boring test

Patience. It should eventually short: "Polymer electrolytic capacitors, tantalum as well as aluminum polymer capacitors are polarized capacitors and generally requires the anode electrode voltage to be positive relative to the cathode voltage. Nevertheless, they can withstand for short instants a type dependent reverse voltage for a limited number of cycles.[54][55] A reverse voltage higher than the type-dependent threshold level applied for a long time to the polymer electrolyte capacitor leads to short-circuit and to destruction of the capacitor."

Now all one needs to do is decode what the "type dependent threshold level" means for polymer caps. Sigh.

The [54] and [55] refererences are both to tantalum caps. They are known to fail at reverse voltage, and sometimes at rated positive voltage.

I'll just let my test keep going. I only need to run them at -5 in real life.

This is interesting:

Both anode and cathode seem to have Al2O3 coatings.

My 56u 25V Panasonic cap stands over 110 volts in the positive direction. Maybe I'll crank one up to some outrageous reverse voltage and see what happens.

Promises, promises... It says that they self heal, maybe they can self heal in the reverse direction too.

George H.

Please note the differences in coating thickness. Reverse leakage does not 'form' this layer, as the chemistry just isn't there as it is in Al electrolytics.

The original Oscon product from Sanyo indicated a 20% temporary or 10% continuous reverse voltage rating. I'm not sure what subsequent vendors claim.

Normal leakage currents have a two orders of magnitude response time, over hundred hour periods, for a stepped voltage (though this is speeded up at elevated temperatures). Rated leakage on first use is only specified after one hour at the application voltage.

While the normal failure mode in these parts is age/stress related to produce reduced C and higher ESR, a short circuit failure mode is predicted for forward or reverse overstress, surge charge or discharge current, or simple mechanical abuse.

Parts with 35 or 50 volts rating approach your requirement, without abuse. I'm not sure what your exercise is intended to prove, as qualification of even small numbers of parts outside of their specified limits is likely to be impractical.

RL

If there is no active chemistry, sustained reverse voltage will not degrade the main dielectric layer either. The only long-term trend in leakage current that I'm seeing is decline, so maybe nothing is being eaten away. There is a jump in leakage current when polarity is reversed, but that drops back down with a time constant of hours. I *said* it was boring.

I tested some other polymer part a while back (180u 6.3V UnitedChemCon) and it failed hard at around 11 volts. Apparently different structure from the 56u Panasonic.

Why do you suggest that?

I'm designing basically a many-channel bipolar power supply that has to run from +15 to -5 out, and a big cap across each output would be handy.

We use lots of parts outside of specs, or make up our own specs when the manufacturer doesn't provide them. Testing, maybe to destruction, is one tool.

I wonder what the current profile looks like on a reverse biased tantalum cap. (I had a 35V one last for years at -5V.... intermittent use.)

George H.

Measure it!

For you.. anything.

I flipped it over, at +5V the leakage is ~100 times smaller.

George H.

The 56uF 25V Panasonic polymer cap dies (shorts) at -25 volts!

That Tek 11802 has a CRT display.

Does it have that vintage tube "warmth", though?

That's an important question

The box does warm my office a bit.

It's a magnetic-deflection vertical-raster-scan display, really digital. The video generator board is a couple of square feet of TTL.

Great scopes. I have one 50 GHz head, several SD24 TDR heads, and a dual-channel 3 GHz probe sampler with fraction-pF capacitance.

I left the tant at -5V for an hour or so.. I thne turned it up to

-10V.. lasted for a few minutes.

GH.

Sure warms up the lab, in cooperation with the HP 8566 and 35665A. When more than a few are running, I have to open the windows even in the winter.

Cheers

Phil Hobbs

At one trade show, Tek had a small box that accepted the SD-series sampling heads and interfaced to a PC, making a screen-less sampling scope. They never marketed it. That would be a cool project, a box that used the cheap old SD heads to make a USB oscilloscope. The hardest part would be to find the connector.

I changed it to drive the caps from a function generator. At 1 Hz,

I'll run that for another month or two. That frees up my power supply and DVM and adds a festive Christmas-tree-light effect to my office.

Nice. I'd buy one.

Cheers

Phil Hobbs

SD-25 sampling head internals: Nice photos.

SD-25 rear connector photo:

Looks like a 36W4 connector. ITT/Cannon catalog: There's a photo of the connector on the cover, Pg 1. See Pg 46 and 47 for 75 ohm version and Pg 225 for the 50 ohm variant. Search the catalog for "36W4" as there are other connectors with the same pin arrangement, but with different mounting hardware. I can help assemble the full part number later. You won't like the price.

Better photo of the connector:

Pg 55 has the non-right-angle version. Looks like the connector number is: DDMV36X4SNA197 (50 ohm) DDMV36C4SNA197 (75 ohm) both without screw locks or board locks. My guess(tm) is that it's probably 50 ohms but this needs to be checked.

Only $40 to $65/ea (ouch):

Asking price for sampling heads on eBay vary from $150 to $400: but are selling for much less:

Drivel: Beware of seriously overpriced extension cables: