# Null detector

Discussion in 'General Electronics Chat' started by gorgondrak, Jul 17, 2015.

1. ### gorgondrak Thread Starter Member

Nov 17, 2014
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Voltmeter Impact on Measured Circuit
Chapter 8 - DC Metering Circuits

If the 24 volt battery were higher that the adjustable voltage source wouldn't that pull current through the null detector even it was the same voltage as the voltage across r2 given that the 24 volt battery positive terminal would pull electrons through the adjustable voltage source positive end?

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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1,066
The entire premise of a "null detector" is that there is some voltage that the "adjustable voltage source" can be set to that causes zero current to flow through the "null detector", which is usually a high resistance voltmeter.

In the stated problem, what is the voltage at node1?

What voltage at node2 would make the current through the null detector zero?

3. ### crutschow Expert

Mar 14, 2008
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The battery voltage is not the voltage of interest, it's the voltages at Node 1 and Node 2, as Mike noted.

4. ### alfacliff Well-Known Member

Dec 13, 2013
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what is the reason for two batteries? couldnt it be done with only one?

5. ### BillB3857 Senior Member

Feb 28, 2009
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IF the null meter were set to NULL with the 24V battery supply set to 24V and for some reason the 24V battery supply were to increase (or decrease) in voltage, then current would flow through the meter. The direction of current flow would depend upon whether the 24V increased or decreased. The voltage of the adjustable voltage supply on the right side of the null meter would not be 24V in either case, as pointed out by others. To the OP, based upon the resistor values shown, what should the adjustable supply be set for to give a null with 24V from the battery supply?

6. ### AnalogKid Distinguished Member

Aug 1, 2013
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While this could reflect a real world situation, this is a teaching problem to demonstrate concepts in circuit theory.

ak

7. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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Think of the meter as a fixed resistance and the adjustable source as a 'dynamic' drop...

Best regards
HP

8. ### crutschow Expert

Mar 14, 2008
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3,393
It depends upon what the circuit is for, but a single voltage is used in a Wheatstone Bridge circuit as seen here:

In this circuit the Null point (from point D to point B) is determined solely by the relative resistance values of the 4 resistors and not affected by the voltage applied to the bridge.

9. ### BillB3857 Senior Member

Feb 28, 2009
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Also, to the OP, most NULL meters are what is called ZERO CENTER meters. That mean that with no current through the meter movement, the needle is at the center point of the scale. Many will also have a small mirror positioned on the scale to help eliminate parallax errors.

Aug 13, 2015
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11. ### ataro New Member

Aug 13, 2015
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Sorry for the jumping of this old thread to back to the top. I've not sured if I can reply to this thread with my question or to open a new thread.

However, my question is about the adjustable voltage source. it is stated here: http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/ that in some circuits designs, there is a potentiometer which control on the voltage level applied by the second voltage source. I'm very confused about the direction of the electron flow in this circuit.

To my best understanding, the electrons flow should be like in this photo:

So if this is true, how is it possible that it will be zero electrical current from point 1 to point 2?

12. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Electrical current can flow between two points only if there is a potential difference between those points.

Aug 13, 2015
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