Voltage Regulators:

An unregulated power supply consists of a transformer (step down), a rectifier and a filter. These power supplies are not good for some applications where constant voltage is required irrespective of external disturbances. The main disturbances are:

1. As the load current varies, the output voltage also varies because of its poor regulation.
2. The dc output voltage varies directly with ac input supply. The input voltage may vary over a wide range thus dc voltage also changes.
3. The dc output voltage varies with the temperature if semiconductor devices are used.

An electronic voltage regulator is essentially a controller used along with unregulated power supply to stabilize the output dc voltage against three major disturbances

1. Load current (I_L)
2. Supply voltage (V_i)
3. Temperature (T)

Fig. 3, shows the basic block diagram of voltage regulator. where

V_i = unregulated dc voltage.

V_o = regulated dc voltage.

Fig. 3

Since the output dc voltage V_Lo depends on the input unregulated dc voltage V_i, load current I_L and the temperature t, then the change ΔV_o in output voltage of a power supply can be expressed as follows

V_O = V_O(V_i, I_L, T)

Take partial derivative of V_O, we get,

S_V gives variation in output voltage only due to unregulated dc voltage. R_O gives the output voltage variation only due to load current. S_T gives the variation in output voltage only due to temperature.

The smaller the value of the three coefficients, the better the regulations of power supply. The input voltage variation is either due to input supply fluctuations or presence of ripples due to inadequate filtering.

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