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Transistor switch driving a relay

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A transistor switch is used to allow a 12 volt relay to be operated (turned on and off) by a small input voltage, Ein.

The voltage level of the input can be changed by sliding the black arrow- head up and down on this vertical scale. Changing the input voltage causes changes in the base current, collector current, and in the collector voltage. When the current in the relay coil (i.e. the collector current) exceeds a certain value, the relay switches on. The diode reduces the large transient voltages that are produced when the current through an inductor (the relay coil) is changed quickly by switching (called "inductive kick").

To see the current flow throught the transistor, click on the "Show current flow" button.

You can change the input (base) resistor, the relay coil resistance, and the power supply voltage (Vcc). Just click on the number with the mouse pointer and edite like any text field. You can also inspect and modify the specifications of the transistor - click on the "Show specs" button on the left; this display a table of several specifications (such as the transistor's current gain, beta) that you can modify.

Student Questions

1. Launch the Hypercard stack "Transistor switch".

2. Inspect the screen layout . Click on the "What do I do?" button and the "Explain" buttons and read the pop-up help fields.

3. Vary the input voltage by sliding the slider along the scale. Observe the behavior of the relay (ON or OFF) and describe the operation.

4. What is the voltage across the relay when it turns off and on? At what input voltage does the transistor turn the relay on and off?

5. What does "transistor is saturated" mean?

6. What "good" does the transistor do in this circuit? That is, what evidence is there that the transistor switch is providing an amplification function?

7. Estimate the beta (current gain) of the transistor from the currents displayed.

8. Check your estimate of beta by clicking on the "Show specs" button to display the table of transistor specifications. Change the beta to another value and determine how this change effects the operation of the circuit.

9. Suppose you wanted the relay to turn on for input voltages above 3 volts and off for input voltages below 3 volts. How could you change the component values in this circuit to do this (assuming that the transistor beta is 100)?

10. For a given value of Vcc and relay coil resistance, what value of collector voltage causes the power dissipated by the transistor to be the greatest? (Vary the input voltage and observe the power dissipated). Test your hypothesis at other values of Vcc and relay coil resistance.

11. Did you find that this computer simulation was helpful in any way in furthering your understanding of this circuit? Why or why not?

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This page is maintained by Prof. T. C. O'Haver , Department of Chemistry and Biochemistry, The University of Maryland at College Park. Comments, suggestions and questions should be directed to Prof. O'Haver at
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