Day 10 - Mosfet and fault finding

Theory

MOSFET(metal oxide semiconductor field effect transistor)
Mosfet has higher input impedance. Like bipolar FET's come in two polarities, P-channel and N-channel, corresponding to PNP and NPN bipolar transistors. They also come in enhancement mode and depletion mode types, specifying what happens when the gate voltage is zero. An enhancement mode FET will be turned off with no voltage at the gate; like a bipolar transistor, it requires a bias voltage to turn it on. A depletion mode FET will be turned on with zero gate voltage. The only way to turn it off is to apply a voltage of opposite polarity to the one that will increase current flow.

Symbols

Signals applied to the Gate.

MOSFETs are used as switches, permitting the microprocessor to turn power on and off to various parts of the circuitry.

ref: http://www.youtube.com/watch?v=Te5YYVZiOKs

http://www.youtube.com/watch?v=j47Yk7bJbxw&feature=related

Practical

Fault finding

Create an open circuit on the marked "x" as show above the circuit diagram and diagnose the circuit.

Diagnosed results before testing

At node A - All the leds are off because there is no positive connection to the leds.

At node B - Everything is fine as normal conditions except the yellow led always off. Because the voltage of op-amp pin.no 2 is floating(around 0 volts), the output is always positve power supply voltage which is the voltage of op-amp pin.no 4. Therefore it always overrides the yellow led to light off.

At node C - When the sensor input voltage is above 0.63volts, the yellow led doesn't turn off.

At node D - The red led always lights off. Because there is no connected ground to the red led.

At node E - The red led always lights off. Because the voltage of the op-amp pin.no 6 is floating(around 0 volts), the output is always positive power supply voltage which is the voltage of op-amp pin.no4.

At node F - The yellow led always lights off. Because the voltage of the op-amp.no 9 is floating(around 0 volts), the output is always positive power supply voltage which is the voltage of op-amp pin.no8.

At node G and H - The green led is always off. Because the voltage of the op-amp pin.no 13 is always 0 volts, so the output voltage is always connected to the positive power supply voltage.

The yellow led is always on. Because the voltage of the op-amp pin.no 10 is always 0 volts, so the output voltage is always connected to the ground.

The red led is always off except that the sensor input voltage is above 9.1volts. Because the voltage of the op-amp pin.2 and 5 is 9.1volts.