- 1). Draw the schematic for your op amp regulator such that it includes the cathode of a zener connected to the positive input of the op amp and the anode of the zener is connected to ground. Include a transistor whose base is connected to the output of the op amp, whose emitter is connected to one end of resistor (RZener) and whose emitter is connected to one end of another resistor (RLoopFeedback2).
Design the schematic such that the other end of resistor RZener is connected to the positive input of the op amp and the other end of resistor RLoopFeeback2 is connected to the negative input of the op amp. Also include another resistor (RLoopFeedback1) that is connected between the negative input of the op amp and ground. - 2). Set the value that the op amp voltage regulator will regulate as required for your application. For this example an output voltage of 5 volts is selected. Select a zener diode voltage that is 50 percent of the value of the desired output voltage (2.5 volts for this example) Divide the output voltage by the zener diode voltage and then subtract 1 from this value to obtain the ratio of RFeedback1 to RFeedback2 required to produce 5-volt output — in this case, since 5/2.5 is 2 and 2 -1 is 1, the required feedback resistor ratio is 1.
- 3). Choose the resistor values such that the value of RFeedback2 and RFeedback1 is the same since their ratio is required to be 1.
- 4). Subtract the zener voltage from the voltage that is applied to the input of the voltage regulator — at the collector of the transistor — and divide this result by the nominal current required for the zener diode to obtain the value of RZener.
- 5). Enter the schematic of your design into a circuit simulator program such as SPICE and simulate the design for different types of op amps, transistors, zeners and resistor load levels. Optimize the component values based on the electrical specifications required for your voltage regulator.
- 6). Use precision values for the feedback resistors if you want to set the output regulation value precisely Use feedback resistor values of at least 1,000 ohms to keep circuit current levels low.
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