Column: Optical isolation


Optically- isolated 0-5V analogue


input modules


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


I


n this month’s column we will focus on two optically-isolated 0-5V analogue input modules, both with inputs of +12V DC. Figure 1 shows module 1 for use with the ADC input of a 5V


microcontroller, connected as shown in Figure 2. Tis circuit is of Positive Unipolar Photovoltaic Isolation Amplifier 1 (PUPIA1)


design, used with a high-linearity analogue optocoupler (HCNR201) for photovoltaic isolation. Te circuit’s transmitting part (to the right of HCNR201) is isolated from the 0-5V analogue voltage input signal VIN


leſt of HCNR201). If VIN


Te LM358P-1A’s output is connected to (to the is subjected to electric surge or


electrostatic discharge on its external terminal connections, it will be protected by the TVS (transient voltage suppressor), also shown in the circuit. Diode D1 is used to protect the circuit from accidental reverse polarity on VIN


A ferrite bead is connected in series


with the input path to add isolation and decoupling from high-frequency transient noises. Te operational amplifier, LM358P-1A, is protected by external Schottky diodes. Even if internal ESD protection diodes are used, applying external diodes is a wise policy since they lower noise and offset errors. Dual series Schottky barrier diodes


D2 and D3 divert any overcurrent to the power supply or ground. Te op-amp, with a +6.26V supply voltage, acts as a voltage limiter; it is connected as a buffer amplifier (voltage follower) and provides high input impedance.


the input of the PUPIA1, which limits its voltage. Here, the PUPIA1 consists of the following components: 1. Te receiving part of the circuit: R1, R2, C3, LM358P-2A.


2. HCNR201 high-linearity analogue optocoupler.


3. Te transmitting part of the circuit: P1, R3, C4, LM358P-3A. In the design, the signal-receiving part


.


can be powered by +6.26V or +12V, and the transmitting part by +12V, isolated from the receiving part. PUPIA1’s output is connected to the non-inverting input terminal of the buffer amplifier LM358P- 3B, which supplies VOUT We assume VIN


. ≤ VIN ≤ 5.00V, VOUT When 5.01V ≤ VIN


= 0-12V. When 0.00V = VIN


≤ 12V, VOUT


; see Figure 3. will be


a value from 5.01V to 5.07V, because of LM358P-1A. It can be seen that input voltages up to 12V do not cause any circuit damage, and output voltages are from 5.01V to 5.07V. For the circuit, VIN


adjusted by P1 so that VOUT


was set at +5.00V, = +5.00V.


Module 2 Module 2 (Figure 5, with its MCU connections shown in Figure 6 and prototype board in Figure 7) is also a PUPIA1 design, in a similar set up to module 1, except, here, R4, D4 (10V Zener diode) and C5 provide a 10.00V reference voltage from a +12V power supply. Tis 10.00V reference voltage is then divided by resistors R5 and R6 to obtain a +6.26V voltage, connected to the non-inverting input of the buffer amplifier LM358P-2B with output fixed at +6.26V to source current to 20mA. LM358P-1A, with its +6.26V supply voltage, acts as a voltage limiter, providing high input impedance. In this design, the receiving and


transmitting parts of the circuit are powered by separate +12V, with VIN


0.00V to 12V. When 0.00V ≤ VIN VOUT


= VIN . When 5.01V ≤ VIN = ≤ 5.00V, ≤ 12V, VOUT


will be a value between 5.01V and 5.07V, due to LM358P-1A. Module 2’s setup is similar to that of


module 1. For proper operation make sure that R6/(R5+R6) = 62.62%.


Figure 1: Optically-isolated 0-5V analogue input module 1, for use with the ADC input of a 5V microcontroller Continues on next page > www.electronicsworld.co.uk March 2022 11


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