The picture shows a fuel injector circuit, components are 2x LED, 4x resistors, 2x transistors and 2x power supply (12v, 5v). Task is to calculate what resistors are need for this circuit to turn on without harming any components. A LED needs 1.8v and 20mA to turn on this means that when circuit is on, 1.8v will be used by the LED, therefore it is necessary to take 1.8 from 12 leaving 10.2v which makes voltage for calculations. now to find out the resistors needed so LED lights and flickers. R=V/A with this, resistor for circuit will be known. R=10.2/0.02 (20mA) = R =510 Ohms. So 510 Ohms resistor is needed so that LED can turn on. Now the transistor, picture shows that its a NPN ( arrow pointing out ), with multimeter on diode check mode one of the three terminals will be common or will give readings when touching other terminals. Readings will be close but one will be slightly higher which will be the emitter and other as collector. Collector will need to be connected in series with LED as shown in picture. Now is to calculate what resistor is needed so that transistor doesn't start smoking (blown). now to calculate resistor, a transistor needs 0.6v to flow from base to emitter for it to turn on, which means 0.6 is subtracted from 5 = 4.4v just like before this is the voltage for calculations. now data sheet from www.fairchildsemi.com shows that in the saturated region the transistor will have IC=10mA IB=0.05mA with this I double IC and IB because I know that I need 20mA following through IC due to LED which then gives me IC=20mA IB=1mA. With this I can calculate resistor needed, R=V/A. R= 4.4/0.001= R =4400 Ohms and this is the resistor needed in series with base of the transistor. next task was to give a 50% buffer so I doubled it again as shown R= 4.4V/0.002A= 2K2.
Next was to build circuit on lochmaster 3.0
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| This is the circuit on a breadboard. |
LED 1.8v / 20mA
R1,R2 12-1.8=10.2 / 0.02= 510 Ohms R = V/A.
BC547 Transistor wwwfairchildsemi.com (Data sheet)
VCE
(sat) Collector-Emitter Saturation Voltage IC=10mA, IB=0.5mA Max of 250mV
IC=100mA, IB=5mA
R3,R4 5V- 0.6= 4.4/ 0.01= 4400 Ohms R= V/A
50% = 4.4/ 0.02= 2200 Ohms. (Ohms law)
Power supply 12V and 5V.
REFLECTION:
When parallel circuit was built on breadboard and connected to power supply (12v, 5v), only one LED was lighting up and with a closer look, found that LED was backwards (positive and positive= off). after correcting it, I found that both LED were turning on but weren't blinking simulating pulsing of injector with was the task. When looking closer, I had both bases of each transistors connected on the the same buzz bar showing that continuity needed to be broken and when done the frequency machine when changed was pulsing simulating the switching on/off of injectors which was the task.
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