Freaky Amazing DMM?!

Damn you reminded me.

Hah ! Bootstrapping ! You have to be careful in critical apps but I did one 24 yrs ago with 100M input Z. Very few app notes mention it. In fact I reckon I worked this one out for myself.

Graham

Looks like you bought a good 'un ! There are some bargains out there if you search.

Graham

Probably the input of the AM240 has a non linear devices (maybe just the input protection diodes). If I'm right, when you connect a capacitor the non linear device works like a RF detector that charges the capacitor to a voltage lesser than its forward voltage level. The multimeter wires work as the antenna in this case. Try to twist the multimeter wires together and see what happen.

Have fun. Massimo

Whenever I read very small voltages I make sure that my meter leads are twisted.

Twisted pairs on a source and return has amazing noise abatement capacity.

I know the issue you are talking about. HV capacitors charge themselves when their leads are left free, but I'm not sure you can state for sure that they grab the "free ions" of the air, since many of them are not polarized capacitors, so you should answer this question: why +ions should attach to one lead and the -ions to the other? Let me know your point of view about it. Have a nice day. Massimo

It was invented in World War II to get rid of the cable capacitance in hydrophones--the original name was 'ghost repeater'. It's a great help with frequency response but generally doesn't do much for the SNR. Of course in a scope that usually isn't such a worry.

Cheers

Phil Hobbs

If the cap is left out in strong electromagnetic fields such as caused by local radio stations or wifi etc. then it's possible for the cap to slowly charge. These are issues a researcher learns to overcome. I use two layers of metal shielding (large, and small) where one is non- magnetic (for high frequency shielding) and the other is magnetic (for low frequency shielding). Almost all of the time the Mylar cap will

*not* charge even when left outside by itself, and I live smak dead in Los Angeles, California. :-) Although it's common to see electrolytic charge by themselves when left out. Try placing the electrolytic inside a heavy shielded area (faraday cage) and one may see a different result. Although it's possible for electrolytics to charge even inside a shielded area for obvious reasons.

Regards, Paul

The AM240 is not charging the cap per say. So far it has merely tried to *maintain* the caps charge. So far, regardless of the caps charge, or its polarity, the AM240 has tried to maintain the charge.

I've never seen a DMM do this before. Maybe it's possessed. ;-)

PL

.

sual

Yeah, Mylars are good, but even they are imperfect. I've spent probably too much time testing for dielectric absorption over the years. Although Mylars have hardly no dielectric absorption. I guess there are caps better than Mylars in terms of dielectric absorption. An air gap cap, but how big would it have to be to make 4.7uF?

PL

If so, that's easy to fix. Just use a Hex Inverter. A 74HC04 should do the job.

I hope this helps...

--
Guy Macon

In a radio hobby magazine of years back, there someone gave details how to make a gravity detector. More of a gravity detector of other worlds, planets, etc. I gave it a try, seems like it was some kind of capacitor integrator circuit. So I built it and hooked it up to a stripchart. What I got totally surprised me. I got symetrical waveforms of periods of minutes or hours. Maybe the damm thing worked. The theory of the article suggested when objects lined up, they produced varying fields at the detection point. I went no further, but should have measured a full day.

greg

Gee I think I found it.....

greg

Sure, mylars leak, and the meter has some bias current too, however small it might be. If you hit the magic voltage the two might even cancel.

Tom Bruhns posted some remarkable polypropylene cap leakage measurements to SED a few years back. Garden-variety 0.1uF caps had 50-year time constants.

Cheers, James Arthur

ge.

usual

s

In the first link he mentions 50 years for a 0.1uF cap. That comes to

1.6e+16 ohms! Here's the datasheet of my Mylar capacitor or very similar-- don't know the manufacturer of my 4.7uF Mylar -->

The spec shows an insulation resistance of >=3D10 Gohms (20 =B0C, 100 VDC,

60 s), and >=3D2 Gohms (20 =B0C, 500 VDC, 60 s).

Insulation resistance -->

I guess it's possible to make a cap with 1.6e+16 ohms, but I would tend to first believe it's due to either dielectric absorption or small signal rectification of electromagnetic signals. As you know, any two atoms forms a junction. There are a lot of impurities in capacitors, thus forming diodes, albeit poor diodes. It's not really possible to have all of the poor diodes counter act each other out, which is probably why a cap, even a good cap can produce a DC voltage in a good electromagnetic field. In the post he says that he did not place the experiment in a shield. From my experience that's normally unnecessary, unless you're near a wifi or radio station, but you never know. It's difficult to say, but my best guess, and it's just a guess, that he was seeing dielectric absorption, and perhaps a bit of DC voltage produced by rectifying electromagnetic signals.

My quick and dirty test of my 4.7uF Mylar showed 5.25Gohms parallel resistance, but I didn't spend much time logging the data. It's probably a lot higher given time to relax. It's possible the insulation resistance would have increased over time.

Paul

They wouldn't "attach", however, if a free ion hits a lead, it will become stored on that node's plate. Same is true if a person charged with an electrostatic field walks by. Generally both leads will see equal pressure, but if one 'sees' the field and the other doesn't, electrons can be added to that plate.

The main cause of capacitor charging up without stimulus is the memory effect that the dielectric media has. One can discharge a freshly charged cap to 'zero' volts. Check again in a few minutes and it will again show some charged voltage.

I'd say clean the cap carefully with alcohol, soak it at voltage for a few days, then measure leakage.

But if you think it's getting charge from the air, a shielded box would easily answer the question.

Meanwhile, either way you got a nice deal on a DMM with a very high input impedance.

Ain't nuthin' wrong with that.

Cheers, James Arthur

Several ways of doing it. In mine I required a high DC input impedance ina non-inverting amp with gain. I returned the 'positive input load resistor' to a tap in the feedback chain. Does no good for DC offset though. But my application wasn't ultra critical in that repect.

Graham

Hi Paul, lovely place LA, I had been there a long time ago for a vacation, and I promised myself to be there again. I better read your issue (I'm Italian and some time I can misunderstand, but to be honest when I read your message yesterday I was at office during the coffe break and I get too little time to give a "right thought" to your question). I'm not sure, but read this application note from Maxim about the switched capacitors ADC.

If your DMM has this kind of converter, it's probable that in the low- voltage-high-impedance mode it enters directly into it. Figure 3 shows how it works, and the related paragraph explains it. As you can read there, after all the caps are charged with the Vin voltage, the ADC switches the MSB cap to Vref leaving the other to GND, so the output logic compute the bit value just comparing the capacitive divider with 1/2Vref. The following less significat bit is computed the same way just removing the previous cap (this is not explained there but it shoud be done, otherwise it can work) and switching the next largest cap to Vref. The Dummy cap is needed to get the LSB. It's my opinion that the caps maybe charged someways during the conversion by the Vref applied to the divider depending on the dispersion characteristic of their values. This could explain the behaviour of you DMM. Don't you? Have a nice day in LA. Massimo