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Old 13th September 2006, 05:10 AM   #1
vax9000 is offline vax9000  United States
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Default How do I know that my measured AC voltage is precise?

I only have a cheap ($3.99) digital multimeter. I measured that the AC voltage from my wall outlets is 125V. How do I know whether it is because that the multimeter is not precise, or it is because the voltage is truely high?
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Old 13th September 2006, 05:20 AM   #2
jarthel is offline jarthel  Australia
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cheaper vom won't be as accurate as a fluke but I would think there wouldn't be a huge difference from reading using fluke and your cheap VOM
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Old 13th September 2006, 06:34 AM   #3
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Default Re: How do I know that my measured AC voltage is precise?

Quote:
Originally posted by vax9000
I only have a cheap ($3.99) digital multimeter. I measured that the AC voltage from my wall outlets is 125V. How do I know whether it is because that the multimeter is not precise, or it is because the voltage is truely high?

Mains voltage fluctuates a lot over the day. If you don't like it, wait an hour and try again.

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Old 13th September 2006, 12:42 PM   #4
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A word about cheap DVMs: I bought several of those little yellow DVMs (ďCentechĒ brand) that go on sale occasionally at Harbor Freight here in the US for $3.99 and sometimes for just $2.99 (!). Iíve compared their readings to simultaneous measurements made by my ďbig gunsĒ, a Fluke 6 1/2 digit thermal true rms lab DVM and an HP 6 1/2 digit true rms lab DVM. The cheap units are surprisingly accurate, especially on the DC scales. But AC measurements (at 60Hz only!) are also quite accurate. The good accuracy probably comes about because of the single laser-trimmed IC used. I keep on hand a large number of DVMs, as well as a couple of good old analog VOMs (Simpson and Triplett). Sometimes I will hook up several meters simultaneously to various voltage points in a project circuit, so I can monitor voltage changes throughout as I tweak things. This is not only faster, but possibly safer since you donít need to move test leads around in a hot circuit. The cheap DVMs come in handy here. I also scatter the cheapies around: one at work, one in the trunk, etc. To a newbie, I would recommend investing in one decent hand-held DVM (Fluke 87, etc.) plus a few cheapies.

And yes, Jan is right about fluctuating line voltages. Try measuring the line voltage during the day and then late at night.
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Old 13th September 2006, 01:03 PM   #5
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I also use the cheap Harbor Freight $4 meters. I have tested them by comparison to other meters costing a lot more. As Brian said the cheap ones are plenty accurate on DC, 50 or 60 Hz AC and ohms. They are not accurate for making an audio power measurement above 400 Hz. Even the good meters start to show inaccuracies above 1KHz or when the distortion is high regardless of what the printed specs say.

If your line voltage reads 125 volts, it probably IS 125 volts, or surely is between 124 and 126 volts. This seems to be common practice in the last few years. I have lived in the same house for 28 years and the line voltage has slowly crept up from 118 volts to 125 volts during that period. Most vintage electronics (and some modern power transformers) were designed for 110, 115, or 117 volts. You get higher than expected voltages when plugging it into a modern AC outlet.

The first picture on my "meter use" page shows several cheap meters compared to an expensive handheld, and a vintage but recently calibrated Fluke benchtop meter. They all read within one count, which is as good as you can expect, and plenty accurate for vacuum tube work.

http://www.tubelab.com/MeterUse.htm
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Old 13th September 2006, 02:48 PM   #6
Sherman is offline Sherman  United States
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My mains voltage seems to be between 122 and 124 most of the time. That is with a true RMS meter.

I generally have to add a few percent to the voltages that PSUD II calculates for just about any PS I design.
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Old 13th September 2006, 09:40 PM   #7
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George,

Nice page on meters. I hadn't come across that page on your site before. We have the same attitude about metering!

(I sent you a private email to your tubelab domain about a Dynacord amp - did you see it?)
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Old 13th September 2006, 10:04 PM   #8
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Quote:
Originally posted by Brian Beck

And yes, Jan is right about fluctuating line voltages. Try measuring the line voltage during the day and then late at night.
Even more interesting -- try measuring the THD of the line at various times of the day AND during the year.

As suggested -- those older HP and Fluke lab true RMS meters are wonderful to have around, as are the Simpson VOM's
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Old 13th September 2006, 10:34 PM   #9
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Old 13th September 2006, 10:36 PM   #10
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Default a question about panel meters

This question may be outside the scope of the original question, and is not directly about tube audio, but all the talk about using multiple meters leads me to ask it.

I have a "breadboard" set up made from an old chassis, a lot of 5 way binding posts, and terminal boards with tube sockets on them. For measuring plate current, screen current, plate voltage, screen voltage etc I have a bunch of "cheap" digital meters that I connect to specific binding posts. I wish at the time I knew about the $4 ones, since I paid $12 to $20 dollars for mine at various times. Anyway my point is that my original plan was to use digital panel meter modules, and build them right into the test set. No wires all over the place etc. Just add range switches and external resistors as needed. But the price of digital panel meters was in the $50 to $100 range as I recall. And as far as I can tell from the specs for these things they are not any more accurate than the cheap meters.

So how is it they can sell entire meters for $4, but you cant get what is essentially the "guts" of one for under $50 ? Or did I just not look hard enough ?

Anybody else using built in panel meters in test set ups, and if so where did you get them ?
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