Application Bulletin «Input Overload Protection For The RCV420 4-20 mA Current-Loop Receiver» (Texas Instruments) - 2

SELECTED OPERATING POINTS
The problem with using a series resistor for input overload
FOR RCV420 AND COMPARATORS
protection is the added voltage drop in the input current loop. A 1125Ω protection resistor in series with the 75Ω internal
RCV420 INPUT RCV420 OUTPUT COMPARATOR INPUT (1) (mA) (V) (V)
current sense resistor would result in a 24V drop at 20mA full 0 –1.250 +0.017 scale input. In most applications the additional 24V burden 2 –0.625 +0.572 can not be tolerated. 4 0.000 1.269 20 5.000 5.563 Another input protection scheme which can be used when 25 6.563 6.950 only a small series voltage drop can be tolerated is to use a 36 10.000 10.000 0.032A fast-acting fuse (such as Littlefuse 217000 series, NOTE: (1) From the 10kΩ, 1.27kΩ summing network. type F) in series with the current loop. This fuse adds negligible voltage drop to the current loop, and blows in less than 0.1s with an overload of 128mA or more. The problem The underrange comparator threshold is set at 0.57V by the with a fuse is that it must be replaced when it blows, and the 10kΩ, 604Ω resistor divider connected to the 10.0V refer- cost of maintenance can be very high. ence. The comparator output goes high when the input to the current loop receiver goes below 2mA. The underrange The active protection scheme shown in this application output is TTL compatible. overcomes the disadvantages of resistor and fuse protection. It uses a solid-state relay for current loop interruption. After The overrange comparator threshold is set at 6.95V by the an interrupt time delay designed to provide a safe 1% 10kΩ, 22.9kΩ resistor divider connected to the 10.0V refer- maximum overload duty-cycle, the circuit resets automati- ence. The output of the overrange comparator goes low when cally. The LH1191 solid-state relay used has a maximum on the input to the current loop receiver exceeds 25mA. resistance of 33Ω which adds less than 0.1V of burden to the With a 36mA current loop input, the overrange comparator current loop at 20mA full scale input. Low receiver burden input is 10V. For a 36mA overload set-point, the overrange allows longer field wiring (with higher resistance) for remote resistor divider can be eliminated by connecting the overload sensors, and extra compliance for “intrinsically safe” barriers comparator input directly to the 10.0V reference output of or other series connected receivers. the RCV420. The solid-state relay is ideal for the resetable protection task. The overrange comparator is connected so its output will go It is inexpensive as compared to a mechanical relay (less than low on overrange to trigger the ensuing active overload $1.00), and because it is solid-state, it will not “wear out” if protection circuitry. If the overload protection circuitry is not cycled continuously. The LH1191 is a single-pole, normally used, the inputs to the comparator can be reversed so its open switch rated for 280V, 100mA outputs. Relay overload output will go high on overload. currents are clamped to 210mA by current limiting circuitry The current sense resistors within the RCV420 are rated for internal to the relay. An extended clamp condition, which overloads of up to 40mA continuous and for momentary increases relay temperature, results in a reduction of the overloads of up to 0.25A with 0.1s maximum duration and internal current limit to preserve the relay’s integrity. 1% maximum duty cycle. Overloads within these limits will Protection circuitry interrupt timing is provided by a 555 not damage or degrade the rated performance of the receiver. timer connected as a monostable multivibrator (one shot). If the 75Ω sense resistor were shorted to 48V (a common Under normal conditions, the output of the one shot is low current loop supply voltage), the overload current would be holding the solid-state relay on by forcing approximately 0.64A producing an I2 • R power dissipation in the resistor of 8mA through the LED as set by the series-connected 470Ω 30.7W. With a package thermal resistance (θ ) of 70°C/W, JA resistor. When more than 25mA flows into the current loop this would result in a theoretical junction temperature rise in receiver, the overload comparator triggers the one shot. With excess of 2000°C. The input resistor would fuse, destroying the values shown, the one shot output goes high, turning off the device, before the package temperature actually reached the LED and the solid-state relay for about one second. At the 2000°C. For input short circuits up to 3V, the current is end of one second the circuit automatically resets turning the limited to a safe 40mA by the 75Ω sense resistor within the relay back on. If an overload still exists, the cycle repeats. RCV420. To prevent possible damage, external means are The overload current applied to the RCV420 is limited to required to protect the RCV420 sense resistors from input about 210mA by the current limit of the solid-state relay and short circuits to potentials greater than 3V. the 75Ω sense resistor internal to the RCV420. The one The least expensive input short circuit protection is a resistor second cycle time of the overload circuitry assures safe 1% connected in series with the current loop at the receiver input. maximum overload current duty-cycle. The input to the Select the resistor so that the input current under short-circuit circuit can be shorted to a 50V power supply continuously conditions will be limited to 40mA by the series combination without damaging or degrading the accuracy of the RCV420. of the protection resistor and the 75Ω current sense resistor A short life-test was performed on a prototype circuit. After internal to the RCV420. Short circuit protection to 48V 168 hours with the receiver input shorted to 50V, there was requires a 1125Ω resistor. no detectable change in receiver accuracy within the 0.005% resolution of the test system. R = (V /0.040) – 75 (Ω) PROTECTION S 2