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A
magnetic relay used with a transistor is often connected with a diode across
it. Explain the function of the diode. |
3
marks |
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1 |
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When the current through the magnetic relay
stops suddenly, a large e.m.f. would be induced across the relay. This
would draw a large current through the transistor (see Fig.a). |
1 |
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With the diode connected (see Fig.b), the induced
current would flow through the diode which is a much lower resistive path.
This protects the transistor. |
1 |
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| b. |
Draw
a circuit diagram for determining the input-output voltage characteristics
of a n-p-n transistor operating in the common emitter configuration. Give
the approximate values of the resistance used in your circuit, explaining
why they are used. |
5
marks |
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Circuit for investigating
the input-output voltage characteristics |
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3 |
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The base resistor is used to limit the base current to a safe value. Base
current should be small because of the current amplification of the transistor.
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The load resistance is to enable a potential drop which depends on the
collector current. When the collector current is large, the p.d. across
RL is large. Then the output voltage across C and E is
small.
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2 |
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| c. |
Draw
a graph of a typical input-output voltage characteristics. With reference
to this characteristic, design circuits for
i)
the thermostat of an electric iron. |
8
marks |
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Characteristic Input-output voltage |
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2 |
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The
thermostat of an electric iron |
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When temperature falls, the resistance of the thermistor
increases. The p.d. across it increases. The input potential of the transistor
increases. A base current flows to saturate the transistor. Since current
flows throught the magnetic relay, the secondary circuit consisting of
the electric iron is turned on. |
2 |
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ii)
a warning system when the temperature is above a certain pre-determined
value. |
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Warning
system when the temperature is above a certain pre-determined value |
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1 |
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When temperature increases, the resistance of the thermistor
decreases. The p.d. across it decreases. The input potential of the transistor
increases. A base current flows to saturate the transistor. Since current
flows throught the magnetic relay, the secondary circuit consisting of
the warning devices (like a light bulb) is turned on. |
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