Column: Optical isolation


Optically- isolated input modules for a 0-5V or 4-20mA to 0-5V signal converter


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


I


n this month’s column we will cover two optically-isolated analogue input modules for a 0-5V or 4-20mA to 0-5V signal converter.


Module 1 (Figure 1) handles DC input voltages from 0V to +12V, or DC input currents from 0mA to 48mA, and requires DC power supplies of +6.26V, +12V and isolated +12V. Module 2 handles the same input voltages and currents but requires DC power supplies of +12V and isolated +12V. Figure 2 shows Module 1’s


connections to an analogue input of a 5V microcontroller. Tis circuit contains the Positive Unipolar Photovoltaic Isolation Amplifier 1 (PUPIA1 – explained previously), with HCNR201 high-linearity analogue optocoupler for photovoltaic isolation. Te circuit’s output, to the right of


the HCNR201, is isolated from VIN , to


the leſt. Here we assume that when the voltage mode is selected, VIN


In voltage mode (S1 open), when 0.00V ≤ VIN


5.01V ≤ VIN


≤ 5.00V, VOUT ≤ 12V, VOUT


= VIN is between


0 and 12V, and when the current mode is selected, IIN


is between 0 and 48mA. . When will be a value


from 5.01V to 5.07V, due to the electrical characteristics of the LM358P-1A. In current mode (S1 closed), when 0mA ≤ IIN


≤ 20mA, VOUT When 20mA < IIN = (IIN . 250)V. ≤ 48mA, VOUT vs VIN and IIN will be a


value from 5.01V to 5.07V, again because of the LM358P-1A. Te curves VOUT


are shown


in Figure 3; it can be seen that for input voltages to 12V and input currents to 48mA the circuit suffers no damage. VIN


and IIN can be subjected to electric


surge or electrostatic discharge on the circuit’s external terminal connections, but the TVS (transient voltage suppressor) provides highly-effective protection. D1 protects the circuit from accidental reverse polarity of VIN


or IIN . Jumper S1 (shown here as a switch)


switches between current and voltage, with R1 = 250Ω used to adjust the levels. When S1 is open, the input can be 0-5V; when closed, the input can be 0-20mA. A ferrite bead in series with the input path


adds isolation and decoupling from high- frequency transient noises, whereas external Schottky diodes protect the op-amp. Even with internal ESD protection diodes, it’s recommended to use external diodes since they lower noise and offset errors in the circuit. Dual series Schottky barrier diodes D2


and D3 divert overcurrents to ground or the power supply. With its +6.26V power, the LM358P-1A op-amp acts as a voltage limiter, provides a high input impedance and is connected as a buffer amplifier (voltage follower). Its output is connected to the PUPIA1’s input, limiting the input voltage. Here, PUPIA1 consists of:


1. Input section: R2, R3, C3, LM358P-2A. 2. HCNR201 high-linearity analogue optocoupler.


3. Output section: P1, R4, C4, LM358P-3A. In this design, the circuit’s input section is powered by +6.26V and +12V; likewise, the output part is powered by another +12V power supply, isolated from the input. PUPIA1’s output is connected to the non-inverting input of the buffer amplifier LM358P-3B. Output voltage VOUT


is thus Figure 1: Module 1 14 September 2022 www.electronicsworld.co.uk


obtained from the output of LM358P-3B. Table 1 shows some example input and output voltages and currents for Modules 1


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