Carey Foster Bridge Construction & Working Principle

Hi friends, this post provides an information about Carey Foster bridge. We will also learn how this bridge can be used to determine the resistance in detail with deriving an expression for determining resistance.

Carey-Foster bridge Experiment

We have already seen some basic methods for measuring medium resistances. Carey foster bridge is the method used for measurement of medium resistances. Carey foster bridge is specially used for the comparison of two equal resistances. The circuit for Carey-Foster Bridge is shown in figure below. A slide wire having length L is included between R and S. resistance P and Q are adjusted so that the ratio P/Q is approximately equal to R/S. this can be achieved by sliding contact on slide wire.

Carey foster bridge circuit

Carey Foster Bridge: Working Principle

The working principle of Carey Foster bridge is similar to the Wheatstone bridge. The potential fall is directly proportional to the length of wire.This potential fall is nearly equal to the potential fall across the resistance connected in parallel to the battery.

Description:

Let l1 be the distance of the sliding contact from the left hand end of the slide-wire of Carey foster bridge. The resistance R and S are interchanged and balance is again obtained. Let the distance is now l2.

Let r= resistance/unit length of slide wire

For first balance,

For second balance,

Comparing,

Where l1 and l2 are balanced points when slide wire is calibrated by shunting S with a known resistance and Sâ€™ is value of S when it is shunted by a known resistance. Thus Carey foster bridge can be used to measure the medium resistance.

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Full wave bridge rectifier:

This post provides an information about full wave bridge rectifier. We will also see its working principle and advantages and disadvantages.

Full wave bridge rectifier circuit diagram

In full wave Bridge rectifier a transformer and four diodes are used. During the positive half cycle of secondary voltage, the diodes D2 and D4 are forward-biased, but diodes D1 and D3 do no conduct. The current is through D2, R, D4 and secondary winding.

During the negative half cycle, the diodes D1 and D3 are forward-biased, but diodes D2 and D4 do not conduct. The current is through D1, secondary winding, D3 and R.

The load current is in the same direction in both half-cycles. Therefore a unidirectional (d.c.) voltage is obtained across load resistor.

Average voltage = Vdc = 0.636 Vp = 2Vp/p

Where Vp = peak value of secondary voltage.

Since each diode conducts for only half cycle, the current rating (Io) of the diodes must be at least – half of the dc load current. i.e. 0.5 Idc.

Each diode must withstand a peak inverse voltage equal to the peak secondary voltage. PIV = Vp.

Therefore the PIV rating of the diodes must be greater than Vp. As the output is a full-wave signal, the output frequency is double the input frequency.

The maximum efficiency of bridge rectifier is 81.2%

Advantages of full wave bridge rectifier:

• Centre tap on the secondary of the transformer is not necessary.
• Small transformer can be used.
• For a bridge rectifier circuit PIV per diode is one-half of the value for each diode in a full-wave rectifier.

Disadvantages of bridge wave rectifier:

• In this type two extra diodes are used.
• The voltage regulation is poor.

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