Re: Proper Ohm/Volt Level for an Analog Voltmeters and Multimeters?

snipped-for-privacy@earthlink.net wrote in news: snipped-for-privacy@4ax.com:

Well,this is not really accurate;the "ohms/volt" or impedance of the meter determines the -loading- or effect the meter has on the circuit under test.When you connect a meter a certain amount of current is drawn from the circuit by the meter,and the higher the meter's impedance,the less current is drawn,and the less effect the meter has on the circuit's performance.Consider the meter as a high value resistor being parallelled with your circuit.A 10 volt range on a 20Kohms/V meter has an effective R of 200K ohms.If this is connected across a 200K resistor,it will effectively make the resistor a 100K resistor,and drastically affect(change) the circuit,making your measurement inaccurate.(The same goes for scope probes.)On a 50K/ohms/V meter,the meter acts like a 500K resistor in parallel with your 200K circuit resistor.If you use a 10Megohm DMM,that meter would have far less effect.(The DMM's input Z is determined by it's input divider,not the basic impedance of the meter like an analog meter.

*Accuracy* of any particular meter is determined by the quality(precision or tolerance) of the meter's components,and it's resolution,or how fine a measurement it can make.

However I was

Most digital multimeters(DMM) have a 10 Megohm input impedance,some are even higher.If you are measuring low impedance circuits,a lower impedance meter has little effect,as it's parallelling a small value R with a very high R.

The analog meter may not respond fast enough to see every 'spike' in the device under test.That's what oscilloscopes are for.

--
Jim Yanik
jyanik-at-kua.net

Hello!

Thank you for the help! And, quite, I would be the first to say that I have so much to learn! :-)

If I understand correctly, then, the 20K VOM would be just fine as a analog meter for most applications, and that if I want precise measurements for "spiking" the best choice would be an oscilloscope. Thus the tandem of a 20K VOM and an oscilloscope would be fine. However for general multimeter usage, since the oscilloscope will catch the "spikes" better, the proper choice would be a DMM. Is this correct?

Thanks again! I really appreciate the help!

Always,

Sam

A pretty good rule of thumb is "do what everyone else does unless you know better." These days, people use digital DMM's for most purposes, and analog multimeters are relatively uncommon.

(I know a bunch of the oldtimers on this group are about to say "hey, I use one every day"; to which, let me just say that I used an ancient HP vacuum tube AC voltmeter just a couple of days ago. And I sometimes fantasize about buying a Simpson 260 for my audio work. But a quick look at the catalogs will tell you which ones are more common nowadays.)

A quality DMM is much cheaper and easier to find than a quality analog meter, these days. For that matter, a cheap DMM is cheaper than a cheap analog meter, and probably better able to take being tossed around in a toolbox.

An oscilloscope is a whole different beast. You talk about wanting to measure "spikes"; there are a lot of things you might mean by that. Analog meters do tend to have faster response than digital meters, but they're still not fast enough to see the sort of spikes that digital circuitry can cause. For that you would indeed need an oscilloscope; but before you go out and get one, you might want to do a bit of research into what a scope does and how to use it. There were several threads on this NG recently instigated by a fellow who went out and bought a used scope without a clue of how to use it, and couldn't even figure out whether or not it was working. A scope is NOT a "turn it on and connect the test leads" device, like DMM's often are.

Most of the better analog VOMs are rated at 20,000 ohms/volt (ie: 50uA at full scale). Most of the DMMs are rated at 10Megohms. Since a DMM samples and there is a long time between samples, they usually miss a "slow spike" more often than not. But a VOM needle will "jump" or otherwise fluctuate. However, if the "spike" is a relatively fast and "low level", both meters will miss it. Fast, regular pulses will show on a VOM as a DC level which is the average value, and if one knows the rep-rate, one could calculate the approximate peak level and pulse width. So, in special cases, they can compliment each other.

VOM: "Hi there, you're looking particularly good today!"

DMM: "Thanks, so are you!"

--
John Fields

Hello all,

Thank you for all of the replies. I appreciate it.

In this case I was reacting to some recent readings which indicated that it would be a good idea to have an analog VOM to see spikes that a DMM would miss. After some looking into the issue I was simply coming away with being unsure of the necessary sensitivity that a VOM would have to be.

Perhaps this opens into a broader issue that I have been concerned about, namely trying to learn practical electronics from books. While I would love to take courses in Electronics Engineering Technology time has just not allowed me to do so. I am hopeful this will change in the near future, but until then I will have to rely on books and hope that I did my research correctly and therefore am learning correct information.

Since I already have a DMM it would seem, if I understand correctly, that the VOM would make a good educational compliment to the DMM but would not really be useful for everyday troubleshooting. And, when my prowess grows in electronics, that I will be best served by the DMM and an oscilloscope. Is this correct?

So much to learn!!! :-) Hopefully it will be an interesting and enjoyable journey.

Thanks again, all!

Always,

Sam

If you want to look at spikes, you need an oscilloscope, _not_ an analog multimeter. Use your meter for what it's intended, and use the 'scope for what _it_ is intended.

snipped-for-privacy@earthlink.net wrote in news: snipped-for-privacy@4ax.com:

Yes.

--
Jim Yanik
jyanik-at-kua.net

questions

impedence

was

order

megaohms,

Most digital multimeters have an input impedance of 10 megohms. For your application, I would recommend a DMM having a high speed bar graph in addition to the digital readout. These bargraph displays are about 10 times the speed of the digital portion and will almost always reveal the presence of transients.

If the spikes are very short you'll have to break down and get an oscilloscope.

Norm Strong

8-)