# Determining The Accuracy of Amp Meter

Discussion in 'General Electronics Discussion' started by XJohnDoe001x, Jul 16, 2018.

1. ### XJohnDoe001x

17
0
Dec 25, 2014
Friends, I would like know if my amp meter is accurate and so I think that if I:

Put a voltage across two Resistors, divide the voltage across one resistor by the resistance of the same resistor to get the actual current then put the amp meter in the circuit, then compare the reading to the actual current calculated.

Is this a good way to determine the accuracy of my cheap amp meter?

2. ### hevans1944Hop - AC8NS

4,647
2,169
Jun 21, 2012
Why two resistors? Just use one resistor with a variable power supply, placing the ammeter in series with the resistor and power supply. Measure the resistance of the resistor. Turn on and Increase the power supply output until the ammeter reads full scale. Measure the voltage drop across the resistor and divide this measurement by the previously measured resistance. Voila! The result is the full-scale current calibration for your ammeter.

If your power supply doesn't provide enough voltage output to drive your ammeter to full-scale with whatever resistor you have selected, reduce the value of the resistance and try again. Wash, rinse, and repeat until you find a power supply voltage and series resistance value that will drive your ammeter to full scale.

davenn likes this.
3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Another approach, is you have a knows good ammeter is to place both in series while you do the resistor and power supply trick. If they read close to the same then they as as accurate as each other.

4. ### XJohnDoe001x

17
0
Dec 25, 2014
Two because with one you will actually be reading the voltage of the power supply no matter how much resistance.

With one of two (or more) you will get the voltage drop caused by the current times the value of that resistor.

At least it sounds good in my head. I took electronic a few lifetimes ago and so I am starting up ghetto style until I can build up a satisfactory work bench. My power supply(s) is various wall worts and a couple of 12 volt batteries from a Pronto Sure step M91 power chair.

I have a Fluke 77 that I bought for \$5 at a yard sale, the current reading does not work. But I believe that it still reads volt and ohm accurately I also have a Harbor Freight multimeter that does read ohm but I don't trust it until I test it .

5. ### XJohnDoe001x

17
0
Dec 25, 2014
I suspect that some of my understanding of electronic principles have Stellar drift, so, I ask the stupid questions until the stupid disapates from my cranial region

6. ### Kabelsalat

168
29
Jul 5, 2011
How much current are you planning to test?

What is the brand and name of the amperemeter? Could it be just easy to download its paper from the manufactors web site and just see what it says?

Do you have access to a more accurate ampere meter to test against? That is what I would have done. And just make a spread sheet where you write down the readings from both meters as you increase the current. Use an XY graph should give a straight line of dots if the meter is true - in oposite case if the dots are spread all over, then you cannot trust the values at all.

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
No, a single resistor is all you need.

8. ### hevans1944Hop - AC8NS

4,647
2,169
Jun 21, 2012
Not true. The voltage of the power supply is divided across the resistor and the resistance of the ammeter connected in series. You don't know the resistance of the ammeter, but you do know that whatever current is flowing in the ammeter (as a result of the power supply voltage) is also the same current that is flowing through the single resistor. Therefore, measuring the voltage across the resistor and dividing by its resistance will provide you with the current through the resistor, which is also the same current that flows through the ammeter you are testing.

There may be very little voltage drop across the ammeter, depending on its construction, but there will be some, and this will subtract from the power supply voltage, leaving the remainder of the power supply voltage to appear across the series resistor.

9. ### davennModerator

13,947
1,989
Sep 5, 2009

as Steve said, no that isn't correct in that you ONLY need one resistor the voltage drop doesn't change for the same total value of resistance
regardless of if there is 1 or 100 resistors adding up to that total value

2 x 100 Ohm resistors is going to give the same total voltage drop and current flow as a single 200 Ohm resistor

5 x 10 Ohm resistors is going to give the same total voltage drop and current flow as a single 50 Ohm resistor

Dave

10. ### Harald KappModeratorModerator

11,993
2,809
Nov 17, 2011
... as a single 200 Ω resistor

13,947
1,989
Sep 5, 2009

edited

12. ### Harald KappModeratorModerator

11,993
2,809
Nov 17, 2011
This may help:

The measurement of the voltage incurs an error depending on the quality of the voltmeter, e.g. 1%
The resistance of R1 has an error, too, e.g. 1 %
The resulting error in the computed current is ~ 2% If the value displayed by your ammeter is within the nominal current +- 2% it is good.