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DC Bridge Circuits
19 questions By Tony R. Kuphaldt
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Question 7 of 19
Identify the most important qualification for the “null” meter used to balance a bridge circuit. In other words, describe what type of meter we would be looking for if we were to select one for use as a “null” meter. Describe why this particular quality is important.
Reveal answerAbove all, a null meter must be sensitive.
Notes:Discuss with your students the definition of “sensitivity” with regard to meter movements, and why null meters have to be sensitive in order for the bridge circuit to be accurately balanced. If your students have studied meter movement design, you may wish to challenge them with a question on exactly how a null meter movement might be constructed (i.e. what would have to be done to maximize its sensitivity?).
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Question 8 of 19
What will happen to the voltage between points A and B if the power supply voltage increases?
Reveal answerVAB will remain the same as Vsupply increases.
Notes:This question highlights another important concept of bridge circuits, namely that balance is irrespective of supply voltage.
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Question 9 of 19
Explain how this bridge circuit is capable of being “balanced” for any values of R1 and R2:
Reveal answerThe potentiometer acts as a complementary pair of resistors: moving the wiper one direction increases the value of one while decrease the value of the other. Thus, it forms a voltage divider with an infinitely adjustable division ratio of 0% to 100%, inclusive.
Notes:This question showcases one more use of the potentiometer: as a voltage divider used specifically to balance a bridge circuit for any arbitrary values of fixed resistances. If students have difficulty seeing how this is possible, you might want to try representing the pot as a pair of fixed resistors (R3 and R4), the wiper position determining the balance of those two resistance values (Rpot = R1 R2).
