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Vom meters.
Topic Started: Saturday, 10. December 2011, 20:27 (738 Views)
Meissen
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Hi,
I need a vom for future radio based projects.
To be honest I am lost as to where to start. Shall I go cheap as chips, or arm and a leg?
Digi or Ana The choice is vast.
Whats yours and why?
Thanks in advance,
Jean-pierre 73
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Techy
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Hiya Jean-pierre

The original 'old school' VOMs had a high input impedance so as not to load the circuit under test while making voltage measurements. They generally had a thermionic (valve/tube) device at their heart, and had around 10 Meg-Ohms input impedance.

For high voltage measurement (several tens of thousands of volts) on TV CRT anodes etc, it was possible to use a multiplier probe, literally a probe with a passive resistor chain divider inside which effectively 're-scaled' the meter i.e. 100Volts FSD would become 10,000 Volts full scale deflection (and so on).

In our workshop I use cheap and cheerful ones from Maplins in the UK. They are around six quid each, and sometimes they sell them at a special offer price of TWO for six pounds!

They are 0.5% accurate, which is more than enough for most uses around the workshop.

You could get yourself a Fluke multimeter, but they are approaching a thousand pounds these days when purchased new. I have used both over the years, but still find that the cheaper ones are almost as good and MUCH lighter on the wallet!

One point I would say though, if you are going to use the meter around RF circuitry it might be better to use analogue meters rather than digital ones, as digital one can mis-read quite badly when exposed to RF fields or have RF present on their input.

Analogue meters don't always have a high input impedance though, only maybe 50,000 OPV ad opposed to 10Million OPV for digital ones. You can sometimes find what are called FET input meters, and these combine an analogue meter movement with a high impedance FET buffer. Tandy's used to sell them years ago, again they do still pop up from time to time on eBay and the like.

A good middle path in a workshop can sometimes be to use an oscilloscope for making measurements.

That's my thoughts anyhow :dance:

73, Mark.
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MadMark1
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163 Charlie Tango 020
I use one of these , Its the same meter different name, What i like about it is its the best of both worlds so to speak, Its a Digital meter but its also got a 'Bar graph' that works like a Analogue meter, Its great for seeing fast fluctuating signals that are just a jumble of numbers on the Digital screen, Its also good for peaking circuits as again you can see were it is highest on the bar graph, This meter also measures Temperature so its good for checking how hot a heatsink is and how hot componants are getting. It also reads frequency upto 10 mhz, Plus it links to a PC via usb so you can save measurments and you have a big monitor screen to see it all on.

http://www.alibaba.com/product-gs/228846977/Digital_Multimeter_Multimeter/showimage.html



I also use a rapid electronics meter i had it for more than 15 years now and its my work horse, Its the meter i reach for first, Again its a digital meter, Un-complicated and very easy to use.

I also have a few old analogue meters, There good to use with RF probes for signal tracing etc.



Mark
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Meissen
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Thanks Mark and Mark,

Very informative ,and usefull information.
I will probably pick one up tommorow. At least now I feel armed with some idea : -}
All the best,
Bon chance and good dx 73

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