Transcript transistors

Show-me board revision quiz
1.
2.
3.
4.
Draw the symbol for an LED
What happens to the resistance of a thermistor
as it is heated up?
What happens to the resistance of an LDR if
light intensity decreases?
How is the supply voltage, Vs, related to the
voltage across two components in series, V1 and
V2?
What we will do today:
•
•
•
•
State that a transistor can be used as a switch.
Name the two types of transistor.
Draw symbols for the two types of transistor.
Investigate how voltage dividers work when using
LDRs and thermocouples.
Transistor
•
•
•
1.
2.
•
A transistor can be used as a switch.
A transistor can be either ON (conducts
electricity) or OFF (does not conduct)
There are two types of transistor that we must
know about:
MOSFET
npn – transistor
We must be able to identify these transistors
from diagrams.
1. MOSFET
•
•
•
A MOSFET has three main parts: Gate; Drain;
and Source.
They switch on at approximately 2 V.
It has the following symbol:
2. npn transistor
Where
•
b is the base
•
e is the emitter
•
c is the collector
•
The npn transistor switches on at approximately
0.7 V.
2009 Qu: 13
2011 Qu: 10
2007 Qu: 11
Voltage Dividers
• When dealing with thermistors and LDRs in
voltage dividers we only need concern ourselves
with the voltage across the lower component, we
will call this Vo, the output voltage.
LDRs – show me board
•
•
Draw a voltage divider circuit with
an LDR and a variable resistor to
show how we could investigate the
voltage across the variable resistor.
Hint: Make variable resistor Vo,
Voutput
LDR at top
•
With a circuit
set up as
shown, we can
investigate Vo
in both light
and dark
conditions:
Condition Vo (V)
Dark
Light
LDR at top
•
•
•
As L ↑, RLDR ↓
So VLDR↓ making VVR ↑
Therefore Vo ↑
LDR at bottom
Condition Vo (V)
Dark
Light
LDR at bottom
•
•
•
As L ↑, RLDR ↓
So VLDR ↓
Therefore Vo ↓
Thermistor at top
•
With a circuit
set up as
shown, we can
investigate Vo
with in both
cold and warm
conditions:
Condition Vo (V)
Cold
Warm
Thermistor at top
•
•
•
As T ↑, Rtherm ↓
So Vtherm↓ making VVR ↑
Therefore Vo ↑
Thermistor at bottom
Condition Vo (V)
Cold
Warm
Thermistor at bottom
•
•
•
As T ↑, Rtherm ↓
So Vtherm ↓
Therefore Vo ↓
2007 Qu: 11
D
2009 Qu: 13
A