Milliamp and micro amp reading differ

Thread Starter

Robesim

Joined May 1, 2017
144
Hi,

On my multimeter i can not select milliAmps. I can only select MicroAmps and Amps (10 amp fuse port). If i turn the dial to microamps, after reading 1 milliamp on my digital multimeter, i get 300 microamps. Why do the milliamp and microamps reading differ so much?
 

BR-549

Joined Sep 22, 2013
4,928
When the milli amp shunt got burned out.....some of the other scaling circuits might have been affected. He He

Or it might be the resolution of the meter. 300 ua is one third(pretty close) of one ma. Would you prefer your meter not to tell you when you are on the amp scale......that nothing is there until you have at least 1000 ua?(at least one full ma)

Assuming the 1 ma reading on amp scale is lowest digit.

Does that make sense? Datasheet on meter?
 

ebp

Joined Feb 8, 2018
2,332
With a digital meter, current is measured by measuring the voltage across a resistor. The higher the maximum current that can be measured, the lower the value of the resistor. If your manual doesn't tell you what the value of the resistor is, you can sometimes estimate by assuming the resistor will produce a voltage, at full-scale for the range, equal to the most sensitive voltage range for the meter - but this isn't always the case. In an analog meter, all or part of the current, depending on the range, will flow through a coil of wire that produces a magnetic force proportional to the current and that current causes the needle to move by acting against the force of a spring (in the most common multimeter type). Sensitive analog meter movements are/were often 50 microamps at full-scale.

Measuring devices always affect the circuit that is being measured. When you try to measure a small current you will be adding a fairly high resistance in series with the circuit and producing a voltage drop across that resistor. Sometimes that can considerably reduce the current in the circuit.

I'm not quite clear on what you are doing. It sort of sounds like you are measuring with two different instruments - one an analog multimeter and one a digital multimeter.
 

AlbertHall

Joined Jun 4, 2014
12,190
Connect both meters in series when measuring the current. This way you know for certain that the same current is flowing in both meters and so a comparison of the readings is known to be meaningful.
 

Thread Starter

Robesim

Joined May 1, 2017
144
With a digital meter, current is measured by measuring the voltage across a resistor. The higher the maximum current that can be measured, the lower the value of the resistor. If your manual doesn't tell you what the value of the resistor is, you can sometimes estimate by assuming the resistor will produce a voltage, at full-scale for the range, equal to the most sensitive voltage range for the meter - but this isn't always the case. In an analog meter, all or part of the current, depending on the range, will flow through a coil of wire that produces a magnetic force proportional to the current and that current causes the needle to move by acting against the force of a spring (in the most common multimeter type). Sensitive analog meter movements are/were often 50 microamps at full-scale.

Measuring devices always affect the circuit that is being measured. When you try to measure a small current you will be adding a fairly high resistance in series with the circuit and producing a voltage drop across that resistor. Sometimes that can considerably reduce the current in the circuit.

I'm not quite clear on what you are doing. It sort of sounds like you are measuring with two different instruments - one an analog multimeter and one a digital multimeter.
No. i'm measuring with one Digital multimeter. If i put the dial om amps and the red lead in the 10 amp fuse port,
With a digital meter, current is measured by measuring the voltage across a resistor. The higher the maximum current that can be measured, the lower the value of the resistor. If your manual doesn't tell you what the value of the resistor is, you can sometimes estimate by assuming the resistor will produce a voltage, at full-scale for the range, equal to the most sensitive voltage range for the meter - but this isn't always the case. In an analog meter, all or part of the current, depending on the range, will flow through a coil of wire that produces a magnetic force proportional to the current and that current causes the needle to move by acting against the force of a spring (in the most common multimeter type). Sensitive analog meter movements are/were often 50 microamps at full-scale.

Measuring devices always affect the circuit that is being measured. When you try to measure a small current you will be adding a fairly high resistance in series with the circuit and producing a voltage drop across that resistor. Sometimes that can considerably reduce the current in the circuit.

I'm not quite clear on what you are doing. It sort of sounds like you are measuring with two different instruments - one an analog multimeter and one a digital multimeter.
No. i'm measuring with one Digital multimeter. If i put the dial om amps and the red lead in the 10 amp fuse port, i read 1 milliamps. If i measure the same circuit with microamps setting i get 300 microamps instead of 1000 microamps. The maximum current listed in the manual of the meter is 4000 microamp. Do i have to buy a meter with dedicated microamp and milliamp settings on the dial to get more accurate readings?
 

