A-277 Precision Resistor, 250 Ohm

Many devices that read analog voltage inputs like data acquisition systems, data loggers, and other analog to digital (A/D) boards and units are designed to read a 0 to +5 volt DC level. However, sometimes it is necessary to read a sensor or other device with different output characteristics because that sensor is the only device available or convenient to use. The following describes how to use different types of outputs with 0 to +5 volt analog inputs.

One common standard for output devices is to produce a 4 to 20 milliampere (ma) output to represent anywhere from no signal to a full scale signal, (with 0 ma being an indication of an error condition, like a broken wire.)

Since a 250 ohm resistor will produce a 1 to 5 volt DC signal across it when 4 to 20 ma flows through it, the resulting signal can be connected directly to a 0 to 5 volt analog input. In fact, some 4 to 20 ma input devices are simply that - a 0 to 5 volt input combined with a precision value 250 ohm resistor.