Electronics > Beginners

Fake polypropylene capacitors from China?

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Krampmeier:
I ordered some polypropylene foil caps from china, which I wanted to use in a ripple filter for a PWM DAC. I chose this type of caps because the dielectric absorption of other capacitor types can increase the settling time of the DAC significantly. I have already seen that effect with the tantal caps I have in my circuit today.

I compared the chinese caps to some others I found in the lab, and got some surprising results.

Method:
I set a Keithley 2461 sourcemeter to 1 µA constant current and a voltage limit of 2 V. Then I discharged the capacitors by shorting them and then charged them with the SMU. The pictures show the current when the capacitors reach the voltage limit. I think that with an ideal capacitor, the current would drop to zero very fast. Any residual, decreasing current would indicate dielectric absorption.

I used 3 different 1 µF capacitors, and 2 470 nF parts of the chinese caps in parallel, which also resulted in about 1 µF (the capacitance was above the nominnal value).
As you can see from the attachments, the chinese capacitors seem to have less DA than the X7R and the tantal capacitors, but very much more than the Wima MKS4 (polyester) ones. I used to think that PP caps should be even better in this respect than polyester.

Am i just wrong in my assumptions or conclusions, is my method wrong, or have I been betrayed?

Here's a link to the "PP" caps I bought: https://www.amazon.de/gp/product/B07J2ZTSGH/ref=ppx_yo_dt_b_asin_title_o09__o00_s00?ie=UTF8&psc=1

edpalmer42:
I'm not an expert on this.

Wikipedia shows a different method of measuring dielectric absorption.  https://en.wikipedia.org/wiki/Dielectric_absorption . 

This method doesn't really disagree with yours, it's just looking at discharge behaviour rather than charge behaviour.  It's also  easier to derive a number from the measurement just by measuring the 'phantom' voltage.

Ed

TimFox:
When I check capacitors to see if they are polypropylene, I use a good LCR meter (DE5000) and measure the Q factor.  Genuine polypropylene (and polystyrene) should have Q around 1000 (a practical limit for this measurement), but polyester (Mylar) is roughly 100.

Sylvi:
Hi
Polypropylene and polystyrene are pretty much ideal dielectrics. In audio they exhibit zero distortion where polyester adds third-order harmonics.

Polyprop also has that nice self-healing effect making it ideal for use in power supplies.

Polystyrene is next to impossible to get in anything but tiny values.

Buying from China takes some care. There are good and bad vendors, so don't paint them all with the same brush. For a critical performance component it is safest to use reliable broadline distributors like Digikey and Mouser as they work hard to assure that their sources are reliable.

Gyro:
If this is for a one-off project then decently large value Polystyrene capacitors are available NOS from ebay sellers in the former Soviet Union. These tend to be close tolerance and good quality, for little money.

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