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Old 31st January 2008, 12:19 AM   #1
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Default Is a VTVM useful?

I have a Fluke 189 with 10Mohms input impedance. I have been told that using a non-VTVM for measuring things (like the PI voltages) are incorrect, and I should use a VTVM.

No really - do I need one? Or is my Fluke good enough.

If so - what is a good one to get?

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Old 31st January 2008, 12:26 AM   #2
SY is offline SY  United States
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Most VTVMs (do they even make them any more?) have an 11M input impedance. You're fine with the Fluke.
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Old 31st January 2008, 12:41 AM   #3
poobah is offline poobah  United States
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I don't know if anyone even makes them anymore...

There are circuits where the 10M impedance can throw things... the key is being aware of it... "grid leak circuits" come to mind.

Unless you want to spend 10K or so on a cal lab style meter... I think your more or less stuck with 10Meg these days. Even the big meters go to 10M on the 200 Volt and up scales.

Keep in mind, that circuits designed with impedances this high are probably a pile of trouble for a host of other reasons. Noise and interference wreek havoc.

Just finger oils are enough to screw these circuits as well.

Plenty of VTVMs on Ebay though...
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Old 31st January 2008, 02:32 AM   #4
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Whut they said.  In the olde dayes, the virtue of a VTVM was it's negligible measurement load; most VOMs were 10K/V or 20K/V, and you had to work arithmetic to figure out how your meter load was affecting the reading.  Nowadays, I don't use my VTVM all that much unless it's a moving reading where an analog (meter with a needle) readout can show me what the trend and movement are, and the flashing digits on the Fluke are just confusing.

I still like my EICO, though.  It's such a piece of dawgpyle, construction-wise, and yet it does such a good job.  Pain in the neck probe, though.

No, in my opinion, you don't actually need one.  As an electrogeek you might want one, though.

Aloha,

Poinz
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Old 31st January 2008, 02:33 AM   #5
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I tossed my VTVM years ago because after living in south Florida for 50+ years it absorbed too much moisture. I could not keep it from drifting, and it never gave a stable reliable reading. The 10 meg input impedance is a good thing, but a high impedance digital meter can be had which is far more accurate and stable than a VTVM could ever be. The extra 1 meg that a VTVM provides is 10%, about the accuracy of a VTVM.

Beware of some of the cheap (under $10) digital meters. The input impedance of these can be under 1 meg, and varies when you change ranges. I have a Fluke DVM, but I only use it when I suspect that the readings on my $50 handheld meter are incorrect. I use the $5 meters in most of my experiments. I don't cry when I blow one up. The readings on the cheap meters get really off once the battery gets weak. Replace the battery when the low battery icon comes on even if the meter appears normal.
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Old 31st January 2008, 02:35 AM   #6
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Quote:
I still like my EICO, though. It's such a piece of dawgpyle, construction-wise, and yet it does such a good job. Pain in the neck probe, though.
Mine was an Eico kit that I built when I was about 10 years old. Thats probably why I kept it so long.
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Old 31st January 2008, 02:43 AM   #7
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Well, then, you know about EICO instructions.  I was fresh off three DynaKits (which had the best instructions in the universe), and it was a real wakeup call.  I wasn't ten, I was a twenny five stoned hippie, which is probably a rough equivalent, though.  I tried the signal generator as well, and never did get it working right.  Now, I would know how to get that 60Hz peak out of the output.

Ah, EICO,

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Old 31st January 2008, 03:22 AM   #8
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i use mine for repairs all the time. it's an old Eico that I reconditioned. I do mostly instrument amp repairs, and having an honest to God needle is useful in debugging modulation circuits and the like.
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Old 31st January 2008, 03:25 AM   #9
poobah is offline poobah  United States
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I would have to agree...

I don't think I have one anymore... but there is something nice about a needle sometimes. Kind of a low frequency scope...

You don't need VTVM for a needle though. SIMPSON made the all classic I believe... the model name escapes me right now...

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Old 31st January 2008, 04:53 AM   #10
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260.

I gots one of those too.  Another nonstandard probe connection, the probes cannot be subbed into or out of any other meter, so like the EICO, it has its own set of dedicated probes. Reall PITA.  It does, though, have the low impedence (relatively heavy load) of other VOMs.

You can drop it off the roof, though, and just go pick it up and plug it in.  It's like those Ma Bell phones we had when were kids.

Aloha,

Poinz
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