# current measurement

Discussion in 'Electronic Basics' started by prad, May 7, 2006.

how can we measure the current flowing in a circuit without using
ammeter
can we have to use a voltage to current convertor?
if so what are the possible solutions to do so?
regards

2. ### Tim WilliamsGuest

Magnetic field?

3. ### Ralph MoweryGuest

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

4. ### ChrisGuest

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

5. ### Phil AllisonGuest

"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.

http://www.farnell.com/datasheets/51396.pdf

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

.......... Phil

6. ### redbellyGuest

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

Mark

7. ### ChrisGuest

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.

Cheers
Chris

8. ### Phil AllisonGuest

"Chris"

** These the ones you mean ?

http://www.allegromicro.com/datafile/0750-050.pdf

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

9. ### Phil AllisonGuest

"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

10. ### Pooh BearGuest

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

11. ### Pooh BearGuest

The ohms measurement on most DVMs is not greatly accurate.

Graham

12. ### JamieGuest

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

13. ### redbellyGuest

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

14. ### Phil AllisonGuest

"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 **** OFF !!

........ Phil

15. ### ChrisGuest

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

16. ### Pooh BearGuest

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

Graham

17. ### ChrisGuest

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

18. ### Phil AllisonGuest

"Chris"

** 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

19. ### redbellyGuest

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

20. ### redbellyGuest

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

Thanks Phil,

Mark