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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Nov 2017, 10:43 pm   #1
buggies
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Default Use of X7R ceramic capacitors for coupling?

I have searched this forum for XR7 ceramics in relation to stuffing old capacitors without finding a thread so far.
Could anyone point me to information or does anyone have an opinion on their use? Their cost is attractive and the insulation resistance is 1 GOhm according to this datasheet:-

http://=http://docs-europe.electroco...6b81528e4d.pdf


The radiomuseum site has this:-

http://www.radiomuseum.co.uk/stuff.html

"Chip capacitor, Kemet 1210 Arcshield HV 100nF Ceramic Multilayer Capacitor, 500 V dc X7R Dielectric ±10%; 3.2mm x 2.5mm x 2.1mm plus small piece of Veroboard."
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Old 12th Nov 2017, 10:55 pm   #2
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Default Re: Use of XR7 ceramic capacitors for coupling?

X7R not XR7!

Unless you want accurate capacitor values, it should be fine. X7R is a medium-permittivity ceramic, it varies a bit with temperature but not as much as Z5U.
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Old 12th Nov 2017, 11:00 pm   #3
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Default Re: Use of XR7 ceramic capacitors for coupling?

Presume this is the correct link.
http://docs-europe.electrocomponents...6b81528e4d.pdf

No experience with them but note the warning on the spec about them not being used for AC line filtering and pulse circuits.
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Old 12th Nov 2017, 11:25 pm   #4
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Default Re: Use of XR7 ceramic capacitors for coupling?

Keep in mind that X7R and some other ceramic dielectrics exhibit significant voltage coefficient and are mostly not suitable for coupling (unless the value is chosen while keeping the bias voltage in mind AND the signal is relatively small AND you don't mind some degree of distorsion).
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Old 12th Nov 2017, 11:50 pm   #5
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Default Re: Use of XR7 ceramic capacitors for coupling?

Quote:
Originally Posted by Nuvistor View Post
Sorry about that - still to get the hang of forum (old USENET guy).
Also X7R...

I do remember something about voltage changing the capacitance but not sure how much distortion this would cause - will google tomorrow.
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Old 13th Nov 2017, 10:11 am   #6
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Default Re: Use of X7R ceramic capacitors for coupling?

Bateman's measurements of capacitor distortion got a THD of 0.145% for a 10nF X7R. That was for an intermodulation test with 100Hz and 1kHz signals at 4V and an 18V bias. That is astonishingly high. A COG ceramic (low k) of the same value and under the same conditions gave 0.00009% (0.9ppm)

The lowest distortion capacitor type he measured was for extended foil polystyrene and polypropylene at 0.00005% (0.5ppm).

There are links to his articles on all this, including the equipment needed to make these measurements here https://linearaudio.nl/cyril-bateman...sound-articles

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Old 13th Nov 2017, 10:38 am   #7
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Default Re: Use of X7R ceramic capacitors for coupling?

This is well known about. Maxim wrote a lot about it in capacitor selection guides for PC audio.

The problem is that the small class 2 MLCC's are piezoelectric so based on various voltage dependent factors they change in size and therefore capacitance.

X7R's are designed for decoupling, bulk capacitance and low criticality coupling because they are cheaper and smaller than C0G/NP0 types. I don't know why anyone wouldn't use C0G/NP0/film for coupling to be honest.

Also a lot of X7R's are quite fragile and the SMD ones crack and explode.
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Old 13th Nov 2017, 1:35 pm   #8
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Default Re: Use of X7R ceramic capacitors for coupling?

Bear in mind that coupling caps only have significant signal voltage across them near and below the bass rolloff. For uncritical applications such as AM receivers they should be fine.
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Old 13th Nov 2017, 1:41 pm   #9
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Default Re: Use of X7R ceramic capacitors for coupling?

I remember Cyril Bateman's articles about capacitors in Electronics World!

To be honest, using a crummy capacitor for coupling shouldn't be a problem. AC voltage across the capacitor ought to be negligible, so who care's if it's a distorted negligible?

The trouble starts if it's not just coupling, but also filtering, so there is significant voltage across the capacitor - such as traditional RC intervalve coupling, at low frequencies. Then, capacitor effects start to kick in.

For audio, it's plastic film for me, every time!
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Old 13th Nov 2017, 11:30 pm   #10
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Default Re: Use of X7R ceramic capacitors for coupling?

Each usage case will be different but you can often use small SMD X7R caps as a DC block when coupling wideband RF circuits together. eg for passing wideband signals across the range of a few kHz through to a few GHz. It's usually fine to do this for smallish signals and the insertion loss will be quite low across this huge frequency range. I've used SMD X7R coupling caps at RF at fairly high RF power without any significant issues.

As MrBungle says, you do have to be careful how you handle some ceramic SMD caps. They can form internal stress cracks if they are mildly flexed. A lot depends on the package size/shape. Usually it's the long skinny ones like the 1206 package that can crack the easiest. It's generally a bad idea to put a 1206 coupling cap on an RF trace pointing towards (and close to) an RF connector on a PCB for example. The act of connecting the coax/connector to the PCB can stress and fracture the cap leading to intermittent operation and inevitable failure. I've seen some examples of 1206 caps where all of the caps on a PCB cracked and failed very quickly after the PCB was handled.

Some 1206 caps can be frighteningly fragile. Others can be very robust. The old 50V Murata 100nF X7R 1206 50V SMD caps used to be very rugged indeed. They would appear to survive fairly harsh abuse even if treated badly with a soldering iron during rework. But the most fragile ones I've seen were high voltage 1206 SMD caps. They were laughably fragile and the end terminations would crack off as if they were as brittle as a coffee granule. Your Kemet 1210 packaged chip caps should be a bit more rugged due to the chunkier shape but even the fat ones can crack. It all depends on the internal construction I think.
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Last edited by G0HZU_JMR; 13th Nov 2017 at 11:55 pm.
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Old 14th Nov 2017, 12:03 am   #11
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Default Re: Use of X7R ceramic capacitors for coupling?

Decoupling-grade capacitors like X7R can be disappointing on leakage specs for use as coupling capacitors. Some materials are microphonic as well.

X7R is not a unique material, it's a code for temperature coefficient and range.

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