Question about measuring the current in a circuit .

[SH@RK]

Hi

I have a question about measuring the current in a circuit

"" Why there is a big difference when I change from mA to A range in the same circuit ? ""

I have a 10watt LED and tried to find the current it derive and faced by this problem

I used tow DMM and the same result.

The voltage is the same in each try (8.11 V DC) .

The circuit should look like this



See the photos

First :- Mastech

At the mA range it reads 65.5 mA
At the  A range it reads 0.14  A = 140 mA

So it is far away reading (140-65.5 = 74.5 mA )

The right side is the "mA range"




Second :- Fluke 87V

At the mA range it reads 108.2 mA
At the  A range it reads 0.148  A = 148 mA

So it is far away reading (148-108.2 = 39.8 mA )

The right side is the "mA range"





I am thinking that the resistance of the DMM changes between the mA & A ranges .

So is my thinking is true ??

And which reading is the accurate to find the power delivered to the LED ??

Thanks

[Rerouter]

your facing the problem of burden voltage, your milliamp range will use a rather large resistance to give a bigger signal, so it will drop more voltage across itself and cause your discrepency, if you want to confirm this, have 1 meter read the current on the 2 different ranges, and have the other read the voltage across the first meter,

[Psi]

I am thinking that the resistance of the DMM changes between the mA & A ranges .

So is my thinking is true ??

Yep, the meter measure current by reading the voltage drop across an internal resistor, so that voltage is lost.
The result is your load (LED) gets less volts and so draws less current than it would without the meter.

This current sense resistor is a different value for the low and high current ranges, so the current detected will be different.

The 10A range will be more accurate but still not 100% correct

[IanB]

I am thinking that the resistance of the DMM changes between the mA & A ranges

Yes, it surely does. Use a second meter to measure the resistance of the first meter on various resistance ranges and be illuminated.

One way to avoid this problem is to use Dave's uCurrent device.

[SH@RK]

Thank you all

your facing the problem of burden voltage, your milliamp range will use a rather large resistance to give a bigger signal, so it will drop more voltage across itself and cause your discrepency, if you want to confirm this, have 1 meter read the current on the 2 different ranges, and have the other read the voltage across the first meter,

As you said the resistance at mA is larger than A range

The voltage in both cases was about 8 V deliverd by the Power supply

The voltage accross the DMM was not stable (raising by the time LED became warmer)

First :- Mastech

At the mA range it reads 0.965 V and the resistance is 6.45 owm
At the  A range it reads  0.332  V and the resistance is 0.23 owm

Second :- Fluke

At the mA range it reads 0.185 V and the resistance is 1.9 owm
At the  A range it reads  0.098  V and the resistance is 0.0~0.1 owm (was not stable btween 0.0-0.2 but mor at 0.0-0.1)

will read the other replies and post again later

thanks