AlbertHall

Joined Jun 4, 2014
12,190
No. i'm measuring with one Digital multimeter. If i put the dial om amps and the red lead in the 10 amp fuse port,


No. i'm measuring with one Digital multimeter. If i put the dial om amps and the red lead in the 10 amp fuse port, i read 1 milliamps. If i measure the same circuit with microamps setting i get 300 microamps instead of 1000 microamps. The maximum current listed in the manual of the meter is 4000 microamp. Do i have to buy a meter with dedicated microamp and milliamp settings on the dial to get more accurate readings?
Changing the meter range will change the resistance of the meter which will then change the current flowing in the circuit. How much the current will change depends on the rest of the circuit and in particular the voltage which is driving that current.

What is the circuit in which you are measuring this current?
 

Thread Starter

Robesim

Joined May 1, 2017
144
Changing the meter range will change the resistance of the meter which will then change the current flowing in the circuit. How much the current will change depends on the rest of the circuit and in particular the voltage which is driving that current.

What is the circuit in which you are measuring this current?
After i measured a circuit and found the inconsistent readings, i tested the meter. I tested the meter with different values resistors in series.

Ex. Powersupply voltage 1 volt

For 4.7k resistor calculated 0,2mA (210 uA) meter showing 143 uA
For 1k resistor calculated 1mA (1000 uA) meter showing 300 uA

Do i need a multimeter with separate uA and mA settings to read accurate, because my meter only has readings for Amps and microamps?
 

kubeek

Joined Sep 20, 2005
5,776
What you need is to know the circuit you are measuring and know how the resistance of your meter will affect the value that you are trying to measure. Or use something like uCurrent from Dave Jones that doesn´t have any voltage drop across the meter.
 

MrChips

Joined Oct 2, 2009
27,695
Your meter appears to be showing the correct current readings.

Take your results and do some further analysis. What do the results tell you about the internal resistance of the meter?
 

ebp

Joined Feb 8, 2018
2,332
[EDIT} Ooops! Just noticed MrChips sent you off to do the calc's yourself. :oops:
My calc's are between the === but set to white; select to reveal

===
By my calculations, the internal (series) resistance of your meter is about 2293 ohms.

1V / (4700 + 2293)Ω = 143 µA
1/(1000 + 2293) = 295 µA

===

These values are well within the tolerance of the resistors unless they were 1% types.
 
Last edited:

Thread Starter

Robesim

Joined May 1, 2017
144
What was the resolution of your 1 ma measurement?
Your meter appears to be showing the correct current readings.

Take your results and do some further analysis. What do the results tell you about the internal resistance of the meter?

There is internal resistance and it is not linear. My meter should read
210 uA and it reads 143 uA , This means we have less current passing so there is a voltage drop and that means we have resistance.

If it should read 1000 uA and it reads 300 uA, there is also a voltage drop and also resistance.

But we can clearly see that the voltage drop is not linear, so internal resistance is not a fixed number.
Question, is this so for all multimeters?? i don't have a lot experience with multimeters. The one i have is the first one. I have attached the specs of the multimeter.
 

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MrChips

Joined Oct 2, 2009
27,695
How did you arrive at the conclusion that the internal resistance of the meter is not linear?
This is not correct. Go back and determine the internal resistance of the meter. With this information you will get an informed knowledge on how to apply the meter appropriately.

Is this so for all multimeters?

The simple answer is, yes, what you are observing is typical of all meters. The discrepancy is not entirely the fault of the meter. It is with the technique used to make the measurement. It is possible to design and construct a meter that does not have is anomaly.
 

crutschow

Joined Mar 14, 2008
31,129
But we can clearly see that the voltage drop is not linear, so internal resistance is not a fixed number.
Question, is this so for all multimeters??
For most multimeters, yes.
The meters change current range by changing the internal series resistor.

To minimize the effect of this internal resistance change, user a higher voltage supply with an external resistor in series to generate the test current (e.g. 10V and 10k to generate 1mA).
That should give you a much smaller difference when you change current ranges.
 

Thread Starter

Robesim

Joined May 1, 2017
144
How did you arrive at the conclusion that the internal resistance of the meter is not linear?
This is not correct. Go back and determine the internal resistance of the meter. With this information you will get an informed knowledge on how to apply the meter appropriately.

Is this so for all multimeters?

The simple answer is, yes, what you are observing is typical of all meters. The discrepancy is not entirely the fault of the meter. It is with the technique used to make the measurement. It is possible to design and construct a meter that does not have is anomaly.

Ok i calculated internal resistance again and it is approximately 2k, It varies a little, that's why i said it's not linear.

