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Old 18th Jun 2003, 9:50 am   #1
wireless_paul
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Default Analogue to Digital Multimeters

Can someone please tell me how to convert the voltage readings that are given in a Circuit which are normally taken on a 20,000 ohm meter (or 1,000 ohm as on some older Radios) to the correct reading that should be obtained on a Digital meter. I have seen this in a publication once before, I think you are supposed to put a resistor across the Test Leads (what value??) when you take the reading.
I have always used a Analogue meter, but must admit in the last year or so have used a Digital meter more often than not.
Thanks.
Paul
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Old 18th Jun 2003, 12:22 pm   #2
Duke_Nukem
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Default Re: Analogue to Digital Multimeters

Yes, you need to add a resistor across the lead.

The voltage measurements quoted on data sheets will be of the form " Readings taken on the 250V range of a 20K/Volt meter " .

This means that the meter should look like a resistance of 250 * 20K =5MOhm (call this 'X'). Now you need to find out the impeadance of your digital meter on the appropriate voltage which should be in its manual. No idea what the value would be but lets pretend for the moment it is 20MOhm (call this 'Y').

Take the product XY = 20 * 5 = 100
Take the difference X-Y = 20 - 5 = 15

The paralell resistor you need is the ratio of the two = 100/15 = 6.67MOhm.

If you dont have the meter's manual, and don't have a second meter that can measure the digtal meter's resistance then this can still be worked out using loading effect - will explain if needed.

TTFN,
Jon
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Old 20th Jun 2003, 3:18 pm   #3
wireless_paul
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Default Re: Analogue to Digital Multimeters

Instead of putting a resistor across the test leads, would it be possible to draw up a conversion table/graph. On the lines of 200 volts on a 20,000 ohm Analogue equalls 220 volts on a digital meter.
Paul
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Old 20th Jun 2003, 3:22 pm   #4
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Default Re: Analogue to Digital Multimeters

Napalm me if you like, but surely if you're going to have a look up chart, it would be easier to use an analogue meter rather than a DMM?
I only say this because when I'm making measurements I like to just 'point and shoot' rather than faff around.
Mind you I tend to use a DMM for everything.
Don't forget lost of trader sheets state that measurements taken with an AVO7, so you would need two charts.
cheers
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Old 20th Jun 2003, 7:57 pm   #5
Duke_Nukem
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Default Re: Analogue to Digital Multimeters

A conversion chart is not possible as the effect of the meter on the circuit depends upon the impeadance of that circuit. So I'd expect a digital meter and analogue meter to give the same HT voltage readings as, say, a trader sevice sheet, but get differences measuring, say, a resistor-fed screen-grid voltage.
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Old 20th Jun 2003, 8:39 pm   #6
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Default Re: Analogue to Digital Multimeters

To duplicate loading you need to know voltage range of original meter . If a 250 volt range with 1000ohms/volt meter a resistor of 250K would be needed across leads of a digital meter. If originally read on a 100 volt range a 100K resistor is needed.

For most work you can ignore loading of digital meters. Many have input resistance in the 10 meg range.

Norm
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Old 22nd Jun 2003, 1:54 pm   #7
joe
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Default Re: Analogue to Digital Multimeters

Digital meters usually have a higher accuracy than analogue, and certainly a higher resolution (Try seeing a change of 10mV on an AVO) They often have other little goodies built into them such as audible continuity , frequency meter, diode forward voltage reading, and I have an Iso-Tech with a built in capacitance meter.
On the down side an analogue meter is far better for adjusting something to a peak (or trough) and if you have a mixture of AC & DC as for example an HT rail with a load of hum on it then a digital meter will probably not give a stable enough reading to be of any use at all.
I appreciate all that a DVM offers and use one most of the time but I wouldn't like to be without my old AVO8

Joe
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Old 21st Jul 2004, 1:00 pm   #8
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Default Re: Analogue to Digital Multimeters

If you want to simulate the circuit loading effect of an Avometer Model 7, it's worth remembering that the advertised " sensitivity " of 1000 ohms per volt applies when the divide by two button is pressed (doubling the sensitivity of the meter so that the scale reading should be divided by 2).

In the 'normal' mode, with the button unpressed, the meter draws 2mA for full scale deflection, hence the sensitivity is 500 ohms per volt.
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Old 21st Jul 2004, 3:37 pm   #9
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Default Re: Analogue to Digital Multimeters

FWIW
Avo 7 on 100v range=1 meg ohm, which just happens to equal the input resistance of most oscilloscopes..........
Personally I would save brainwork and buy an AVO7! They tend to be fairly cheap because not many people think they are suitable for electronics, being 1000 ohm/volt. They are certaily cheaper than AVo8.

Tim
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Old 27th Jul 2004, 7:11 pm   #10
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Default Re: Analogue to Digital Multimeters

Hi

I'm going to get a load of flak for this

I know a lot of folk are wedded to AVO’s various – I’m not that old so I’m definitely not

My advice is to forget about " retro " meters completely. Get yourself a good second hand scope on e-bay, 50 Mhz is adequate and will probably cost you well under £100 these days. The Philips PM 3215 is an excellent “workhorse” for low frequency work – I have used one for years, they also don’t contain a multitude of “specialist” IC’s the higher performance Tektronix beasts do, so can be mended easily. Also buy a good DMM.

The scope will show you exactly what's imposed on the DC (assuming you are not at the front end of TVs and FM radios) and enable you to do adjustments that involve " peaking " . The DMM will tell you exactly what the voltage is. A bit of circuit analysis will tell you if the voltage read by the DMM is correct without having to allow for any " loading " of the measuring instrument. The DMM will not stop the circuit from functioning correctly as a " obo " meter would if used in a high impedance area Also, no parallax errors, bent needles, and broken toes (when you drop the ruddy thing on them).



Chris

PS

If you want to economise, forget about the meter, just use the scope
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Old 27th Jul 2004, 9:29 pm   #11
Tim
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Default Re: Analogue to Digital Multimeters


I use my scope quite a bit. It is a very versatile piece of equipment and can tell you much more than a simple meter.
Mine(a Solartron CX1400 1967) is about as old as I am.
I would say a scope is more useful for solid state electronics, repair of complex equipment or building and designing circuits.than vintage radio work, but does have it's uses.
If anyone is thinking of investing in a scope, get a probe to go with it. Probes are particularly useful in high impedance circuits(such as valve stuff) and also enable the scope to measure higher voltages.
Personally I wouldn't be without one!
Tim
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