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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