Column: Optical isolation


Two optically- isolated input modules for a -5V to +5V 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 -5V to +5V to 0-5V signal converters, with DC input


voltages varying between −12V and +12V, powered by different power supplies: +5.00V, +6.26V, −12V, +12V, isolated +12V for module 1 and −12V, +12V and isolated +12V for module 2.


Module 1 Figure 1 shows the −5V - +5V to 0-5V signal converter for use with the ADC input of a 5V microcontroller; the module’s connections are shown in Figure 2. Te circuit is based on the Positive Unipolar Photovoltaic Isolation Amplifier 1 (PUPIA1 – as explained in a previous month), with an HCNR201 high-linearity analogue optocoupler for photovoltaic isolation. Te circuit’s output, i.e., to the right of HCNR201, is isolated from VIN


,


to the leſt. Tis design is used to level-shiſt the


bipolar −5V - +5V analogue voltage signal to provide a unipolar 0-5V input signal. When −5.00V ≤ VIN


≤ +5.00V, this circuit


acts as a non-inverting summing amplifier with the transfer function:


12 June 2022 www.electronicsworld.co.uk


PUPIA1 is connected to the non-inverting input terminal of buffer amplifier LM358P- 3B; the output voltage VOUT


is obtained Since R1 = R2,


from its output. We assume that the input voltage range


VIN Te design adds VIN


a gain of 1. VIN


and +5.00V with can be subjected to electric surge


or electrostatic discharge on the external terminal connections, but adding a transient voltage suppressor (TVS) gives it good protection. Diode D1 is used to protect the circuit from accidental reverse polarity of VIN


, and a ferrite


bead in series with the input path adds isolation and decoupling from high- frequency transient noises. Resistors R1 and R2 divide VIN


into voltages from


0-5V. External Schottky diodes protect the op-amp; even when internal ESD protection diodes are provided, using external diodes lowers noise and offsets errors. Dual series Schottky barrier diodes D2 and D3 divert any overcurrent to the power supply or ground. Te operational amplifier LM358P-1A,


with bipolar supply voltages, provides a high input impedance and is connected as a buffer amplifier (voltage follower). +5V and R1, VIN


and R2, and LM358P-


1B, with bipolar supply voltages, act as a non-inverting summing amplifier. Diodes D3 and D4 ensure that when −12.00V ≤ VIN


< −5.00V, VOUT


< −5.00V, VOUT VIN


+5.01V ≤ VIN


is −12V to +12V. When −12.00V ≤ VIN = 0.00V. When −5.00V ≤


≤ +5.00V, VOUT = (VIfrN ≤ +12V, VOUT


+5V) / 2. When will be equal


to a value from 5.01V to 5.07V, due to the op-amp’s characteristics. See the relationship between VOUT


and VIN in Figure 3. For input voltages to 12V there’s no


damage to the circuit, which outputs voltages from 5.01V to 5.07V; see Table 1. To calibrate the setup, set VIN and by adjusting P1 bring VOUT


Module 2 Figures 5, 6 and 7 show the optically- isolated analogue input module 2 with its connections to the ADC port of a 5V microcontroller. As before, the circuit is also PUPIA1 based, with a high-linearity analogue optocoupler (HCNR201) providing photovoltaic isolation. Tis design is used to level-shiſt the


bipolar −5V - +5V analogue voltage signal to provide a unipolar input of 0-5V. When −5.00V ≤ VIN


≤ +5.00V, this circuit acts as a


non-inverting summing amplifier with the transfer function:


= 0.00V.


Te operational amplifier LM358P-2A, with a +6.26V supply voltage, acts as a voltage limiter and is connected as a buffer amplifier (voltage follower). Te amplifier’s output is connected


to the input of PUPIA1, with its input limited by LM358P-1A. In this design, PUPIA1 consists of the following: 1. Input section: R3, R4, C4, LM358P-3A. 2. HCNR201 high-linearity analogue optocoupler.


3. Output section: P1, R5, C5, LM358P- 4A. In this design the circuit’s input is


powered by +5.00V, +6.26V, −12V and −12V power supplies. Likewise, its output is powered by +12V that’s isolated from the input. Te output of the


Since R1 = R2,


to +5.00V, to +5.00V.


VIN


and + 5.00V are added with a gain of 1. R6, D5 (10V zener diode) and C6


provide a 10V reference voltage from a +12V power supply. Tis 10V is then divided by resistors R7 and R8 to obtain +5.00V, which can source current to 20mA. Likewise, the 10V reference voltage


is also divided by R9 and R10 to obtain +6.26V. Next, the +6.26V reference voltage is connected to the non-inverting input of buffer amplifier LM358P- 3B, whose output is fixed as a +6.26V reference voltage capable of sourcing to 20mA.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48