Measuring Voltages Higher Than Vref - A Practical Example - Arcom RC-210 Operation And Programming Manual

Measuring Voltages Higher Than Vref - A Practical Example

The voltage applied to an A/D input cannot exceed Vref, which is 5.00 volts by default.
The first step is to decide what the range of actual voltages will be measured:
Suppose the Low end of expected actual voltage 10 volts, and the High end of the expected actual voltage is 18 volts
Since we realize that applying a voltage to the RC-210 higher than the programmed reference voltage will "pin" the meter face, we need
to "scale" the actual voltage. This can be accomplished using either a pair of resistors as a voltage divider, or a variable resistor to get
just a portion of the 18 volts as the maximum value to connect to the ADC input. Using a 5-K pot would be a good choice.
What you want is to have the High end of voltage range (18 volts) to be less than Vref. If Vref is 5.00 volts, you might choose to set the
resistor divider (or variable resistor) to produce 4.5 volts as the "sensor" voltage when the actual voltage is 18 volts.
If we look at the math, we see that 4.5 volts is 18 volts divided by 4 so the "sensor" needs to provide ¼ of the actual voltage. If the
power supply is currently providing 13.8 volts, divide this by 4 to get 3.45 volts. If you are using a variable resistor (as shown in the
Hardware Reference Manual), hookup your "sensor" to the battery, and adjust the pot to produce 3.45 volts on the wiper. This "sensor"
will work fine with the ADC input. If the battery voltage ever gets to 18 volts, the wiper of the pot will see 4.5 volts, below the Vref of 5.0
volts, which is what is desired.
We then need to program the meter face used. For this example, we will use Meter 1 and assume the Voltage Reference Value has
been left at the factory default of 5.00 volts:
*2064 1# 1# 250# 1000# 450# 1800#
This programs Meter 1 to use the voltmeter meter face and tells the controller that when the sensor is 2.5 volts, this means the actual
voltage is 10 volts, and when the sensor (ADC input) is 4.5 volts, the actual voltage is 18 volts. Note that each of the calibration points
is the actual measured value multiplied by 100 then rounded to the nearest whole value. This is done because there is no "decimal"
point on the DTMF pad, so the controller assumes that the "real" values are whatever you put in, divided by 100.
Using the above example, you should be able to accommodate just about any external sensor.
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