Digital Circuits

Electromechanical Relay Logic

19 questions By Tony R. Kuphaldt

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Question 4 of 19

Identify each of these relay logic functions by name (AND, OR, NOR, etc.) and complete their respective truth tables:

Reveal answer

Notes:
In order to familiarize students with standard switch contact configurations, I like to given them practice with identification and truth tables each day. Students need to be able to recognize these ladder logic sub-circuits at a glance, or else they will have difficulty analyzing more complex relay circuits that use them.
Question 5 of 19

Predict how the operation of this relay logic circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Pushbutton switch A fails open:

Relay coil CR2 fails open:

Relay contact CR1-1 fails open:

Relay contact CR2-1 fails shorted:

Relay contact CR2-2 fails shorted:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Pushbutton switch A fails open: Lamp 1 always energized, lamp 2 simply becomes inverse status of pushbutton switch B.

Relay coil CR2 fails open: Both lamp 1 and lamp 2 simply become inverse status of pushbutton switch A.

Relay contact CR1-1 fails open: Lamp 1 simply becomes same status as pushbutton switch B.

Relay contact CR2-1 fails shorted: Lamp 1 always energized.

Relay contact CR2-2 fails shorted: Lamp 2 simply becomes inverse status of pushbutton switch A.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
Question 6 of 19

Though many electronics students and professionals alike associate semiconductor components with the word “digital,” electromechanical relays are also digital logic (on or off) devices. In fact, some of the first digital computers were built with electromechanical relays as their active elements.

In what ways are electromechanical relays similar to semiconductor logic gates? In what ways do the two digital technologies differ?

Reveal answer

Like semiconductor gates, electromechanical relays have but two states: energized and de-energized (1 and 0). Like gates, the contacts of relays may be interconnected to perform standard logic functions such as AND, OR, NAND, NOR, and NOT.

Notes:
This question provides a good opportunity to review electromechanical relays: how they work, what they are used for, etc.