Lecture Text (DC2)

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Parallel Circuits Lecture (Text Only).

In the parallel circuits the applied voltage is the same across each parallel branch, and the current in each branch equals the applied voltage divided by the resistance of the branch.  The total current in a parallel circuit is always more than the current in any one branch even if one branch carries most of the current. This is true because the total current is simply the sum of the branch current.  This is usually obvious to must student.  What is implied by this is not always obvious to many students is the relationship of the total resistance to the branch resistance.  The combined resistance of resistors in parallel is always less than the smallest branch resistance.  For example, if you had 100,000, 10,000 and 1000 ohm resistor in parallel, the combined resistance would be less than 1000 ohms.  If you connected an ohmmeter across the 1000 ohm resistor you should measure slightly less than 1000 ohms.  If you measure across the 10,000 or 100,000 ohm resistor you should also measure slightly less than 1000 ohms.  If you measure across the 1000 ohm resistor and read more than 1000 ohms, than you can be certain the 1000 resistor is out of tolerance or open. If you don't measure the same resistance across all three resistors than they are not really connected in parallel anymore, and you should start looking for an open wire or land or a cold solder joint at one of the resistor nodes.

Of course you can only make ohmmeter measurements when circuit and or system power is off.  You may not realize that it is also better to make resistance measurement with the circuit board out of the electronic system.  For example, if you had 1000 ohm resistor in series with a 2000 ohm resistor and one resistor went to ground and the other resistor went to 5 volts.  Then, when you measured across both resistors you would get a reading of 3000 ohms if the board were out of the system.  However, a measurement made on the same circuit board when circuit board is in system might be much lower.  That is because even though the power is turned off you might still read much less than 3000 ohms.  That is because the power provides a parallel pathway for current to flow from the 5 volt bus to ground.  That is the power supply is in parallel with the 1000 and 2000 ohm series combination.  

 

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