Dc amps

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Dennis Alwon

Dennis Alwon

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Someone asked me if a standard amp meter will be able to read amps when it is DC current. I thought amps were amps but I know squat about DC.

I have a feeling this may get too technical for my tiny brain.
 
Dennis Alwon said:
Someone asked me if a standard amp meter will be able to read amps when it is DC current. I thought amps were amps but I know squat about DC.

I have a feeling this may get too technical for my tiny brain.
Click to expand...

It will kind of, but only for the brief time it takes for the clamps to snap shut around the conductor.

Most AC ammeters rely on a transformer action. Most DC and AC/DC ammeters (newer ones at least) use a Hall Effect sensor.
 
It also depends on what you mean by a "standard" ammeter. To me, that term conjurs up panel meters, not handheld testers. Jim is correct about the Hall effect sensors in DC clamp-ons.
 
LarryFine said:
It also depends on what you mean by a "standard" ammeter. To me, that term conjurs up panel meters, not handheld testers. Jim is correct about the Hall effect sensors in DC clamp-ons.
Click to expand...

I was thinking handheld clamp on meters. This guy was looking at an ideal meter but it did not give the indication it could read DC current.

Am I understanding if I use the hold feature and clamped it around a DC circuit it will read it. If so is it accurate?
 
Dennis Alwon said:
I was thinking handheld clamp on meters. This guy was looking at an ideal meter but it did not give the indication it could read DC current.
Click to expand...
No, if it doesn't specify DC amps, it won't read them.

Am I understanding if I use the hold feature and clamped it around a DC circuit it will read it. If so is it accurate?
Click to expand...
I've never heard that, and I wouldn't rely on it.
 
080903-1804 EST

Dennis:

There are many different kinds of meters. Excluding digital, and electrostatic, all meters with moving needles are basically current meters. Of these the ones used for voltage measurement will operate with relatively low current. For example a Simpson 260 or 270 has a 50 microamp movement and 250 MV at 50 microamps. Other meters may be designed with a 50 MV full scale value.

One reference:
"Basic Electrical Measurements", Melville B. Stout, Prentice-Hall, 1950. Does not appear to be digitized at the University of Michigan yet.

Of the moving needle type meters there are:

Permanent-magnetic moving-coil (d'Arsonval) --- Weston, Simpson, Tripplett and others.
This is basically a DC instrument. It can be made into an AC instrument with a rectifier. Because of rectifier drop not good for AC current without using electronics. Does not measure AC RMS but rather the full wave rectified average value. May be calibrated for RMS of a sine wave (Simpson 260). Page 408 of Stout.

Electrodynamometer --- Weston and others.
This is a fixed and moving coil meter and measures true RMS of either AC or DC or the combination. Page 418 of Stout.

Moving Iron --- Weston and others.
Page 423 of Stout.

Thermocouple Instruments.
Basically a thermocouple connected to a DC meter with the thermocouple in a heater. Thus, this is AC and DC and RMS. These are considered more accurate over a wider frequency range than the hot-wire meter.
Page 432 of Stout.

Electrostatic Instruments
AC and DC and RMS.
Page 439 of Stout.

Hot Wire
Not mentioned by Stout but has a wire that expands with increase in current and connected to a needle.
AC and DC and RMS. Useful from DC to high frequencies, maybe 30 mHz.

http://www.practicalphysics.org/go/Experiment_698.html;jsessionid=alZLdQlAHb1

The following is from the University of Michigan library as digitized by Google. Note the date 1905.
In tonight's paper is another article on "Google book-digitizing well under way".
There are already 1.5 million books digitized and ultimately the total will be about 7.5 million. The project started in 2004. The rarest of books are done by the U of M rather than Google.
http://books.google.com/books?id=gZ...i=book_result&resnum=5&ct=result#PRA5-PA35,M1

The Ann Arbor News article.
http://www.mlive.com/annarbor/stories/index.ssf?/base/news-29/1220452853114420.xml&coll=2

Note: aircraft circuit breakers manufactured by Mechanical Products used a Nichrome wire as the sensing element in the breaker.

Various electronic instruments will have AC or DC capability, and in some cases will measure the combination.

.
 
The meter I am talking about is here. If you click specifications it only shows ac current but ac and DC voltage. It is the 61-744.

What would you need true RMS for?
 
