Question

A) You are in a situation where you need to control a load that draws 1A from an approximately 12V supply. The control signal that you have available is a 0-1V signal capable of sourcing 100mA. Choose a type of transistor to implement this control and show a circuit diagram for how you might hook it up.

B) For this problem, the load and its power supply are the same as in the previous problem, but the control comes from a CMOS logic gate (the output voltage swings between 0 & 5V and it is capable of sourcing 4mA of current). Choose a single transistor (one of the generic types that we talked about in class) to implement the control and show a schematic of how you might hook it up.

C) In this problem, a load with the same current draw and power supply are being used as in problem 2 (1A at ~12V) but the load is such that one terminal needs to be tied to ground (this is very common in automotive applications). In this case, the control circuit only swings between 0 and 2V and is capable of sourcing only 1mA. Design a circuit to implement this control.

Limit your design to components that you have experience in using in class and operating at typical specifications (i.e., rules of thumb and state assumptions). Show in numbers how the selected transistor will perform the task (use datasheet).

Answer 1

There can be many possible designs but I have used one using BJT. Rc is load resistor drawing 1A current and the control signal is passed through resistor Rb. Base current I_b controls the operation of the transistor and thus controls the current through load Rc.

Relevant equations:

Vcc-IcRc-Vce=0..........................ref. datasheet

12-1*Rc-0.2=0

=>Rc=11.8 ohm

Now,

Vs-IbRb-Vbe=0

1-0.1*Rb-0.7=0

=>R_B=30 ohm

Final design:

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Thanks!