places where people say that the ESR of ceramics is so low that it is usually ignored... My readings are 1 to 2 ORDERS of magnitude higher than what people say ceramic caps should read.  Given that my meter reads (relatively) accurately for other caps, is there something else going on here?  Does anyone else have a similar LCR meter that might verify this?  Any suggestions for alternative testing that might help? 

~Justin
 

Offline tautech

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Re: Strange Readings (HIGH ESR) from Ceramic Caps
« Reply #1 on: December 21, 2016, 08:15:14 pm »
Welcome to the forum.

I think your test frequency is too low for ceramic caps.

Typically they're used to suppress HF components adjacent to VDD pins on IC's, test again @ 100 KHz.
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Offline phantomEcho

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Re: Strange Readings (HIGH ESR) from Ceramic Caps
« Reply #2 on: December 21, 2016, 08:20:10 pm »
Thanks for the prompt reply!  I had wondered about the appropriate frequency for testing them.  Sadly my test meter only allows 120 (twice our mains frequency) hz, and 1kHz.  Motivation to get a better LCR meter I guess!   :-+
 

Offline tautech

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Re: Strange Readings (HIGH ESR) from Ceramic Caps
« Reply #3 on: December 21, 2016, 08:51:41 pm »
I just checked a couple of 0.1uF 0805 ceramics @ 1 KHz and 10 KHz and the ESR reading was slightly lower @ 10 KHz @ ~2.1  \$\Omega\$.
To another extreme, some 10 uF I have measure under 50 m \$\Omega\$ but with more stable readings of under 20m \$\Omega\$ @ 1KHz.

Tested with SMD Smart Tweezers.
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Offline phantomEcho

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Re: Strange Readings (HIGH ESR) from Ceramic Caps
« Reply #4 on: December 21, 2016, 09:31:08 pm »
Hello again, and again thank you for the reply.

     The fact that you were able to get such low readings at 1kHz is worrying.  Admittedly these are NOT SMD components, rather Radial Lead Through Hole components.  They have numerous values, I tried some standardized '104' and '103' caps and all of them had (at best) readings of ~28 Ohms ESR at 1kHz. 

     Like I said, i've used this meter for hundreds of different caps over the years (bought it in 2011) and it's always been accurate, yet every ceramic cap I read is significantly too high. 

47uF Electrolytic = 1.2 Ohms @ 1kHz, where datasheet says 1 ohm.


1nF Ceramic = 27 Ohms @ 1kHz, closest online reference says it should be ~300 mOhms or less.

The only reference I could find (found: http://sound.whsites.net/articles/capacitors.htm) suggests that 100kHz is a great frequency to test them at, stating that at lower frequencies the Capacitive Reactance swamps meter accuracy with values of less than 10s of uF.    I'm thinking more and more that your assessment about frequency of test is quite right.  Additionally it would explain why your SMD Tweezers Meter and SMD parts were accurately measured at the lower frequencies where my rather crude meter may simply be being swamped out at these low values.   :-DMM

Given that the meter is accurately measuring the CAPACITY of the caps, I'd think this theory fits the facts.  Any thoughts?
Not certain what's going on.  Thanks again for taking the time to try to help.

 

Offline TimFox

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Re: Strange Readings (HIGH ESR) from Ceramic Caps
« Reply #5 on: December 21, 2016, 11:17:16 pm »
Note that for ceramic and film capacitors, roughly (to first order) the Q of the capacitor is approximately constant over a broad frequency range, and therefore the ESR = X/Q will vary with frequency, since X = 1/(2 pi f C).  Specifically, a small capacitance will show a high ESR at low frequencies.
C0G = NP0 ceramic and polypropylene film capacitors have very high Q, roughly 1000 or higher, in the audio range.  X7R ceramic and polyester film capacitors have lower Q.  Polyester is about 200.
See  http://www.vishay.com/docs/26033/gentechinfofilm.pdf
Note that the dissipation-factor values  D = 1/Q  vary by less than 10:1 for a frequency range of 1000:1.  For polypropylene ("KP" in the graph), the variation is less than 4:1 over the frequency range of 1000:1.  If ESR were constant, then Q or D would vary by 1000:1 over the frequency range of 1000:1.
 


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