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Digital volt meter?

FrankD. on Sun August 06, 2006 10:12 PM User is offline


Digital volt meter?


I have been putting off buying a new multimeter for way to long.

What brand do you use and any suggestions?


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FrankD.

Edited: Sun August 06, 2006 at 11:56 PM by FrankD.

NickD on Mon August 07, 2006 12:13 AM User is offline

Your most common types of DVM's use dual slope integration, a scheme whereby an operational amplifier connected as an integrate charges a capacitor at a given controlled current rate depending on the voltage applied, the greater the voltage the greater the charge on that capacitor. This capacitor is then discharged at constant current rate where discharge time controls a gate for a frequency counter, the larger the charge, the longer the gated time, the higher the count on the counter. Since the same clock is used for the sampling as the discharge time, the clock isn't critical that was the key to the dual slope patent issued way back in the 60's. But in reality, the clock is critical for making precise AC measurements and must be an precise multiple of the cycle time of the frequency of the voltage you are measuring. Pure DC doesn't make any difference, but in AC makes a big difference if you are measuring complete cycles or several complete cycles plus a fraction of another cycle, the degree of that fraction is what causes the errors.

Practically all have 10 Mohms of input impedance, a standard, but do have several HP's that have a flat infinite impedance for zero circuit loading for critical work.

Another standard is the number of digits for the resolution of the meter, most are 3 1/2 digits that is confusing to most people, first off, should read, 1/2 3 digits, but doesn't look handy for marketing purposes, the 1/2 doesn't mean a half, but indicates the first digit is one out of two combinations, either a 0 or 1, the 3 indicates 3, 0-9 digits can be displayed for a total range scale of 0 to 1999 meaning one part out of 2,000 resolution. My HP's either have 4 1/2 or 5 1/2 digits with the latter from 0 to 199999 or one part in twenty thousand.

Accuracy varies all over the place even in the 3 1/2 digits DVM's, one part in 2000 reflects an accuracy statement of 0.05% accuracy, would except at least an accuracy of plus or minus 0.01% plus or minus one digit. Stay away from meters that don't specify the accuracy.

Practically all are capable of reading positive or negative voltages unlike the older analog meters where you have to reverse the leads. If the red lead pops up the "-" sign in front of the first digit, the voltage is negative.

Cheaper meters use a radiometric form of resistance measurement where an unknown resistance is compared to a fixed reference resistor in the meter, these supply practically zero measuring current and not very good in determining whether a switch contact is good or bad. Better meters feed a constant current to the unknown resistance with the resultant voltage measurement displayed in ohms. These can also be used to check the forward bias voltage of diodes or transistor junctions, where the former cannot, but some of the cheaper meters do have that extra diode mode position, I favor BK Precision for this reason and is my favorite for automotive work, small, but yet complete.

Nice to have a 10 ampere range, but even with the most expensive meters, have to buy some Empro shunts for the higher currents.

DVM's also feature temperature, transistor, capacitor and frequency measurements, but most are crap, prefer a meter dedicated to those parameters.

Hall Effect current probes are nice so you don't have to break in the circuit, the best use a neutralizing technique where the Hall Effect is used to switch at a zero flux, the actual current is measured by a coil that has a current used to neutralize the flux in the core produced by the measured current using the Hall Effect that can be zeroed out checks that zero state.

Just some things to look at, but I normally grab my BK Precision for automotive work, cheap, and if I break it, it won't break me for what my Ballantines and HP's cost.

bohica2xo on Mon August 07, 2006 12:46 AM User is offline

Wow Nick, a lot of information there!


I use an assortment of meters, depending on what I am poking at. If it looks like it may have more that 48vac, I may start with a wiggington voltage tester...

My "knock-around" DVM is a Fluke 77 that I have had for 15 years. Usually accurate enough for most basic tests. I have a Fluke 189, and really like the datalogging feature. Also have several accesories, like the AC / DC clamp on current probe, and the thermocouple attachment. The 189 reads type K T/C's directly.

If I did mainly automotive work, I would probably have a Fluke 78, with the duty cycle, rpm, & other specialized functions.

Of course, there are other instruments too, like my Biddle Megger, and the 2 channel Omega datalogger...


It really depends on what you plan to do with it.


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"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

k5guy on Mon August 07, 2006 1:02 AM User is offline

I use either a Fluke or a Keithley DVM. Heck, for automotive work, even a cheap Triplett works. If I need to get serious, I use an O-scope.

