Electronics > Projects, Designs, and Technical Stuff

active low-pass filter from Valhalla 2701C DC standard

<< < (5/5)

SilverSolder:

--- Quote from: Kleinstein on March 01, 2020, 08:36:19 am ---
--- Quote from: SilverSolder on March 01, 2020, 01:28:30 am ---Maybe we could try soaking the capacitor at 10V for a really long time, then disconnect the supply and measure the capacitor voltage as the first point...  - then, let it sit for a long time to leak down on its own...   finally, we come back later and measure its voltage as the second point, and calculate the leakage current based on the difference between the two measurements?  (in a temperature stable environment)

--- End quote ---

This is a reasonable method to get the real capacitor leakage. The difficult (but not impossible) part here is the switch to connect the voltmeter. It should be low leakage, probably switching the voltmeter between the cap and a 10 V source.
With enough time (e.g. 1l hour) the temperature is no longer that critical: 1 pA would still be 3.6 mV/hour and this equivalent to some 3.5 K in temperature change. Chances are one would do a few more than just 2 readings from the capacitor. So more like soak to 10 V for over night, measure for a few seconds second, wait 1 minute, measure, wait 1 hour , measure, wait 1 hour, measure.

--- End quote ---

I'll have to measure the leakage of some of the GPIB switches I have here...  but maybe it isn't necessary with a switch at all?

Since it is relatively few measurements, the "switch" could simply be connecting a grabber for the measurement, then disconnecting it again.  There is no leakage when there is no switch...   and the grabber could be connected to 10V when not used to measure the cap?



David Hess:

--- Quote from: SilverSolder on February 29, 2020, 12:01:16 am ---
--- Quote from: David Hess on February 28, 2020, 09:10:43 pm ---In high impedance and high temperature applications, film capacitor leakage can become a factor.  Mylar (polyester) capacitors have higher leakage than the other types for example.

Film capacitor leakage is also a factor in precision applications.
--- End quote ---

What would you expect from a good film cap e.g. 1uF with 10V on it...   more than 1pA?
--- End quote ---


--- Quote from: Kleinstein on February 29, 2020, 08:29:49 am ---1 pA is not much current. So I would not be surprised to see more than 1 pA of leakage at 10 V. Even a reed relay may show more leakage.
It may be possible to get capacitors that good, but I would take it for granted.
--- End quote ---

When I was designing my high temperature sample and hold, I figured that I needed a leakage specification at high temperature of better than 50 picoamps, at about 4 volts, because other sources of leakage, input bias current and clamped low leakage diode leakage, could be that high.  With a qualified 0.47 microfarad polypropylene capacitor, I did significantly better than that at high temperature.  At room temperature, droop was much better than 1 microvolt/second so less than 0.5 picoamps of which most was from the capacitor.

"Qualified" means that I bought a bunch of polypropylene capacitors types from different manufacturers and tested them all.  I did the same for the clamped low leakage diode which was just a 2N3904 collector-base junction.

SilverSolder:
So, I tested a couple of capacitors using the method above (soak at 10.00000V overnight,  disconnect the supply in the morning and let sit for an hour, take the first measurement, let capacitors sit on the bench with nothing connected for 9 hours or so, then take a second measurement).

C1 -  Large Mica capacitor, rated 0.25uF at 100V
Start Voltage at 08:39 was  9.71413V
Stop Voltage at 17:49 was 8.04941V
Delta:  9.16h,  1.66472V
Lost Q:  0.41618E-6 Coulomb
Avg Leakage current: 12.6pA

C2 - Polycarbonate capacitor, rated 5uF at 50V
Start Voltage at 08:40 was 9.95304V
Stop Voltage at 17:50 was 9.79666V
Delta:  9.16h,  0.15638V
Lost Q: 0.7819E-6 Coulomb
Avg Leakage current: 23.7pA


Do the numbers seem reasonable, or did I mess up the math...

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod