Home    Industrologic, Inc. (636) 723-4000    Products Page


Using a 0-5 Volt DC Analog Input to Read Other Signal Levels


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.


Reading Strain Gauges

Strain gauges produce such a low voltge signal that they normally require an amplifier circuit. Industrologic offers signal conditioning amplifiers designed specifically for strain gauges.



Reading 4-20 ma Signals

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.

One source of precision resistors is Huntington Electric, Inc. at http://www.heiresistors.com. Part number ALSR-1-250-1% is a 1 watt 250 ohm 1% silicon coated wire wound resistor.

The following diagram show how to connect a 4 to 20 ma output device and a resistor to a 0 to 5 volt input.

4-20ma to 0-5vdc circuit


Variations in the value of the resistor will cause variations in the voltage drop, so the exact resistance of a particular resistor as well as its change in resistance with temperature should be considered. A precision resistor with stable temperature characteristics is desirable.

To obtain the accuracy desired, tests can be made to compare the current loop device's output with the actual voltage read by the analog input. The resistance value can then be adjusted for greater accuracy, or the voltage read by the analog input can be adjusted mathematically. To obtain the repeatability desired, measurements should always be taken at the same temperature as the tests were performed.

Note:
Since 249 ohm 1% resistors are more common than 250 ohm high precision resistors, it is likely that you will want to use this value instead. Using this value has the added advantage of having less chance of the voltage at the analog input be over 5 volts, which would be over the 5 volt upper limit of the input, and be "off scale".

If the resistance of the 249 ohm resistor is at the lower end of the tolerance range (246.51 ohms), 20ma will produce 4.9302 volts at the analog input, which is around 1.41% low.

If the resistance of the 249 ohm resistor is at the higher end of the tolerance range (251.49 ohms), 20ma will produce 5.0298 volts at the analog input, which is around 0.59% high.

Another way to obtain the required 250 ohms (or at least closer to 250 ohms) in a precision resistance is to use two 499 or 500 ohm precision resistors in parallel.



Reading Higher DC Voltages

If a device produces a voltage that varies over a range greater than 0 to 5 volts, a voltage divider can be used to reduce the voltage to within the range of 0 to 5 volts. Any variation in the output device's voltage will create a proportional variation in the adjusted voltage at the output of the divider circuit.

The formula for a voltage divider is:

Vout = Vin * (R2 / (R1 + R2) )

R2 is typically 1K to 10K ohms, and R1 is selected by using the voltage divider formula.

For example to reduce a 0 to 15 volt signal to 0 to 5 volts the formula would be:

5 = 15 * (10K / (20K + 10K) )       (Use 20K for R1 and 10K for R2)

The following diagram shows how to connect the resistors to form a voltage divider between the output device and the analog input.

voltage divider circuit


This method of reducing an output device's voltage is practical only within a certain range of voltages. For voltages over 50 volts safety may become an issue.

If the correct resistance values are used, the voltage divider circuit can at the same time be electrically scaling the voltage so that it does not need to be scaled mathematically.

Variations in the value of the resistors will cause variations in the voltage division, so the exact resistances of the resistors as well as their change in resistance with temperature should be considered. Precision resistors with stable temperature characteristics are desirable.

To obtain the accuracy desired, tests can be made to compare the input voltage with the actual voltage read by the analog input. The resistance values can then be adjusted for greater accuracy, or the voltage read by the analog input can be adjusted mathematically. To obtain the repeatability desired, measurements should always be taken at the same temperature as the tests were performed.

Another way accuracy can be improved is to use a potentiometer instead of or in addition to the two fixed resistors. This will allow the precise analog input voltage to be set when a known voltage is applied to the voltage divider input.



Reading Switch and Relay Contact Closures

Normally a switch or other contact closure would be read by a digital or logic level input, but if a spare analog input is all that is available, it can be used to read a contact closure by making the contact closure part of a simple voltage divider cicuit.

A "pullup" resistor can be connected from +5 to the input to make the input read a positive voltage. (The exact voltage read will depend on the analog input's circuit resistance characteristics.) The switch can then be connected from the input to ground to cause the voltage read to be very close to zero when the contact is closed.

The following diagram shows how to connect a resistor and a switch or other contact closure so that it can be read by an analog input.

switch to analog input circuit





www.industrologic.com
log file