Application Note AN2301 SPLIT LOOPS WITHOUT A LOOP SPLITTER Reduce costs using a low-cost single loop isolator Users often specify dual-output 4-20mA loop splitters in their system applications. Common reasons include: The need for an output boost to drive several loads at once. The need to run independent isolated current loops to two different systems. These needs often can be met with a simple, low-cost single loop isolator. Heres how to save money! Application 1: Boosting and splitting a transmitters output. Many (not all) readouts on a 4-20mA loop use 250 ohm dropping resistors on their inputs; that is, they drop 5 volts at 20mA. Typical 4-20mA transmitters may drive 600, 800, 1000 or at most, 1200 ohms. This limits you to between two and four 250 ohm loads on a loop. Application 1 uses a JH4300C plug-in isolator to drive five loads. In the example, the source transmitter needs to drive only the local display plus the JH4300Cs input. In fact, since the JH4300C uses only a 62 ohm input resistor, you could put at least one more load in series with the display. The JH4300Cs output, which can drive 1200 ohms (24V), drives the remaining four loads. Note that this arrangement provides two isolated loops, so that the output may be grounded separately from the source transmitter if necessary. You might, for example, choose to ground the controllers input to minimize noise and common mode pickup. _ + _ + OUTPUT INPUT JH4300C ISOLATOR Application 2: Boosting and splitting a 2-wire transmitters output. Similar in principle to Application 1, this example illustrates the use of a JH5050 isolator with a 2-wire (loop-powered) transmitter. (The JH5050 is similar to the JH4300C but in a DIN-rail-mount housing.) The application includes an intrinsic safety barrier for hazardous (explosive) area protection; however, the concepts are equally valid without the barrier. Typically, the loop is driven from a 24 volt dc supply. + INPUT _ + OUTPUT _ JH5050 ISOLATOR The 2-wire transmitter drops 10 to 12 volts, the readout 5 volts, the JH4300C 1-1/4 volts and the barrier a few more. The use of a barrier forces the loop to be grounded. Again, the isolator provides a second, isolated loop capable of driving up to 24V of additional loads. When are dual-output loop splitters required? Primarily, when both loops must be isolated and the source transmitters output is not. This is most obvious in the above 2-wire application where the safety barrier forces the input loop to be grounded. If there is a need for grounding elsewhere in the same loop (for example, if one of the readout devices inputs must be grounded), then a dual-output isolator must be used. Even without a barrier, the minus 24V supply is grounded in many systems. The input transmitter itself can sometimes cause problems if its input is not isolated. The most common example is a nonisolated transmitter used with a grounded thermocouple. If the input loop is also grounded anywhere else in the system, unwanted currents will flow and the reading will be incorrect. www. .com Application 3: Isolators with built-in loop excitation. This application is conceptually identical to Application 2 but uses an isolator having a built-in 24V loop supply. Models JH4305 (plug-in) and JH5040 (DIN-rail mount) include the isolator and the 24V supply all in one instrument. The input loop current flows from the positive supply to negative through the System 1 input, barrier (optional), transmitter and the 62 ohm resistor. Like the JH4300C and JH5050, these isolators can power up to 1200 ohms of external load. HOW ISOLATORS WORK Models JH4300C and JH5050, like many isolators, convert the input to pulses which are optically coupled to the output. The output circuit converts the pulses back to DC, proportional to the input. The 4-20mA input flows through a 62 ohm resistor, which converts it to a voltage input (1/4 to 1-1/4 volts) to an integrator. The integrator works in conjunction with the DC-to-pulse conversion circuitry to produces pulses whose duty cycle (width) are proportional to the input. The pulses drive an LED opto-isolator. Output circuitry, powered from a separate transformer winding, amplifies the isolators output pulses to a precise 5-volt level and filters them to create a voltage proportional to the input. The output amplifier converts the voltage to 4-20mA. Zero and span trimpots allow precise adjustment of the output, or may be used to fine-tune overall system characteristics. ISOLATORS AND TRANSMITTERS FROM JH TECHNOLOGY PLUG-IN JH4300C 4-20mA Isolator JH4305 Isolator with Loop Excitation Supply DIN RAIL JH5050 4-20mA Isolator JH5040 Isolator with Loop Excitation Supply A full range of other input styles is available FIELD MOUNT FT5000 DC Input Isolator FT5040 Isolator with Loop Excitation Supply Sarasota, FL USA (800) 808-0300 / (941) 927-0300 / Fax (941) 925-8774 www.jhtechnology.com e-mail jhtek@jhtechnology.com