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Latch Circuits
26 questions By Tony R. Kuphaldt
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Question 22 of 26
Identify at least one component fault that would cause the “Q” LED to always stay on, no matter what was done with the input switches.
For each of your proposed faults, explain why it will cause the described problem.
Reveal answer- NOR gate U2 output failed high.
- Wire break between “Reset” switch and resistor R2 (although if this was the only fault it may allow the Q LED to energize at power-up, just not de-energize after the “Set” button had been pressed).
Follow-up question: explain why the nature of the problem rules out the possibility of the only fault being something related to the feedback connections between U1 and U2.
Notes:Latch circuits can be confusing due to their use of positive feedback. Questions such as this are important tools for helping develop your students’ understanding of latch circuits.
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Question 23 of 26
A very common form of latch circuit is the simple “start-stop” relay circuit used for motor controls, whereby a pair of momentary-contact pushbutton switches control the operation of an electric motor. In this particular case, I show a low-voltage control circuit and a 3-phase, higher voltage motor:
Explain the operation of this circuit, from the time the “Start” switch is actuated to the time the “Stop” switch is actuated. The normally-open M1 contact shown in the low-voltage control circuit is commonly called a seal-in contact. Explain what this contact does, and why it might be called a “seal-in” contact.
Reveal answerEven though the “Start” and “Stop” switches are momentary, the “seal-in” contact makes the circuit latch in one of two states: either motor energized or motor de-energized.
Notes:Motor “start-stop” circuits are very common in industry, and apply to applications beyond electric motors. Ask your students if they can think of any application for a circuit such as this.
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Question 24 of 26
A student decides to build a motor start/stop control circuit based on the logic of a NOR gate S-R latch, rather than the usual simple “seal-in” contact circuit:
The circuit works fine, except that sometimes the motor starts all by itself when the circuit is first powered up! Other times, the motor remains off after power-up. In other words, the power-up state of this circuit is unpredictable.
Explain why this is so, and what might be done to prevent the motor from powering up in the “run” state.
Reveal answerWhat you have here is something called a race condition, where two or more relays “race” each other to attain mutually exclusive states. This is a difficult problem to fix, but the solution (and yes, there is more than one valid solution!) invariably involves “rigging” the race so that one of the relays is guaranteed to “win.”
Notes:Analyze the power-up states of this circuit with your students, and the “race” condition will become apparent. Such problems can be very difficult to locate and fix in real life, so it is good to expose students to them early in their education, and in contexts where the circuitry is not too confusing.
