Column: Optical isolation


Optically-isolated analogue output module for a 0-5V to 4-20mA signal converter


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


I


n this series of articles about signal coverters, this month we will present the seventh – and last in the series – output module, this time for a 0-5V to 4-20mA converter that provides current from 4-20mA


and requires two DC power supplies: +12V and isolated +12V. Figure 1 shows the module, which


connects to the DAC output of a 5V microcontroller; see Figure 2. Tis circuit contains the Positive Unipolar Photovoltaic Isolation Amplifier 3 (PUPIA3), which we covered before, with an HCNR201 high-linearity analogue optocoupler providing photovoltaic isolation. Te circuit’s input (to the leſt of HCNR201) is isolated from its output. Due to the limited current drive capability, the buffer amplifier (a voltage follower) LM358P-1A is used on the DAC output, whose output is connected to the input of PUPIA3. Here, PUPIA3 consists of: 1. Input: R1, R2, LM358P-1B; 2. HCNR201; 3. Output: P1, R3, C3, LM358P-2A. Provided that the input voltage of the


PUPIA3 is limited by 0.00V ≤ VIN ≤ 5.00V,


its output voltage will also be limited between 0.00V and 5.00V. Te output of PUPIA3 is connected to the non-inverting input terminal of the buffer amplifier LM358P-2B.


Design based on XTR116 Tis design is based on the XTR116 from Texas Instruments, a precision current output converter that feeds 4-20mA signals over an industry-standard current loop. It provides accurate current scaling and output current limit functions. XTR116 operates over an extended


industrial temperature range (-40°C to +85°C), offers low quiescent current (200μA), 5V regulator for external circuits, VREF


for sensor excitation (4.096V), low


span error (0.05%), low nonlinearity error (0.003%) and a wide loop supply range (7.5- 36V). Applications include 2-wire, 4-20mA current loop transmitters, industrial process control, test systems compatible with Hart modems, current amplifiers, voltage-to- current amplifiers, and more. Te XTR116 is found in most PLCs at


the heart of modern automation products. It is the least expensive 4-20mA current


loop output IC and it is very accurate. Te XTR116 is a two-wire current


transmitter, where its input signal (pin 2) controls the output current. A portion of this current flows into the V+ power supply, pin 7. Te remaining current flows in the external transistor, BD139. Te XTR116 is a current-input device


with a gain of 100; i.e., IOUT input voltage at the IIN to the IRET


= 100 • IIN . Te pin is zero (referred pin). A voltage input is created


with an external input resistor (P1 + R1). Common full-scale input voltages


range from upward of 1V; full- scale inputs greater than 0.5V are recommended to minimise the effects of offset voltage and drift of the internal operational wamplifier A1. Te XTR116 is designed to use an


external transistor to avoid on-chip thermal- induced errors. Transistor BD139 conducts the majority of the full-scale output current. Power dissipation in this transistor can approach 0.8W with high loop voltage (40V) and 20mA output current. Te XTR116 provides accurate, linear


output up to 25mA. Internal circuitry limits the output current to approximately 32mA, to protect the transmitter and loop power/measurement circuitry. Te XTR116 low compliance voltage rating (7.5V) allows the use of various voltage protection methods without compromising the operating range. A diode bridge circuit, obtained by using 1N4148 diodes, allows normal operation, even when the voltage connection lines are reversed. Te bridge causes a two diode drop loss (approximately 1.4V) in loop supply voltage, which results in a compliance voltage of about 9V – suitable for most applications.


Figure 1: The output module of the optically-isolated 0-5V to 4-20mA signal converter 12 March 2023 www.electronicsworld.co.uk


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