# PC/CP320 Physical Computing

## Output Transducers: Relay

### Background

It may sound strange to speak of a relay as an output transducer, but because it contains a solenoid inside, its operation depends on the operation of this internal solenoid. So investigating a relay as an output transducer really means investigating the internal solenoid.

### Equipment

You will use one of the relays; either one is OK.

Procedure

You will use one of the relays; either one is OK.
1. Determine the functions of the various pins by measurement. Remember there should be two pins for the coil and pins for common, normally open, normally closed and any duplicates. From these measurements and the attached datasheet, which model of the relay do you have?

2. Can this relay operate as normally open, normally closed, or both?

3. What is the coil's resistance?

4. The coil is energized when you hear it click. (You can also measure the resistance between the common and either NO or NC pins to verify.)
• What current is required to consistently energize the coil?
• What voltage is required to consistently energize the coil?

• Observe the ac component of the voltage across the coil on an oscilloscope as you turn it on and off. What do you notice?
• Disconnect the coil from the voltage supply and observe the ac component of the voltage across the supply on the scope. Does it look different from when the coil was connected? If so, how?
• Reconnect the coil, but now put a reverse-biased diode across the coil, (i.e. in parallel with it), and observe the ac component of the voltage across the coil as you turn it on and off. How does it compare to the previous two situations?
From now on, always use a diode across the coil.
5. Normally, the current/voltage required to energize the coil is different from the current/voltage required to keep the coil energized. Decrease the current until the relay clicks back. (As before, you can also measure the resistance between the common and either NO or NC pins to verify.)
• What current is required to keep the coil energized?
• What voltage is required to keep the coil energized?

Demonstrate the relay in operation, showing how you can ensure whether it's "on" or "off".

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