Column: Optical isolation


Three positive unipolar photovoltaic isolation amplifiers for isolating 0-5V signals


By Professor Murat Uzam, Department of Electrical and Electronics Engineering, Yozgat Bozok University, Turkey


I


n this part of the series on optically isolating analogue input/output modules we will focus on three positive unipolar photovoltaic isolation amplifier (PUPIA) designs, one of which as a core isolation


circuit. In the first two PUPIA designs (PUPIA1


and 2), a 0-5V analogue input signal will be received and then transmitted on to a digital system, first inputting it into the ADC of a 5V microcontroller. Te transmitting part is thus isolated from the 0-5V input signal. In the third PUPIA design, we’ll assume a 0-5V analogue voltage output signal is to be produced by a digital system – say, the DAC of a 5V microcontroller – which requires the receiving part of the circuit to be isolated from the transmitting part. For all three designs, I used the HCNR201 high-linearity analogue optocoupler to establish photovoltaic isolation. PUPIA1 and 2 are specifically developed


to be part of an optically-isolated analogue input module, whereas PUPIA3 that of an optically-isolated analogue output module.


PUPIA setups In Figure 1, the PUPIA1 curcuit has its input and output sections powered


by isolated +12V power supplies. Tere are two separate LM358P operational amplifiers and an HCNR201 high-linearity optocoupler. Te input voltage range VIN When 0.00V ≤ VIN output voltage VOUT VIN


is 0.00-12V.


≤ VTH = VIN


≤ 12V, VOUT = VTH


, the isolated . When VTH


, where VTH < is the


threshold voltage, whose value depends on the particular HCNR201 used. Experiments with 21 different HCNR201 optocouplers in PUPIA1 show that 5.22V ≤ VTH


≤ 7.32V; VOUT vs VIN is shown in


Figure 2. A PUPIA2 design is shown in Figure


3. Here the input part of the circuit is powered by a +24V power supply, whereas the output by a separate +12V power supply. As before, the design comprises two separate LM358P op-amps and an HCNR201 optocoupler. In this design, we assume the input


voltage to be 0.00-24V. When 0.00V ≤ VIN ≤ VTH VIN


. When VTH


, the isolated output voltage VOUT < VIN


≤ 24V, VOUT 8.25V; the VOUT and VIN relationship is


shown in Figure 4. Te PUPIA3 design is shown in Figure 5. Here, the circuit’s input and output


= VTH ≤


= .


Experimenting with 21 different HCNR201 optocouplers shows that 5.91V ≤ VTH


Figure 1: PUPIA1, designed by using two separate LM358P operational amplifiers and an HCNR201 high-linearity analogue optocoupler


16 February 2022 www.electronicsworld.co.uk


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