# The Linear Optocoupler Hcnr201 Isolation Circuit

Working Principle

Composition of the output portion of the isolation circuit of The hcnr201 the LED, PD1 and op amp A1 composed of an input portion of the isolation circuit, PD2, and op-amp A2. And isolation circuit input voltages vin, the output voltage VOUT, ??the LED on the current for the IF current generated in the diode pd1 IPD1 current generated in the diode pd2 ipd2

Isolation circuit pd1 negative feedback, When vin input voltage, the output of the op amp a1 led on the current if flowing through, and the change is reflected in the current if input voltage, and drive led light into electrical signalsoptical signal. The light LED issued pd1 detected and generates photocurrent IPD1. Meanwhile, the input voltage Vin will also generate a current flow through R1. Assumed that a1 is an ideal op-amp, no current flows into A1 input side, the flow of the R1 current will flow through pd1 to ground, therefore, IPD1 = vin/r1. The note, ipd1 only depends on the value of the input voltage vin and r1, led light output characteristics. Because of the LED light emitted simultaneously irradiated on the two photosensitive diodes, and pd1 and pd2 identical the ideal case ipd2 should equal ipd1. Define a coefficient k, there ipd1 = kipd2 k approximately 1 ± 5% (determined, when the chips finished). Op amp a2 and resistance the r2 to ipd2 into the output voltage vout vout = ipd2r2, the combination of the above three equations to obtain the relationship between the output voltage and the input voltage: vout / vin = kr2/r1, output voltage vout stability and linear, its gain can be realized by adjusting the value of r2, and r1, usually take the same values ??of r1 and r2.

The isolation circuit resistance r1-limiting role. r3 for control led luminous intensity, which play a role in the control of channel gain. Capacitors c1, c2 is the feedback capacitance, and for improving the stability of the circuit. The role of the operational amplifier (a1) is the voltage signal into a current signal, the operational amplifier a2 optocoupler output current signal into a voltage signal, and to enhance the load drive capability.

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