Problems with internal resistance of DMMs

[DrQuasi]

Hi!

At first: Sorry for my bad english. I'm from Germany. I hope you can understand me.

I'm new here  My Name is Philipp, I'm 18 years old and I'm an electronic engineers (Trainee).

I build a little circuit (please look at the circuit diagram) und I found something strange:

First, I turned on the circuit without a multimeter. All LEDs bright. Nothing wrong.
Then I wanted to messure the current, so I used a Gossen Metrawatt Metrahit One Plus, turned it on (mA) and...
The LEDs are significantly darker. The DMM showed me 7mA.
7mA is very low for four white LEDs...

I decided to take another Gossen Metrawatt Metrahit One Plus. Same Problem: LEDs significantly darker and only 7mA.
Then I used a HT Instruments HT 410. Same again.
Another HT Instruments HT 410. Same again.

Then I measured the internal resistance of the four DMMs and every one has a internal resistance of 50 Ohm.
But for current messurement the internal resistance should be nearly 0 Ohm.

At last I used my Fluke 115. It has a internal resistance of 0.18 Ohm and I measured 55mA. Great.
(The both Batteries of the circuit aren't new/full, so 55mA looks realistic and it's a better result than 7mA)

I don't understand why the both Gossen Metrawatt DMMs and the both HT Instruments DMMs have an internal resistance of 50 Ohm.
OK, I could think: Measuring milliamps = low current = high resistance (circuit) = 50 Ohm or nearly 0 Ohm internal resistance doesn't matter.
Or the 50 Ohm internal resistance looks like a current limit (safety feature).
But in my case it makes a big different!

What would you say? Is an internal resistance of 50 Ohm in the milliamps-range normal?
Is someone else here who has already had this problem?

I'm looking forward to an answer from you!
If necessary I can send you the datasheet of the LEDs or other informations.

Best regards from Germany!
Philipp

[Gyro]

Hello Philipp and welcome. Don't worry about your English, it's very good!

Internal resistance of multimeters on their current ranges is a common problem. As you have identified, it can be quite large on low current ranges. This is not just from the shunt resistor that the meter measures the voltage drop across, but also from the fuse, which is not included in the sensing circuit and so is wasted voltage drop. 50 ohms is probably at the high end of what you might encounter.

EDIT: Encounter on the mA range at least, uA current ranges will have much higher resistances.

There are several things you can do:

1. Measure current on the highest range that will give you sufficient reading accuracy. Higher ranges clearly have lower value shunt resistors. If you can use the 10A range then its fuse will be much lower resistance than the smaller fuse used for lower ranges.

2. Use an external shunt resistor. Equip yourself with a few accurate low value resistors eg 1 ohm and 0.1 ohm, connect these in series with the load and measure the voltage drop across it. NOTE: Only do this when it is safe because you have no protective fuse.

3. Use the 'shunt' resistors that are already there - In your case you have a resistor in series with each LED, Measure the voltage across each and you will get a far more accurate measurement than breaking the circuit and inserting a meter.

I hope this helps you.

[Fungus]

To measure current a multimeter puts in a known resistor then measures voltage drop across it. Then it uses Ohm's Law to calculate current.

But 50 Ohms sounds much too high to me. I think it is a measurement error.

To measure it you need to put the meter measuring the LED current (note milliamps on the meter) then measure the voltage drop across it using a separate multimeter. Use the current/voltage to calculate the resistance.

[pelule]

As you battery voltagee is just 2x 1.5V, ~3V in total, the so called "burden" of you DMM causes the issue. Exactly as Gyro mentioned.
Solution depend on maximum possible current to be expected.
Less than 100 mA:
use a 1.0 Ohn 1% resistor in line (series) with your battery and measure the volatge over that "shunt" with a DMM in Volt. Thus as Mr. Ohm stated in his law: 1mV <=> 1 mA, maximum shunt losses = 10 mW, max. Vdrop = 100mV
More than 100 mA, but less than 1 A:
use a 0.1 Ohn 1% resistor in line (series) with your battery and measure the volatge over that "shunt" with a DMM in Volt. Thus as Mr. Ohm stated in his law: 1mV <=> 10 mA, maximum shunt losses = 109 mW, max. Vdrop = 100mV
Above 1A, but less than 10 A:
use a 0.001 Ohn 1% resistor in line (series) with your battery and measure the volatge over that "shunt" with a DMM in Volt. Thus as Mr. Ohm stated in his law: 1mV <=> 1000 mA, maximum shunt losses = 100 mW, max. Vdrop = 10mV
Have fun.

[Kleinstein]

With many normal DMMs the voltage drop at max current for the range is about 200 mV plus a little for the fuse, but this should not be much in the mA range.  So 20 mA range is expected to have something like 10 Ohm and the 200 mA Range schould be about 1 Ohm.  Poor meters with something like 3000 oder 4000 counts resolution might have a higher drop - e.g 10 Ohms in the 40 mA range.

Some Autoranging meters have an even higher drop, as there is one Shunt for a larger current range - so this might lead to a high drop sometimes more than 1 V.

50 Ohms looks like quite high - are you using the correct inputs ?

[Timur Born]

Taking a look at the datasheet of the One Plus reveals that it seems to have low voltage drops (even compared to newer designs) at all ranges except for 30 mA and 300 mA. Looks like your measurement just fell into these ranges.

[babysitter]

Hi DrQuasi,

I support Gyros variant no. 3 - use the resistors as shunts. You might want to measure their resistance as you cannot expect them to be exactly ten ohm - consider that they get a little bit warm during operation which can push their resistance around a bit.

Also, if you want to measure the internal resistance in current mode by another multimeter and it appears high - consider the probes and cables! Compare to published specifications of your multimeters.

Viel Glück bei der Problemsuche!