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

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

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

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

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...

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

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

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

From: snipped-for-privacy@InfiniteSeries.Org (Archimedes'=A0Lever) On Sat, 17 Jan 2009 00:16:40 -0500, snipped-for-privacy@webtv.net (The Mighty Stupid WontVolt) wrote: =3D=3D>I see thou art imagining things again Archimetard, =A0=A0=A0=A0You see nothing, DonTard Ksmelly. YOU are a retard. the capacitor can just as well repel any such ions, ye are indeed a boob., =A0=A0=A0=A0No. YOU are the boob in this group, DonTurd Ksmelly.. and that twisted leads test ideology thousts posted - ye both are twisted... =A0=A0=A0=A0You are a retard. ANY dope knows that twisted meter leads yield a more accurate reading in an RF rich world like that which we live in when taking small signal readings. =A0=A0=A0=A0A reading taken with 4 feet of open leads can get externally influenced. Twisting them reduces such external influences. You are an idiot that you claim to have been in the industry for so long, yet have no knowledge of such a simple premise. take the measure and deal with it do not try to sensationalize it =A0=A0=A0=A0That was the point, you coxucking retard. You are obviously nothing more than a "self made" appliance repair technician wanna be. With that being the extent of your grasp of things electrical, I have no doubt that you would also have zero grasp of this concept. =A0=A0=A0=A0You are indeed an electrical nobody, Don. }:{) =A0=A0=A0=A0More proof that we know it is you, IDIOT! The Mighty Voltron =A0=A0=A0=A0Get the f*ck out, retard! Ye poor pathetic Troll.

The Mighty WontVolt

From: snipped-for-privacy@bitbucket.gov (Hope=A0for=A0the=A0Heartless)

The evil thing about Voltmeters with very high impedance is they will read induced voltages that analog meters wouldn't. It makes a voltmeter useless for checking for live circuits in a crowded panel.

Arlowe wrote in news: snipped-for-privacy@gmail.com:

Why? If the source has a low resistance/impedance then you're ok as soon as you have good probe contact. If not, then you could always twist the probe leads together a bit to try to make a balanced line to cancel induced noise. Crude, but it would help if the signal to be read was DC or audio AC.

A craftsman never blames tools for his failures. Hackers, on the other hand...

krw explained on 19/01/2009 :

If you work with electricity you had better know the limitations of your tools or you will find them...the hard way.

In article ,=20 snipped-for-privacy@gmail.com says...>=20

m...

That is certainly true (though perhaps your heirs are the ones who=20 will find you), but doesn't modify my statement. In this case, the=20 tool *can* be used as long as the one using it knows what he's=20 doing.

krw used his keyboard to write :

Ok....everything seems to pass right over your head.... I pointed out a limitation of a DMM and you seem to be inferring that I am somehow a hack who blames his tools... BTW> I am not a "craftsman" I am an electrcian.. You don't do what I do for as many years as I have without knowing what the f*ck you are doing.

In article ,=20 snipped-for-privacy@gmail.com says...>=20

com...

No, your incompetence didn't pass by anyone.

It is not a limitation of the tool, only the user. Lower the=20 impedance if it's too high. =20

I'll take your word that you don't do "craftsman like" work. You=20 clearly don't know how.

*I* know what I'm doing. You've clearly demonstrated that you=20 don't.