Column: Optical isolation
Optically-isolated analogue output modules for a 0-5V to –10V - +10V signal converter
By Professor Murat Uzam, Department of Electrical and Electronics Engineering, Yozgat Bozok University, Turkey
T
his month we will discuss two more optically- isolated output modules, this time for a 0-5V to −10V - +10V signal converter. Modules 1 and 2 provide voltages from −10V to +10V, requiring four DC power supplies (+5.00V, −12V, +12V and isolated +12V) for Module 1, and three DC
power supplies (−12V, +12V, and isolated +12V) for Module 2.
Module 1 Figures 1 and 2 show the 0-5V to −10V - +10V signal converter that connects to the DAC output of a 5V microcontroller. The circuit contains the Positive Unipolar Photovoltaic Isolation Amplifier 3 (PUPIA3 – explained before), with an HCNR201 high-linearity analogue optocoupler for photovoltaic isolation. The circuit’s input, to the left of HCNR201, is isolated from its output, at the right.
After adjusting the value of P2, we obtain R4 + P2 = R5, and after adjusting the value of P3, we obtain R6 + P3 = R7:
Becuase of its limited current drive capability, the buffer
amplifier LM358P-1A (a voltage follower) is used on the DAC output. LM358P-1A’s output is connected to PUPIA3’s input. Here, PUPIA3 consists of:
1. Input: R1, R2, LM358P-1B; 2. HCNR201; 3. Output: P1, R3, C3, LM358P-2A. Provided the input voltage of PUPIA3 is limited to between 0.00V and 5.00V, its output will also be between 0.00V and 5.00V. PUPIA3’s output is connected to the non-inverting input of LM358P-3A. Jumper S1 (shown here as a switch for clarity) is used to select between 0-5V to 0-10V signal converter mode (S1 open), or 0-5V to −10V - +10V signal converter mode, when S1 is closed. Here, S1 is closed, a setting used to level shift the unipolar 0-5V input signal to a bipolar −10V to +10V output. When 0.00V ≤ VIN
≤ 5.00V, the two operational amplifiers LM358P-
3A and LM358P-3B, with bipolar supply voltages, provide this transfer function:
Dual series Schottky barrier diodes D1 and D2 divert any overcurrent coming from VOUT
to the positive or negative power
supply rail. A ferrite bead is connected in series with the output path to add isolation and decoupling from high-frequency
Figure 1: Module 1 circuit diagram
14 February 2023
www.electronicsworld.co.uk
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