Component list:
2 x NPN BJT transistors2 x LED's (Light emitting diode)
2 x 470 ohms 0.25 watt carbon film resistors
2 x 1 kOhms 0.25 watt carbon film resistors
Calculations:
Vled=1.78 volts
Iled=20 mamps=0.02 amps
Vs=12 volts
R1=R2=(12-1.78)/0.02=10.22/0.02=511 ohm as we dont have such resistor we can use 470 ohm resistor for R1 and R2
PWM signal=5 volts
Vbe=0.7 volts
B=Ic/Ib, Ib=Ic/B
Ib=0.02/100=0.0002 amps=0.2 mAmps
R3=R4=(V-Vbe)/Ib=(5-0.7)/0.0002=4.3/0.0002=21500 ohm =21.5 kohm
from data sheet we found that to be at full saturation mode transistor require from 0.5 mamps to 5 mamps and maximum amps which can flow through the base is 200 mamps. If we use 21.5 kohms resistor it wont be enough amps at the base and it will not saturate. So as we need it to be at full saturation we will use 5 mamps at the base as it can handle up to 200 mamps and recalculate what value resistor we will use to get that amps at the base.
R3=R4=(V-Vbe)/Ib=(5-0.7)/0.005=4.3/0.005=860 ohm as no such thing as 860 ohm resistor we will use 1000 ohm resistor for R3 and R4
Problems:
When i first connect circuit to PWM signal generator, LED's were flashing OK, but when i connect it to 5 volts constant power source it blew transistor and i get open circuit at Vce. I soldered new transistor and tried to check it again, and blew another one. After inspecting circuit board i have found that i solder resistor R3 and R4 on one track and forget to drill a hole in the track to make resistor restrict amps flow to the base of the transistor. After all job been done circuit started to work as it suppose to.
Explanation:
In this circuit BJP transistor is used as a negative trigger switch to trigger flashes of LED's. Every time PWM signal sends to the base of transistor it closes emitter and collector and create a complete circuit for LED and it flashes for as long as a signal last. If signal intensity increased LED start flashing faster and if decreased it flashes slower. Also signal length can be changed to make light flash longer or shorter. This circuit represent basic injector drive circuit, where LED's changed with fuel injectors and can be controlled how fast, slow or how long to operate.
Voltage drop across Vbe 0.82 volts, that shows that transistor working as it need at least 0.7 volts at the base to work.
Voltage drop across Vce 0.05 volts, shows that transistor in fully open (saturated) condition.
Voltage drop across LED 2.3 volts, shows that LED use 2.3 volts to work.
Voltage drop across Rled 9.62 volts
Voltage drop across Rb 4.18 volts
Reflection:
As this was my first circuit i build i would like to change layout of components and make it more tidy and smaller. And also connect all wires on one side.
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