Column: Optical isolation
Optically- isolated input modules for a 0-10V to 0-5V signal converter
By Professor Murat Uzam, Department of Electrical and Electronics Engineering, Yozgat Bozok University, Turkey
T
his month we will cover optically-isolated analogue input modules for a 0-10V to 0-5V signal converter, with
DC input voltages varying between 0V and 24V, powered by different power supplies: +6.26V, +12V and isolated +12V for module 1, and +12V and isolated +12V for module 2.
0-10V to 0-5V signal converter – module 1 Figure 1 shows the 0-10V to 0-5V signal converter to be used with the ADC input of a 5V microcontroller; Figure 2 shows its connections. The circuit is based on the Positive Unipolar Photovoltaic Isolation Amplifier 1 (PUPIA1 – as explained in a previous column), with an HCNR201 high- linearity analogue optocoupler for photovoltaic isolation. The circuit’s output, to the right of HCNR201, is isolated from VIN
= 0-10V, located to
the left. VIN
can be subjected to electric surge
or electrostatic discharge on its external terminal connections, so to protect
14 May 2022
www.electronicsworld.co.uk
the circuit there’s a transient voltage suppressor (TVS). Diode D1 protects the circuit from accidental reverse polarity at VIN
, and a ferrite bead
connected 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 operational amplifier; 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. The LM358P-1A op-amp, with +6.26V supply voltage and connected as a buffer amplifier (a voltage follower), acts as a voltage limiter, providing high input impedance. The 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, C3, LM358P-2A;
2. HCNR201 high-linearity analogue optocoupler;
3. Output section: P1, R5, C4, LM358P-3A. In this design, the circuit’s input
is powered by +6.26V and +12V. Likewise, the circuit’s output is powered by +12V that’s isolated from the input. PUPIA1’s output is connected to the non-inverting input terminal of buffer amplifier LM358P- 3B. Thus, the output voltage VOUT
is
obtained from LM358P-3B’s output. We assume that the input voltage
range VIN ≤ 5.00V, VOUT
is 0-24V. When 0.00V ≤ VIN = VIN
. When 5.01V ≤ VIN ≤ 24V, VOUT will
be a value from 5.01V to 5.07V, due to the characteristics of LM358P-1A. See the relationship between VOUT
and VIN
in Figure 3. For input voltages up to 24V 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
to
+5.00V, and by adjusting P1 bring VOUT to +5.00V.
Optically-isolated analogue input module 2 Figures 5, 6 and 7 show the optically- isolated analogue input module 2 with its connections to the ADC port of the 5V microcontroller. As before, this circuit is also PUPIA1 based, with a high-linearity analogue optocoupler (HCNR201) providing photovoltaic isolation. Here, R6, D4 (10V zener diode) and
C5 provide a 10.00V reference voltage from a +12V power supply. This 10.00V is then divided by resistors R7 and R8 to obtain +6.26V, which can source current to 20mA. Analogue input signal VIN
can be subjected to electric surge or
electrostatic discharge on its external terminal connections. Resistors R1 and R2 divide the input signal VIN
into a
range of 0-5V. The output of the LM358P-1A
connects to the input of the PUPIA1, which limits its input voltage. In this design, PUPIA1 consists of the following: 1. Input section: R3, R4, C3, LM358P-2A;
2. HCNR201 high-linearity analogue optocoupler;
3. Output section: P1, R5, C4, LM358P-3A. The input and output are powered by
isolated +12V power supplies. PUPIA1’s output is connected to the
non-inverting input terminal of buffer amplifier LM358P-3B, which supplies VOUT
.
range VIN ≤ VIN
10.02 V ≤ VIN
It is assumed that the input voltage = 0.00V to 24V. When 0.00V /2. When will be a
≤ 10.00V, VOUT = VIN ≤ 24V, VOUT
value from 5.01V to 5.07V, due to LM358P-1A. See the relationship between VIN
VOUT and in Figure 3, and some example
voltages in Table 1. For proper operation ensure that R1 = R2 and R8/(R7+R8) = 62.62 %. Set VIN
to +10.00V and then by adjusting P1 ensure VOUT = +5.00V.
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