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FrankD. on Mon August 07, 2006 3:12 AM User is offline



Thank you it is nice to finally find techs willing to discuss multimeter’s.
I was on a couple of TV repair forums and those guys refused to discuss multimeters.

Nick I like how you explaned about the ½ 3 digit.
“Another standard is the number of digits for the resolution of the meter, most are 3 1/2 digits that is confusing to most people, first off, should read, 1/2 3 digits, but doesn't look handy for marketing purposes, the 1/2 doesn't mean a half, but indicates the first digit is one out of two combinations, either a 0 or 1, the 3 indicates 3, 0-9 digits can be displayed for a total range scale of 0 to 1999 meaning one part out of 2,000 resolution. My HP's either have 4 1/2 or 5 1/2 digits with the latter from 0 to 199999 or one part in twenty thousand.”

I would like to have a Fluke 179 but the price and the size is too much. Look at the size of todays cell phones and then look at the size of these multimeters. These things are the size of a brick and could be used to beat some ones head in.

The meter I have now is an old small AW Sperry that measures 5x3x1 but its on its last leg.

The features I like in a meter are AC-DC current to 10A.and auto off. I have killed way too many 9 volt batteries by not moving the dial to off.

BK used to sell a meter like this except in black,
http://www.professionalequipment.com/xq/ASP/ProductID.1775/id.5/subID.326/qx/default.htm

I just looked at BK and they must have lost there minds some of there meters are blue with white rubber boots.

Triplett has a 3303 meter that looks good (not too big) but do not know if they are any good.

Nick what model BK do you have?


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FrankD.

NickD on Mon August 07, 2006 8:55 AM User is offline

BK Precision models 2905 and 2704A. I prefer the 2905 with a power switch on it, can't remember the last time I put a new 9 V battery in this unit, auto shut down units always manage to draw a couple of microamps of standby current. You have to be awake when using any meter.

None of the solid state meters are worth a crap when dealing with high unknown voltages, still use my Heathkit IM-29 that I had for 50 years now, nothing will fry that meter.

Most the of 3 1/2 digit DVM's are based on the Intersil ICL-7606 dual slope IC, regardless of price, you may see the actual chip in the unit, same chip with a different name, or just the chip part of the package surface mounted on the PCB with a drop of epoxy on it. Big differences besides features are the plastic and range switches used on the outside, and whether they use super expensive trimming pots on the inside so you can calibrate the meter. Being very facetious on those nickle calibration pots.

HP makes their own ASIC chips, you don't even want to think about having to replace one, try $1,100.00.

You mention Fluke being pricey, could because you are paying their plant bill in Washington, their engineering department has the sweetest smelling restrooms because the excrement from that department doesn't stink if you know what I mean, but like anyone else, the actual product is made in Asian based countries.

The company is the main source of many of the private branded DVM's:

Tenma

Quite a selection of meters by Tenma, and if you compare their meters with name brand meters selling for 2-3 times the price, the only difference you will note is maybe the color of the case, and in all cases, the name on the front. MCM is always having a sale so if your timing is right, can save another 30% off their prices.

I have yet to see a DVM with an oscilloscope on the front panel that was useful unless you just want to see what a 60 Hz standard outlet waveform looks like and these are very pricey. Still can't beat an old Tek or HP analog scope for troubleshooting the hard stuff.

FrankD. on Mon August 14, 2006 2:25 PM User is offline


Another hand held digital voltmeter question.

On some digital voltmeters when you are in AC or DC volts the meter will count randomly up or down. They will do this until you probe a voltage or touch the probes together. Some manual ranging meters will do the same thing in the lower ranges.

Is this a sign of a very sensitive meter or a sign of a poorly designed meter?

Seams like it would be desirable for the meter to read 0.0000 uV or mV until a voltage is probed.




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FrankD.

NickD on Mon August 14, 2006 3:34 PM User is offline

My meters that have the auto ranging feature also have a switch to turn it off, never liked auto ranging watching the meter playing around. Know what kind of voltage to expect anyway before probing so never use auto ranging.

Even my 5 1/2 digit HP wanders in the 20 mv full scale range that is 19.9999 mv, (watched my decimal point with Tim over my shoulder, LOL.) The least significant digit is tenths of a microvolt and plenty of that stray stuff around. My infinite impedance meters also dance around, but on the higher ranges like 200 mv or 2 volts, very stable with the standard 10 Mohm impedance.

Probably hand soldered PCB's loaded with flux is causing those problems.

FrankD. on Mon August 14, 2006 6:55 PM User is offline


Thanks Nick

Well it is good to know this is normal to pick up stray voltages in the lower settings.


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FrankD.

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