The Open Door Web Site
HOME PAGE BIOLOGY CHEMISTRY PHYSICS ELECTRONICS HISTORY HISTORY of SCI & TECH MATH STUDIES LEARN FRENCH STUDY GUIDE PHOTO GALLERY
ATOMIC and NUCLEAR ELECTRICITY and MAGNETISM MEASUREMENTS MECHANICS OPTICS PRACTICAL WORK QUESTIONS RELATIVITY THERMAL PHYSICS WAVES
ELECTRICITY and MAGNETISM
Google
Custom Search
Using Variable resistors
Variable resistors are still used (this written 2016) in various electronic devices, for example, as volume controls in amplifiers, dimmer circuits for lights etc.  
They have, in many cases, been replaced by digital circuits but some of the same principles still apply.
 
In this circuit, notice that only two of the connections of the variable resistor are being used.
In what follows, we will assume that the maximum resistance of the variable resistor (that is, the resistance between points A and B) is 100W.
When the sliding contact, S is moved to A, the voltmeter will read 6V. In this situation it will be connected directly to both sides of the supply.
 
However, what will be the reading of the voltmeter is S is moved to B?  
   
 
When S is moved to B we have, in effect, this situation.
 
Clearly, the voltmeter will read 3V (see here if in doubt).
 
 
 
We will now consider using all three connections of the variable resistor to make a variable potential divider.  
With S connected to A, the situation is as before, therefore the voltmeter reads 6V.  
   
However, with S moved to B, the wire x that we have added (assumed to have zero resistance) is now in parallel with the 100W resistor.
 
This means that the voltmeter will now read zero.
(NB The circuit symbol for the variable resistor is not usually drawn like this.)
 
Hence a variable potential divider circuit is more useful as a supply of variable voltage.  
However, it should be noted that a system like this does not provide a stable variable voltage supply.  
     
If we connect only a voltmeter, as shown here, we can predict its reading, knowing the position of S.
If we assume S is half way between A and B, then V = 3V.
 
Remember that, if we are using a suitable voltmeter, it will have a resistance that is very much higher than the other components in the circuit.
In calculations, we often consider it to have infinite resistance.
 
If we now connect a component having a resistance of 50W (for simplicity of calculation!) the voltage will decrease significantly.  
   
 
 
 
 
 
 
 
This is because we now have, in effect, the circuit shown in the next diagram.  
   
 
Two 50W components in parallel have an effective resistance of 25W so the voltage indicated by the voltmeter will now be one third of the supply rather than one half as above, so V now equals 2V.
 
 
 
 
Conclusion  
The voltage can still be varied from zero up to the voltage of the supply but it also varies depending on what we connect to the variable potential divider.  
SITE MAP
WHAT'S NEW
ABOUT
PRIVACY
COPYRIGHT
SPONSORSHIP
DONATIONS
ADVERTISING
 

© The Open Door Team
2016
Any questions or
problems regarding
this site should be
addressed to
the webmaster

David Hoult 2017

Hosted By
Web Hosting by HostCentric

 
SiteLock
 
 
Electricity and Magnetism Index Page