Freaky Amazing DMM?!

On Wed, 21 Jan 2009 13:50:29 -0800 (PST), Daniel


No, that's the backhand side of the expression. He doesn't blame his tools because he understands when he makes a mistake it's his. Hackers blame everyone else, and the tools (even if they have the best).
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Rich Grise laid this down on his screen :

No shit... exactly my point. A voltmeter with a high impedance is not the tool to use to test for live circuits....
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Bare Arsed Arlowe wrote: Rich Grise laid this down on his screen : On Sun, 18 Jan 2009 19:35:00 +1100, Arlowe wrote: 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. Then pick the right voltmeter for the job, duh. Hope This Helps! Rich No shit... exactly my point. A voltmeter with a high impedance is not the tool to use to test for live circuits....
Thou art truly an arse, yon point is moot. High Z meters are safer to use on live circuits, the high side construction is part of the best protection against adverse contact or trumped up off scale measurements..
Stop smokin that wacky tobacy, and go puteth some pants on, Fool.
The Mighty WontVolt
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Arlowe wrote:

That depends on what you consider a live circuit.
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wrote:

What's it's input capacitance?
RL
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I don't see it listed on the spec sheet, but I've seen a lot of other similar Amprobe meters that are around 30pF input.
Paul
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wrote:

Most likely an R-C circuit (maybe 10M and 1uF) going into a CMOS analog circuit with almost no leakage typically (at room temperature).
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Paul wrote:

It's almost certainly a bootstrapped input amplifier, with the bootstrap gain slightly over unity.
Clifford Heath.
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Thanks Clifford! I think you nailed it. Anyhow, this is a first for me, and a pleasant surprise to learn of this.
I have no affiliation with Amprobe, but as far as inexpensive ($40) DMM's go, this one seems like a gem. I was going to take it back today at Frys Electronics for the PM51A because it claims 1Gohm impedance, while the AM-240 only says > 100Mohms. Hmmm, 14G is far greater than 100M, lol. I'll keep it.
Paul
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BTW, here's a bootstrapped input amp circuit -->
http://web.telia.com/~u43200663/blocks/bootstrapping.htm
Thanks Clifford.
Paul
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Clifford Heath wrote:

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
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Eeyore wrote:

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
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Phil Hobbs wrote:

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
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Nothing new there, many DMM's have selectable "high impedance" or "HI-Z" modes on the mV range. e.g. the Fluke 87.

The input impedance is so high that the small input capacitance maintains a charge when you disconnect. A bit disconcerting when your meter doesn't read zero, but it comes good when you connect a source.
Dave.
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From: snipped-for-privacy@InfiniteSeries.Org (Archimedes'Lever) On Sat, 17 Jan 2009 00:16:40 -0500, snipped-for-privacy@webtv.net (The Mighty Stupid WontVolt) wrote: ==>I see thou art imagining things again Archimetard, You see nothing, DonTard Ksmelly. YOU are a retard. the capacitor can just as well repel any such ions, ye are indeed a boob., No. YOU are the boob in this group, DonTurd Ksmelly.. and that twisted leads test ideology thousts posted - ye both are twisted... You 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. A 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 That 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. You are indeed an electrical nobody, Don. }:{) More proof that we know it is you, IDIOT! The Mighty Voltron Get the fuck out, retard! Ye poor pathetic Troll.
The Mighty WontVolt
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From: snipped-for-privacy@bitbucket.gov (HopefortheHeartless) <Gay Advocate
On Sat, 17 Jan 2009 00:16:40 -0500, snipped-for-privacy@webtv.net (The Mighty WontVolt) wrote: ==>I see thou art imagining things again Archimeturd, You see nothing, Roy Kingjano. YOU are a retard. Why don't you and Roy get a room and give the rest of us a break from your twisted homoerotic foreplay?
B'cause The Roy is not interested in Gay Trolls, perhaps ye can schedule a romp with Architroll, he's may be just what thou needest.
The Mighty WontVolt
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