Electronics > Beginners
Some noob questions
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Brumby:
You really are overthinking things.

With your circuit setup, just before you make the final connection, both the +ve and -ve probes of the DMM are at 9V with respect to the -ve connection of the battery (I use this reference point as it is pretty much the standard.)  Because there is no difference in potential between the meter probes, it will display 0V ... as expected.

The piece of wire touching the -ve probe of your DMM is also at 9V with respect to the -ve connection of the battery.

Then, when you connect the wire (at 9V) to the -ve of the battery (at 0V), you changed the equilibrium.  The circuit is now closed and current can flow.  How much current will depend on the resistance of the circuit and the voltage of all the points around the circuit will change.  Ohm's Law will now be able to put numbers to each of those.


To be honest, I would suggest you ease up a bit in trying to "understand" things to such a level of detail.  It is only going to confuse you and frustrate your efforts in learning electronics.

Understanding how a bipolar transistor actually works at the fundamental physics level is a classic example.  You don't really need to know in order to design, build and fault-find circuits.  All you really need is to understand the datasheet.

Be kind to yourself and:

--- Quote from: ArthurDent on July 21, 2018, 12:53:29 pm ---Don't overthink this. 

--- End quote ---
Mr D:
Ok, so when you put a MM across the terminals of a battery to measure voltage, there is in fact some current flowing all the way through the MM to the - terminal of the battery, right?

So the MM is basically just a resistor, and as we know the resistance (or, the MM knows it's own resistance in voltage-measure mode) and the MM can measure the current, the MM can, thanks to Ohm's law, tells us the voltage?

So a MM measures current in both voltage-measure and current-measure mode, the only difference being that in the voltage measure mode, the MM has a very high resistance?

 
Brumby:
Correct.

Some people might argue that the DMM is measuring voltage - but it's sort of an indirect process and it takes some current flow to work.
rstofer:
Back up to the days before the DMM and consider the analog Volt-Ohm-Milliammeter (VOM).  In every range, whether measuring resistance, current or voltage, the meter needle was displaced by current flow through the windings of the meter movement.  Always current flow!

When measuring current, the meter resistance was placed in parallel with a shunt resistor (a really low value resistor) such that only a percentage of the measured current was flowing through the meter.  The vast majority of the current was flowing through the shunt resistor.  But the current flow through the meter deflected the needle.

When measuring voltage, the meter resistance was placed in series with a resistor (a much higher value) such that the full scale maximum voltage would produce just enough current to fully deflect the meter.  Side issue:  This conversion factor was given on the meter face as Ohms/Volt Full Scale and a given scale, say 10V, would have a value like 10,000 Ohms/Volt Full Scale - the meter, when on the 10V scale, imposed a 100k Ohm load (10V scale * 10,000 Ohms/Volt) on the circuit.  For high impedance (vacuum tube) circuits, this could produce an error in the measurement.  Cheaper meters had values around 5k Ohms/Volt Full Scale.  DMMs are usually around 10M Ohm no matter the voltage scale.

When measuring resistance, there is a battery to produce the voltage for the meter.  It really measures current (the ONLY thing the analog meter movement really measures) based on the known battery voltage and the meter was scaled to read Ohms.

There is still a known relationship between the meter full scale current and full scale voltage and it is exactly the meter resistance.  When you buy an analog meter (VOM or panel meter) these values are given in the datasheet.

The same kind of thing happens in a DMM except that current isn't necessary to cause a magnetic field to move the needle.  In the case of the DMM, it measures voltage.  In current mode, it measures the voltage drop across the low value shunt resistor.  In resistance mode, it uses batteries to drive the current through the circuit and, one way or another, measures the voltage produced by that current flowing through a known resistor.  In voltage mode, is measures the voltage across a known internal resistor voltage divider.  The actual analog to digital converter measures voltage.

The DMM is always measuring voltage, the VOM is always measuring current.  But it doesn't matter!  Ohm's Law takes care of the details.  So while I say the DMM is always measuring voltage, current is involved somewhere in the circuit (causing a voltage drop across a known measuring resistor) and Ohm's Law is used to understand the relationship.

Mr D:

--- Quote from: Brumby on July 21, 2018, 01:40:05 pm ---Correct.

Some people might argue that the DMM is measuring voltage - but it's sort of an indirect process and it takes some current flow to work.

--- End quote ---

Great, this has been a stumbling block for me! Now it's clear. So actually when we say we measure voltage, what we actually mean is we measure current across a known resistance, and infer the voltage.

And when we measure current with an DMM, it's actually a two step process. First (or maybe second?) it does the voltage measurement, then, as it has the voltage and resistance, it can tell you the current? So in current measure mode, the DMM has a parallel measurement path?

Wait, nope, i still don't understand. Is it possible to measure the current without knowing BOTH the voltage and resistance?

Or put another way: to measure either, voltage, current or resistance, you ALWAYS need to know the other two? Or is one ever enough?
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