When the AUX pin of a stereo is connected to my mobile phone's audio female port, I just measured the voltage at the end of this AUX pin (No load is connected) approximately 2mV using a DMM. But when I measured the current, it didn't indicated any current in the DMM. According to Ohm's law, voltage is current times the resistance. When relating this basic law to my practical case, it doesn't matching. How can it be realized?


closed as off-topic by The Photon, John Rennie, Qmechanic Sep 20 '18 at 7:51

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  • $\begingroup$ You need to make this more clear to be able to get a useful answer. The AUX pin of what? Of the phone? Of a stereo? How did you measure the current? If you didn't read "a uA current" what did you read (no current or a larger than uA current)? $\endgroup$ – The Photon Sep 20 '18 at 2:06
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    $\begingroup$ Also, this is probably a better question for Electrical Engineering rather than Physics (but not until there is enough information provided to make the question answerable). $\endgroup$ – The Photon Sep 20 '18 at 2:07
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    $\begingroup$ was the aux line actually connected to the phone? if the circuit was open, then no current would flow. also when you made the measurement, was there an audio signal flowing between the devices? no signal means no current and no voltage. Finally, were you using the AC or DC range on the meter to take your measurements? By the way, the nominal impedance of the AUX port is 10,000 ohms. $\endgroup$ – niels nielsen Sep 20 '18 at 3:00
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    $\begingroup$ FYI, the question has now been cross-posted to EE. $\endgroup$ – The Photon Sep 20 '18 at 3:01

In electronics, circuits that pass signals to one another are usually isolated by having high input impedance (that is, the next circuits senses the voltage but doesn't take in any current). This is necessary because as soon as you have current, you are modifying the circuit response and thus affecting the signal. You need to ensure that the wires only carries the signal from A to B and B can't talk back. The circuits should ideally behave the same regardless of what they are connected to. Most circuits are low-power devices that would drop their voltage significantly if you wanted to draw any current from them.

Ideally, you can just imagine the next device as a voltmeter reading the signal. Depending on the circuit complexity and intended use, the input can go straight into an op-amp or some other amplyfing unit that separates the high-impedance (low current) input to the processing part. Additionally... once you go to active electronics, there is no such thing as the ohm's law, as you have nonlinear response AND power supply in each stage of the circuit.


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