DMM

[Moe2255]

I am in the process of purchasing a new digital multimeter (I have two sources so far, Jameco & MCM Electronics); some of the meters are "True RMS". I want one to tests electronic circuits that I build, plus to check parts to see if they are ok or not.

What is "True RMS"? Is a meter that doesn't say "True RMS" good enough?

Any suggestions on a good meter (model#)!

Thanks

 

[TheOne]

True RMS is needed if you ever need to measure the "effective" value of voltage or current that is not sinusoidal like triangular, square etc. Other non-True RMS meters will only give the correct value for sinewaves within its frequency measuring range

 

[Dean Huster]

And you know, we've been using meters (VOMs, TVMs, VTVMs, differential voltmeters, ac voltmeters, etc.) for the last 70 or 80 years that only measured the average ac value and displayed the rms value and got along just fine, didn't we? You'll find that MAYBE 1 out of 1000 ac measurements made with an ac meter require the sophistication of "true rms" measurement.

My advice is to buy the meter with the best dc accuracy you can for the money. Don't worry about all the bells and whistles of more functions on the meter than you can count. How many of all those advertised functions on your scientific calculator do you really use? The DMM is the same way. Most of those extra functions are marginal in performance and better performed by an instrument designed for that function.

However, if you intend on using this DMM in high energy circuits, such as working with 3-phase motors, etc., buy a Fluke DMM. They're one of the few that are virually bullet-proof and rated for Category III or IV so they won't blow up in your face if you perform the big "oops" during a measurement.

Dean

 

[stevez]

I'll add what I think I understand about the true RMS meters asking that others verify or correct - while traditional meters are arranged on the assumption that any AC is sinusoidal the DMM with 'true' feature is actually sampling the waveform then calculating the result. If I am correct it would seem that sampling rate might be an important bit of information in some cases. It might be that higher sampling rates cost more money or that a low end 'true rms' meter won't sample fast enough for your application - better check.

In general, I would agree with Dean - have some instruments on hand that are good quality, accurate and reliable and if money is an issue then be willing to give up 'bells & whistles' so that you have a good foundation.

 

[JohnBrown]

If you anticipate building PWM circuits, true RMS could come in useful.

 

[evandude]

if i recall, last time i was at radio shack there was a decent DMM on sale for like $50 or so (down from $70 I think?) that had an RS232 computer interface. seems like a neat toy... could even use it as a cheap oscilloscope for real low-frequency signals, or for datalogging. that feature alone makes it seem worthwhile to me...

I have been using my $40 craftsman DMM for years. it does frequency, capacitance, duty cycle, and all the standard features. probably not as accurate as the real expensive ones but it works well for me...
the radio shack rs232 one seems to be about the same thing, only with a serial link as well...

wow, i also just found this:
https://www.electro-tech-online.com/custompdfs/2004/11/68424.pdf

full info of the RS232 communication of the meter. 4800 baud, 8-N-1, pretty standard. doesn't seem to say anywhere what the actual measurement frequency is... the manual leads me to believe it might only be on the order of 1 per second or worse... looks like it literally just sends the states of each of the LCD segments, the range, etc. but a PIC could easily convert that to numerical values. that would be very interesting, since you could hook it up to a PIC project for datalogging without a PC... hmmmm... I may have to go buy one of these things after all before they go off clearance!

 

[gurgalesci]

What about the ACC's? Or the AMM? The Alti-Multimeter? You can try that. Or how about the TYV's? I found them very useful and cheap if you look in the right places. In Ebay, they sell them for about $50. 8)

 

[zevon8]

I would write down what it is you will do regularily with the meter. Be honest with the list and I bet you will find mostly you just need good DC accuracy, a diode check function, and audible continuity, 80 percent of the time. The other features are nice, and do get used occasionally, but most of the time I find myself just checking the basics.

I would agree with Dean Huster, you will find it hard to beat a good basic Fluke DMM. I have 3, a model 7 that lives in my tool bag for use in the plant on electrical and vehicles.. it does AC or DC by itself, anything under about 4 volts it will read resistance.. great for goof proof testing of voltage and continuity. Another is the model 12, it does capacitance, records min-max, and does a very useful recording of intermittant open/short down to 250mS. Lastly is a model 83, it has many more features, but was expensive, and stays on the bench, when I need more accuracy.

The model 12 and 7 have seen very hard use, in nasty conditions, and have never let me down.

Now that I am done with the Fluke commercial, , ( honest I don't work for them ) look for a good basic meter, spending your money on features you will use.

One thing I never really liked about alot of the " all but the kitchen sink " meters is the big rotary switch with all the contacts and mechanical wear and tear that sooner or later will give you grief as the connections go bad or wear out. ( anyone remember getting yelled at for racing through the dial on the old rotarty tuner TV's when you were a kid? ... applies to those meter dials too... )

 

[Dean Huster]

That's "Huster", zevon8, not "Hustler". Must've had a few odd electrons crash during your post and they messed up that name a little.

Dean

 

[zevon8]

I truly apologize, no offence meant, I was just not paying attention when typing.

 

[Moe2255]

How important is it to get a DMM with an inductance readout? Can I read the inductance in a coil that I've wound myself, when building RF circuits?

 

[TheOne]

Resolution of the inductance function of handheld multimeters is not great. Typical about 1uH. If you play around with RF circuits like Zack, coils will be in the region of 100nH. You need a very specialized Inductance meter to measure coils this small. My Grundig RLC300 have a 100nH resolution and was not cheap!