1. Strain gauge
- When the plastic is stretched the wire also stretches.
-This increases wire’s length and its cross-sectional area
also decreases slightly.
- Both of these changes cause the resistance of the wire
to increase.
2. -The resistance R of a wire of length L and of uniform
cross-sectional area A is given by the expression
R = ρL /A,
where ρ is the resistivity of the material of the wire.
-Assuming that, when the wire extends by a small
amount ΔL, the change in the cross-sectional area is
negligible, and the new resistance will be given by
(R + ΔR) = ρ(L +ΔL) /A,
where ΔR is the change in the resistance.
3. -Subtracting these two expressions,
ΔR = ρΔL /A
or,
ΔR ∝ ΔL (stress)
-Thus the strain which is proportional to the
extension ΔL is also proportional to the change in
resistance ΔR.
-Where the cross-sectional area A is assumed to be
constant.
-Since strain is proportional to the extension, then the
strain is also proportional to the change in resistance.
4. Light-Emitting Diode (LED)
-A LED emits light only when it is forward biased and the typical
maximum current is 20 mA .
-A LED may be damaged if the reverse-bias voltage exceeds
approximately 5 V (called breakdown voltage) but is dependent on the
device.
-Resistor is connected in series with an LED so that, when the LED is
forward biased (the diode is conducting), the current is not so large as
to damage the LED.
5. -The light emitting diode(LED) which is available in many
colours, is a semiconductor device that is very robust,
reliable and dissipates little power.
-A LED with an in-built resistor to protect the LED is often
used to indicate the state of output of an op-amp.
6. Fig. Below is a circuit using two diodes to indicate
whether the output from an op-amp is positive or
negative with respect to earth.
When the output is positive with respect to earth,
diode D1 will conduct and emit light. Diode D2 will not
conduct because it is reverse biased. If the polarity of
the output changes, then D2 will conduct and emit
light and D1 will not emit light. The state of the output
can be seen by which diode is emitting light. The
diodes can be chosen so that they emit light of
different colours.