For Powersupply voltage of 1 volt:

Resistor 20k calculated 50 uA read 45,2 uA internal resistance 2123 ohm
Resistor 4.7k calculated 210 uA read 143 uA internal resistance 2293 ohm
Resistor 1k calculated 1000 uA read 300 uA internal resistance 2333 ohm

So for above 1000 uA i need to switch to the 10 amp fused port, because this multimeter has no dedicated mA setting to choose.

This multimeter cost me $180 with shipping. It's pricey. In my opinion the internal resistance is too high. Let me know what you're opinion is.
 

ebp

Joined Feb 8, 2018
2,332
The meter's internal resistor isn't going to change in value with current and there is, or should be, no other voltage drop, as such. There may be some error due to "input offset voltage" in an amplifier. This does more-or-less appear as a small voltage in series with the resistance. Other sources of error include the tolerance of the resistors you used for testing, and possibly a slight variation in the voltage from the power supply (power supplies sometimes don't regulate very precisely at very low currents - you can check that by measuring its output while testing with some different resistors). The analog to digital converter may also exhibit a bit of non-linearity. Without having a high quality high resolution meter for comparison, it is difficult to determine where the error(s) lie. I have a meter that would probably be helpful, but it cost over a thousand dollars. You can spend a lot more than that on a meter. The voltage reference alone in a high end meter can contribute hundreds of dollars to the cost, but it is "the" component critical to accuracy.

Unless your meter is semi-autoranging, I would have expected more current ranges for the price. I have a few quite inexpensive meters that have several ranges for current.

In any event, you will have come away from this with some understanding of how instruments can make a pretty big change in the behavior of the circuit to which they are connected. It is very valuable to be aware of that. Lots of people lack that awareness, even after they've been in electronics for quite some time. AC measurements are where things get considerably more complicated, with issues of average and RMS values and how the meter responds to different waveforms.
 

MrChips

Joined Oct 2, 2009
27,695
Are you taking all measurements with the meter at the same range setting?
The internal resistance is likely to be different at every range setting.

You have calculated that the internal resistance is 2.3kΩ.
That would be considered high.

My meter shows:
1kΩ at 200μA range
100Ω at 2mA
10Ω at 20mA
1Ω at 200mA
 

Thread Starter

Robesim

Joined May 1, 2017
144
Are you taking all measurements with the meter at the same range setting?
The internal resistance is likely to be different at every range setting.

You have calculated that the internal resistance is 2.3kΩ.
That would be considered high.

My meter shows:
1kΩ at 200μA range
100Ω at 2mA
10Ω at 20mA
1Ω at 200mA
Yes, all measurements were done in the uA range. (meaning if i set the DMM dial on uA). So for a $180 multimeter (let's say $ 120 wthout shipping) i expected a lot more. From you're experience, can you recommend a not very expensive (cheap) multimeter which is good in reading uA, mA and has dedicated uA and mA settings on the dial.
 

Thread Starter

Robesim

Joined May 1, 2017
144
The meter's internal resistor isn't going to change in value with current and there is, or should be, no other voltage drop, as such. There may be some error due to "input offset voltage" in an amplifier. This does more-or-less appear as a small voltage in series with the resistance. Other sources of error include the tolerance of the resistors you used for testing, and possibly a slight variation in the voltage from the power supply (power supplies sometimes don't regulate very precisely at very low currents - you can check that by measuring its output while testing with some different resistors). The analog to digital converter may also exhibit a bit of non-linearity. Without having a high quality high resolution meter for comparison, it is difficult to determine where the error(s) lie. I have a meter that would probably be helpful, but it cost over a thousand dollars. You can spend a lot more than that on a meter. The voltage reference alone in a high end meter can contribute hundreds of dollars to the cost, but it is "the" component critical to accuracy.

Unless your meter is semi-autoranging, I would have expected more current ranges for the price. I have a few quite inexpensive meters that have several ranges for current.

In any event, you will have come away from this with some understanding of how instruments can make a pretty big change in the behavior of the circuit to which they are connected. It is very valuable to be aware of that. Lots of people lack that awareness, even after they've been in electronics for quite some time. AC measurements are where things get considerably more complicated, with issues of average and RMS values and how the meter responds to different waveforms.
I think it has someting to do with other parts of the internals of the meter. A resitsors value doesn't change unless it's a variable resistor or there is a change in temperature. You're right about measurements having effect on the circuit that is measured. Which multimeters from you're experience are good in reading uA and mA and have dedicated settings for these two on the dial.
 
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