current measurement

Magnetic field?

-- Deep Fryer: a very philosophical monk. Website:

What exectally are you trying to do ? YOu can insert a resistor and measuer the voltage across it and calculate the current.

Hi, Pradheep. The easiest way to measure current without an ammeter is by measuring the voltage across a series resistor. This works well for DC and AC (for higher frequencies, you have to get non-inductive resistors). Accuracy is solely dependent on the precision of the resistor and the accuracy of the voltmeter. This becomes rather wasteful of power at higher currents.

Another way of measuring either AC or DC current is to use a Hall effect sensor to measure the magnetic field, and from that infer the current. Commercial hall effect current sensors are available which can give you better than 2% accuracy from DC to about 10KHz. These sensors generally are made to work on larger currents (more than an amp).

If you are measuring AC current, you may also use a current transformer. Accuracy can be better than 1%, and the current and frequency range is usually specified by the manufacturer. Make sure not to leave the secondary of the current transformer unterminated.

Good luck Chris

"Chris"

** The Hall Effect sensors I use ( from Lem) operate up to beyond 100 kHz, have better than 1% accuracy and linearity and resolve down to 1mA or better.

Dunno where your crummy ones are from .....

......... Phil

If your DVM has a decent ohmmeter, you don't need to worry about the resistor's precision.

Mark

Hi, Mr. Allison. I'll use Allegro for industrial current measurement. As I remember, the Lem units are a bit pricey. I'll definitely check them out again now, though.

Thanks for the heads-up and the link, sir.

Cheers Chris

"Chris"

** These the ones you mean ?

That 14 mV noise spec is a real horror !

13% overall accuracy is a worry too.

Shame those poor folk in "industry" are so hard up they cannot afford something decent.

:-0

........ Phil

"Jamie"

** I suppose measuring the voltage at both ends of a current shunt and doing the math IS how a code scribbler would do it.

........ Phil

Rather than pose vague hypothetical questions like this, is there a practical problem you have that you need to solve? Would you care to describe it if so ?

Graham

The ohms measurement on most DVMs is not greatly accurate.

Graham

if your working with AC then a CT (current Transformer) works well. if your working with DC then a hall type current sensor also works well. but maybe that isn't what you want ? if you have the options of inserting something in the line then you could use a shunt type series resistor and measure the voltage at both ends and do the math

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

How inaccurate are we talking about? I would think it's better than the 1% of a precision resistor. If most meters are worse than that, I'm surprised.

Mark

"redbelly"

** Forget it - the Poopie Bear cretin does not deal in facts. ** Absolutely .

** You are being TROLLED by the fourth bear.

The one Snow White told to FUCK OFF !!

....... Phil

Hi, Mark. A Fluke 77 (typical handheld DMM) can be typically expected to get around 1% measurement accuracy for resistors over a couple hundred ohms.

However, current shunt resistors are typically in the low ohm or milliohm range, and it's practically impossible to measure these accurately without a Kelvin measurement.

If you have a precision wirewound resistor (0.1% 100 ohm) and a stable variable DC power supply, you can make a fairly accurate Kelvin measurement with that Fluke 77 by setting the voltage to allow a precise current to flow through both the precision wirewound and the resistor under test. You can then accurately measure the voltage across the test resistor, inferring the resistance to the precision of the 100 ohm resistor and the accuracy of the meter. If you're using a Fluke 77 on the 200mV range and an 0.1% resistor, you might be looking at 0.2% tolerance on your measurement.

Of course, if you have a bench DMM with built-in Kelvin measurements (as I guess you do), that makes your job a lot easier, as you said.

Cheers Chris

What meter do you have ? My Fluke's worst performance is on the ohms range.

Graham

Sadly, yes. :-(

Cheers Chris (hired hand for "those poor folk")

** That is very conservative.

The Fluke 77 spec is for 0.5% basic accuracy and 0.1 ohms resolution.

Means even a 20 ohm resistor can be tested to within 0.5%.

DMMs use a *ratio* method to test resistors - ie the external one is compared with an internal, reference resistor.

The reading is just as accurate as that reference +/- 2 digits on the display.

One can verify the accuracy of the meter ANYTIME by keeping a few 0.1% resistors on hand to check the range in use.

........ Phil

I have an Extech model 380282. (Doesn't have the name recognition that Fluke has, but it's a pretty good meter and cost 300 $US new.)

Mark

That's a good idea, I'll plan to get a few next time I order parts.

Thanks Phil,

Mark