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