Regulated Power Sources

Predict how this power supply circuit’s output voltage will be affected as a result of the following faults. Also note whether or not any other components in this circuit will become stressed as a result of each fault. Consider each fault independently (i.e. one at a time, no multiple faults):

Transformer T₁ primary winding fails shorted:

Transformer T₁ secondary winding fails open:

Rectifying diode D₃ fails open:

Zener diode D₅ fails open:

Zener diode D₅ fails shorted:

Resistor R₁ fails open:

Capacitor C₂ fails shorted:

For each of these conditions, explain why the resulting effects will occur.

Suppose this regulated power supply circuit used to function fine, but now has stopped outputting any DC voltage at all:

Initial diagnostic measurements show there to be full DC (unregulated) voltage across capacitor C₁, and no DC voltage between the transistor base and ground. From this data, where would you suspect the problem is?

Many different types of sensors work on the principle of a variable resistance representing a different physical quantity. One such sensor is the common fuel level sensor used in automotive, marine, and industrial fuel storage applications:

As the fuel level in the tank changes, the float position will change, changing the sensor’s resistance. This resistance change is detected by an electric gauge (a special type of meter), which then provides visual indication of fuel level in the tank.

We must have some accurate way of measuring electrical resistance in order for this scheme to work. One common technique for doing this is to pass a constant current through the sensor resistance and then measure the voltage dropped across it. Since current mirror circuits function as current regulators, and therefore may be used as current sources if supplied with an external voltage, we could use a current mirror to force constant current through the fuel level sensor:

One problem with the circuit shown is that the sensor current will change as the supply voltage ( V) changes. This may be important to us, because the DC system voltage on an automobile may not be very stable, and this could lead to inaccuracies in fuel level measurement.

Figure out a way we could use a zener diode to stabilize the voltage in this current mirror circuit so that supply voltage changes would have minimal effect on the amount of current through the variable-resistance sensor.