Short Answer
Controlled rectifiers are circuits that use power electronic devices like SCR to control the output DC voltage by adjusting the firing angle. Uncontrolled rectifiers use simple diodes and cannot control the output voltage.
Controlled rectifiers are used where variable DC output is required, such as motor speed control. Uncontrolled rectifiers are used where fixed DC output is sufficient, such as simple power supplies and battery chargers.
Detailed Explanation:
Controlled Rectifiers
Controlled rectifiers are power electronic circuits in which the output DC voltage can be controlled. They use controlled devices such as Silicon Controlled Rectifiers (SCR), thyristors, or other gated devices. The main feature of controlled rectifiers is that the output voltage can be varied by controlling the triggering or firing angle of the device.
In a controlled rectifier, the switching of the device is not automatic. Instead, a gate signal is applied at a specific time in the AC cycle to control when the device turns ON. By changing the firing angle, we can control the amount of power delivered to the load.
Controlled rectifiers are widely used in applications where variable DC output is required. They provide flexibility and better control over power systems.
Uncontrolled Rectifiers
Uncontrolled rectifiers are circuits that use simple diodes to convert AC into DC. In these rectifiers, the output voltage cannot be controlled. The diode automatically conducts during forward bias and blocks current during reverse bias.
Since diodes do not require any external control signal, the output depends only on the input AC supply. This makes uncontrolled rectifiers simple and cost-effective.
However, the output of uncontrolled rectifiers is fixed and cannot be adjusted. They are suitable for applications where constant DC voltage is sufficient.
Working Difference
In controlled rectifiers, devices like SCR are used. These devices require a gate signal to turn ON, and the timing of this signal controls the output voltage. By delaying the firing angle, the average output DC voltage can be reduced.
In uncontrolled rectifiers, diodes conduct automatically whenever they are forward biased. There is no control over conduction angle, so the output is fixed and depends only on the AC input.
Thus, controlled rectifiers provide adjustable output, while uncontrolled rectifiers provide fixed output.
Construction Difference
Controlled rectifiers are more complex because they require gate control circuits, triggering circuits, and sometimes feedback systems.
Uncontrolled rectifiers are simple and consist only of diodes arranged in half-wave or full-wave configurations. They do not need any additional control circuitry.
Efficiency and Performance
Controlled rectifiers offer better control but may have higher switching losses due to controlled switching. They are more flexible and suitable for industrial applications.
Uncontrolled rectifiers are more efficient in simple applications because they have fewer components and lower complexity. However, they lack control over output voltage.
Applications
Controlled rectifiers are used in DC motor speed control, industrial drives, battery charging systems with variable output, and HVDC systems.
Uncontrolled rectifiers are used in simple power supplies, adapters, battery chargers with fixed output, and small electronic devices.
Advantages and Limitations
Controlled rectifiers provide adjustable output and better control but require complex circuits and gate control systems.
Uncontrolled rectifiers are simple, reliable, and cost-effective but cannot control output voltage.
Conclusion
The main difference between controlled and uncontrolled rectifiers is that controlled rectifiers allow adjustment of output DC voltage using devices like SCR, while uncontrolled rectifiers use diodes and provide fixed output. Controlled rectifiers are used for flexible power control, while uncontrolled rectifiers are used for simple and low-cost applications.