Design of a Bridge Rectifier

By Frank Guthrie
Tim Edwards: opencircuitdesign.com

Bridge rectifiers change AC voltage into DC voltage. They are used in a wide variety of electronics and most often found in AC powered devices, modern home telephones, simple plug-in power supplies and even in high horsepower motor controllers. Many modern devices have benefited from these solid-state silicon semiconductor devices, as they have replaced the failure-prone metal oxide rectifiers used many years ago.

Function

Bridge rectifiers can be either in a 'half wave' or a 'full wave' configuration. Electrolytic filter capacitors are commonly added to smooth the rectification pulsations. The device can also be used for assuring that DC (battery) power happens in a fixed way even if the battery is connected improperly. In modern telephones, they assure proper connectivity and electrical isolation to the incoming phone lines. Even if the phone wires are miswired, the phone will still work properly.

Types

All common bridge rectifiers incorporate devices called 'diodes' and they are available for single and three-phase AC circuits. A diode is a device that allows electrical current to flow only in one direction. A single phase device will have four diodes and a three-phase device will have six diodes. They come in various configurations to allow for the wide range of voltage and current demands found in today's AC-powered devices. Individual diodes can also be used in place of the single package types. However, special versions, known as SCR Bridges, are available to allow for power control, such as for industrial variable speed motors; 'SCR' means Silicon Controlled Rectifier. These devices will not conduct current until a control signal is present on the SCR 'gate' terminal. The support circuitry is much more complex than with the simpler devices.

Considerations

It is often important to know device specifications in order to properly select an appropriate bridge rectifier. Many criteria can be involved, such as its breakdown voltage, forward current rating, transient current rating, rated temperature range, case size, mounting requirements and many other factors. These details can be obtained from the manufacturer or supplier.

Hobbyists

For hobbyist use, which often involves lower AC voltages and current requirements, a smaller device with a 400 volt and 1 Amp rating would usually suffice. They are readily available and of relatively low cost, please refer to the References section, below. However, if it will be used in a 'war robot', then a much more robust device is required and proper cooling can become critical as currents, especially if a motor is stalled, can be very high.

Cautions

Not so obvious is choosing the proper wire size for the bridge rectifier. It is important to use the 'peak current' or 'stall current', not the 'average current' of the electronic load (like a motor); otherwise the circuit wiring might burn out if there is a failure. A properly rated safety fuse is recommended and it should be placed in one of the AC input lead(s), not on one of the DC output leads, because if the Bridge Rectifier itself fails, then the fuse will protect the circuit.

About the Author

Frank Guthrie is a technical writer living near Chicago. He has 45 years of experience writing consumer and industrial product manuals and creating engineering level documentation. His knowledge of electronic circuitry provides uncommon insights.