# Electronic Circuits and Projects Forum

1. ## Multimeter's ammeter accuracy

Hi

in a simple circuit consisting of an exactly measured 100Ohm/5W resistor as a load
(double and triple checked with different multimeters) a stabilized power source and two
multimeters (one continuesly monitoring the voltage and the other measuring the current passing
through the resistor) at various voltages raging from 1.5VDC to 3.3VDC i've noticed that my calculations
for current had nothing to do with what the multimeter measures, there is a 18%difference.

then i got 5-6 different multimeters from different brands not expensive models but
all common ones, analog and digital and i've also noticed the same thing when measuring current... some
where more accurate and some other less accurate but none of them was less than 10%

seems the problem only exists when measuring current..while resistor and voltage measurment appears
to be very accurate i don't know if such thing is reasonable...

2. You are measuring the current placing the meter in series with and not across the load correct?

<EDIT> Like the attached image? </EDIT>

Ron

3. Also, you are leaving the ammeter in circuit while you measure the voltage aren't you?

If you take the ammeter in/out of circuit, that can change the current and you will get odd results.

JimB

4. i have also noticed those problems, and still don't know why is the circuit gets short circuited when measuring current

You are measuring the current placing the meter in series with and not across the load correct?

<EDIT> Like the attached image? </EDIT>

Ron

yes i do..

Originally Posted by JimB
Also, you are leaving the ammeter in circuit while you measure the voltage aren't you?

If you take the ammeter in/out of circuit, that can change the current and you will get odd results.

JimB

yes...i continuesly monitor the voltage, also i think the accuracy gets
worse when using lower value resistors at the same voltage... (i tried that
too but i'm not sure yet if it is true..)

6. A Current meter has a resistance. When it is in series with a load then its series resistance reduces the current.

7. Originally Posted by Ziddik
i have also noticed those problems, and still don't know why is the circuit gets short circuited when measuring current
A current meter is supposed to be connected in series with the load. It shorts the load (or power suopply) if it is parallel.

8. Originally Posted by audioguru
A Current meter has a resistance. When it is in series with a load then its series resistance reduces the current.

yes but in a modern digital multimeter it's internal resistance at which the current get measued it should be taken into account by it's micro controller when it calculates
the measurement result.. right?

also in analog multimeter i think it is already calculated when the instrument's
tablet gets printed by the factory...

9. Originally Posted by lynx
yes but in a modern digital multimeter it's internal resistance at which the current get measued it should be taken into account by it's micro controller when it calculates
the measurement result.. right?

also in analog multimeter i think it is already calculated when the instrument's
tablet gets printed by the factory...
No to both, inserting the meter increases the circuit resistance, and thus reduces the current somewhat - the meter will read what the current IS, not what it might possibly have been if the meter wasn't there (which would be impossible).

To recheck what others have asked, and to which your answers haven't really been clear enough, you do have TWO meters connected at all times? - an ammeter in series with the load, and a voltage directly across the load (and only the load).

Incidently, this check is how you would accurately measure the resistance of the load.

10. My fairly expensive Fluke digital multimeter calls the voltage loss caused by its current-sensing resistors its "burden voltage". It is 11mV/mA for low currents up to 40mA and is 30mV/A for high currents up to 10A. It can measure up to 20A for 30 seconds max.

EDIT: I forgot to say that the test leads and probes add additional resistance that reduces the current.

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