OMC-042 / 043 / 044 Data loggers
Users
’ manual
page 75
Potmeter input (OMC-043/044 only)
The OMC-043 is equipped with a potentiometer input. This inputs acts as a voltage input, but it uses
passive sensors only. It is designed especially for potentiometers, like the ones in winddirection meters
and angle meters. This input is connected to the internal ADC directely and is translated into a 0 % -
100% value. The user has to connect the potentiometer between Vref (Terminal X1.8) and ground.
Because of the fact of the potentiometer is connected between Vref an ground, the ADC value will
always be the full range (0..4095) INDEPENDENT of the value of the connected potentiometer. So, all
types and all values of potentiometers are supported.
Although all values are supported,we strongly recommend to use high values only. This is because the
lower values are draining more power from the data logger. Values between 100K and 4M7 are
recommended.
A potmeter gives a value what is translated to 0.. 100%, and can be re-scaled to engineering values by
the user. A potentiometer is a variable resistor with three terminals. (see picture below)
Figure 2: Potentiometer
The two outer terminals are connected to the fixed resistor and the middle terminal is connected to a
slider. So, when a potmeter is turned, the resistance between the middle terminal and one of the outer
terminals is changing with the angle of the potmeter. Very often potentiometers are used to regulate a
voltage from 0% to 100 % of the input voltage. In the next figure this well know circuit is drawn.
The output voltage is adjustable from 0% to 100% of Vin, in this case. The OMC-043 data logger has 3
terminals (on connector X1) for connecting a potentiometer to the logger:
X1.6 = GND
X1.7 = 0..100% Resistance
X1.8 = Resistance Reference terminal
So, the circuit above is connected as follows:
IN+ = X1.8
GND = - = X1.6
OUT = X1.7
Another application of the use of the potmeter-input is to connect a PT-1000 temperature sensor to the
potmeter-input. In the appendix is this example explained
The resistance reference terminal is derived from the internal ADC reference and has a 150 ohm
series resistor (to prevent damage in case of a wiring fault). The internal reference has a level of
3.30 Volts. When relative low impedance potmeters are used, there will be a voltage drop across
the internal series
resistor, and the max ADC value won’t be reached. Another reason to use only high
impedance potmeters. Of course, when a sensor has got a low impedance value, you can use it, but you
have to compensate for the voltage-drop.