You need the proper tool for the job. I know Fluke makes a DC ameter and I'm sure there are others.Unless you can operate that hold button in 16.6 miliseconds!!:grin:
 
080903-2029 EST

Dennis:

Although not stated, but implied, the meter uses what amounts to a standard current transformer for clamp-on current measurement. This is not capable of measuring DC current. You would need something like a Hall device current sensor.

An AC meter using full wave rectification of an AC signal measures the average value of the waveform. For a sine wave this is 0.636 times the peak. If one assumes that the waveforms are near sinusoidal, then one can calibrate the meter to read RMS. In other words the scale is modified by the ratio of 0.707/0.636 . On non-sinusoidal waveforms this may produce an incorrect reading.

To electronically measure the RMS value you need to do the equivalent of the instantaneous square of the value, average this, and take the square root.

The RMS measurement of current will give you the heating effect in the wire in which the current is being measured.

Electronic RMS meters generally will not provide a good reading on a very peaked wave form. A hot-wire thermocouple meter would work well on this type of waveform.

.
 
Gar, thank you for all the time and effort you put into your post. They were helpful-- some of it over my head but that isn't hard to do.
 
Dennis Alwon said:
The meter I am talking about is here. If you click specifications it only shows ac current but ac and DC voltage. It is the 61-744.

What would you need true RMS for?
Click to expand...
Look at the PDF brochure on that page. The 61-768 reads AC/DC current.
 
080804-0909 EST

Dennis:

Following is a reference that may be useful to you.
http://www.vias.org/feee/acmeter_02.html

A moving coil DC meter has sufficient inertia that at 50 to 60 Hz and higher it will average the input. At 1 Hz it can probably follow the input.

If we place a full rectifier before the meter, then the meter will measure the average of that rectified input. A 1N5060 diode has a forward voltage drop of about 0.4 V at 50 microamps. A bridge rectifier composed of these would have a drop of about 0.8 to 0.9 . Experimentally you could determine the average value of a sine wave with a DC meter, bridge rectifier, and a known input voltage.

With calculus we can calculate the exact value of the average and RMS values of a sine wave with respect to the peak value.

We can approximate these values quite accurately by calculating a few points on the sine wave. In the calculation below I use 18 points in one quadrant.

Code: 
   First quadrant values of SIN and SIN squared

      Angle   Radians      SIN    SIN^2
               
        2.5   0.0436    0.0436   0.0019
        7.5   0.1309    0.1305   0.0170
       12.5   0.2182    0.2164   0.0468
       17.5   0.3054    0.3007   0.0904
       22.5   0.3927    0.3827   0.1464
       27.5   0.4800    0.4617   0.2132
       32.5   0.5672    0.5373   0.2887
       37.5   0.6545    0.6088   0.3706
       42.5   0.7418    0.6756   0.4564
       47.5   0.8290    0.7373   0.5436
       52.5   0.9163    0.7934   0.6294
       57.5   1.0036    0.8434   0.7113
       62.5   1.0908    0.8870   0.7868
       67.5   1.1781    0.9239   0.8536
       72.5   1.2654    0.9537   0.9096
       77.5   1.3526    0.9763   0.9532
       82.5   1.4399    0.9914   0.9830
       87.5   1.5272    0.9990   0.9981

   Sum of 18 values    11.4628   9.0000

   Average              0.6368   0.5000

   Sq-root of avg                0.7071

   Actual values        0.6366   0.7071
   using calculus
Note the exact value for the average is 2/Pi .

Off hand I do not have a simple experimental way for you to determine the 0.707 value.

Next consider a simple waveform to contrast the great difference between an RMS and average value. Consider a short rectangular pulse of width 1/100 of its period and a peak amplitude of 1. Quite obviously its average value is 0.01. Its RMS value is 1 squared, averaged which is 0.01, and then the sq-root produces 0.1 for the RMS value. This pulse has an RMS value 10 times the average value. The electronic RMS meters will not likely give an accurate reading of this waveform because of their crest factor limitation. This is basically a dynamic range problem. The thermocouple type meter would give a good reading.

Do you need a true RMS meter? This depends on the type of work that you do. Most of the time I do not need a true RMS meter and I usually use a Simpson 270 or a Fluke 27 for most measurements.

Do not hesitate to ask any questions if you do not understand what I have said. It is quite important that you have a fundamental understanding of how an instrument or other devices work to make good judgements on the results of any problem that you encounter.

I would say that 50 years ago that few users of a Simpson 260 or 270 would have known that the meter would give an incorrect reading of the output of a white noise generator, even 20 years ago, and today more people are aware of the existence of so called true RMS meters.

.
 
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