The transistor, as you all know, can act as a Switch. So using "Transistor as a switch" concept, many students do a project on "Controlling Home appliances through a switch".
The first idea which strikes you is : "I should use a transistor as a switch and a relay( since Home appliances need a large voltage supply). Here is the popular circuit .
The basic working of a circuit is
1)when the source voltage is high (Say 3 volts) .Transistor will be on. IC(Collector current) will be large enough to on the relay and hence on the respective home appliance.
2)when the source voltage is low (0v) .Transistor will be off.No current in the relay and hence Home appliance will be off.
So the first question which arises is "why there is a diode D1 in this circuit".
As we know relay consists of a inductor coil.Inductor resists the change in the current.
V=L di/dt. .....(Eqn 1)
In the above case, If transistor is on.there will be a base current and hence collector current.The collector current flows though the relay (inductor).But if suddenly the transistor becomes off. di/dt will be very large.
(Because in a very small time there is large change in current). So the inductor will generate a very high voltage at the collector as given by the equation 1.( Vce will be very high).
The VCE (collector to emitter voltage) will be so high that .It will destroy the transistor.The Collector voltage should not exceed the VCEO rating of the respective transistor .
Now lets see what happens if we dont use Flyback diode.
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| INPUT |
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| OUTPUT |
As you can see the above output is the VCE voltage for the corresponding input (As shown by green probe).When the input voltage falls down.Vce will be very large in range of Kilovolts and much much greater than VCEO(which is the maximum collector voltage allowed) and hence transistor will breakdown and no longer functions as a (Transistor )
With the FLYBACK Diode
- What Flyback diode does is. ---->> when the transistor is ON. It will be reverse biased and doesnt exist in the circuit.
- When the transistor is OFF. The Flyback diode will be forward biased. Because( See red probe,Collector voltage will be very high than the power supply).The flyback diode makes inductor to draw current from itself in a loop until the energy is dissipated in diode and wires.(Flyback diode creates a loop for the energised inductor when the switch is off )
When switch is opened the diode become FB, Allowing it to conduct current in a circular loop
from the positive potential at the bottom of the inductor to the negative
potential at the top (assuming the power supply was supplying positive voltage
at the top of the inductor prior to the switch being opened). The voltage
across the inductor will merely be a function of the forward voltage drop of
the flyback diode. Total time for dissipation can vary, but it will usually
last for a few milliseconds.
See the output voltage Vce after using flyback diode
When the transistor is ON. The Vce will be very less.
When the transistor is OFF. The large voltage generated by the inductor will be dissipated in the flyback diode and resistor within few milliseconds .
And Flyback diode ensures that VCE<VCEO when the switch/transistor is off (earlier this condition was true only when transistor/switch was ON) . Thus ensuring the proper working of the transistor.
Note :Please use Flyback diode while dealing with Relays/inductor in Your projects. Otherwise your transistor may not work again :)
Note :Please use Flyback diode while dealing with Relays/inductor in Your projects. Otherwise your transistor may not work again :)
