I used a B103 potentiometer to split the voltage from the 5v source of my Arduino UNO R3. The output from the third pin was connected to a 330 ohm resistor, and that was connected via an LED back to a GND pin on the Arduino. The LED lit up, as expected.
I then tried to measure the current between the LED and the GND pin, using a Kaiweets KM601 multimeter. The LED stopped emitting light, and the voltage between the shorter leg of the LED and GND rose to, for example, 3.5v.
I would be grateful if someone could tell me which effect I should read up on.
I am embarrassed to say that I just measured again with the meter and everything worked as expected. I got 6.4mA. I must have plugged the leads into the wrong ports. Thank you pointing out my error.
You have to be careful with (direct) current measurements! It's kind-of a pain and it's not done nearly as often as voltage & resistance measurements because you have to break the circuit and insert the meter in series.
In current-mode the multimeter is essentially a short circuit (zero Ohms) and if you connect it wrong you can short something out or you can get excess current through the meter and blow a fuse in the meter. Or you can "change things" so the current is different when the meter is connected!
Normally in this situation, you'd measure the VOLTAGE ACROSS the (known) resistor and CALCULATE the current as Current = Voltage / Resistance (Ohm's Law).
The current through series components is the same, so the current through the LED is the same as the current through the resistor.
Ohm's Law is a law of nature so it's always true, but measuring the voltage across the LED "doesn't work" because the resistance of the LED varies with voltage & current and it's unknown unless you calculate it from the current & voltage, under current conditions.
However, the voltage in series circuits divides, so if we apply 5V and measure 2V across the LED, we know there is 3V across the resistor and we can use the calculated 3V and the known resistance to calculate current.
It’s kind of you to have taken the time. I obviously plugged the probes into the wrong plugs, though I don’t remember exactly what I did.
When I used the 10A plug, I got a reading of 4 mA; when I used the 600mA plug, I got 8.638mA. The voltage after the split was 4.27v.
As I said in my post, I’m new at this game, and I’ve not had ant tuition, so I’m rubbish. But I enjoy it.
I’ve found Vocademy.net where they give free tuition. Perhaps I need to spend a lot of time on that site, but I suppose being careless with probes is just something I need to correct.
