Machine Design

Bridge Circuits

In control applications, operational power supplies are alternatives to other sources, ranging from simple SCR bridges used in electric drills to elaborate servoamps. The benefits of operational power supplies in these situations are high precision and moderate cost.

The easiest way to understand how operational power supplies work is through a comparison with conventional linear supplies. Linear supplies generally include a reference voltage, a feedback arrangement to monitor output voltage or current, a high-gain amplifier, and a series-pass element that regulates the output. The devices regulate output voltage by comparing it to a fixed reference, then changing the output based on load current.

Operational supplies, on the other hand, can be considered high-gain amplifiers. Each contains a bridge that controls a conventional operational amplifier. The amplifier is driven by a sensor whose voltage output is compared to a reference. The resulting difference unbalances the bridge and sets amplifier output.

The voltage that the sensor generates can be thought of as feedback. Thus, operational supplies use feedback to vary or modulate their voltage output. This property allows the supplies to generate signals approximating those of a servocontroller.

The typical device serving as a reference in these supplies is a Zener diode. The substitution of an external source or other transducer for the internal reference allows the supply to change its output in response to external conditions. These conditions may be a change in speed, temperature, velocity, or other similar physical entities.

Because a sensor provides the feedback that sets the supply output, the sensor voltage may be said to program the device. The programming voltage can be produced in several ways. The simplest method is to put a potentiometer and battery, or other voltage source, across the feedback terminals. Or, a resistor can be placed across the terminals, and a separate source then provides a control current.

In most modern operational supplies, a shunt-regulated Zener diode produces a stable reference voltage across a precision series resistor to produce a 1-mA (1,000Ω/V) or 10-mA (100Ω /V) feedback current. The feedback resistor takes the form of a variable control mounted on the supply front panel. This control varies power-supply output voltage.

A remote feedback resistor or transducer can also vary the output. This type of remote control can provide motor-speed regulation. Here, a tachometer or Hall-effect device generates a dc feedback signal proportional to motor speed. In operation, the tachometer output generally connects in series with an opposing dc supply. The difference between the two sources controls the main power supply driving the motor. A change in motor speed generates a tachometer voltage that makes the supply compensate for the error.

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