ANALOG RESISTANCE MEASUREMENT (OHMS)
APPLICATIONS > RESISTANCE MEASUREMENT >
Resistance Measurement with the i100
The i100xx box provides 8 differential analog input voltage channels (14bit) and 8 voltage output channels (i.e. for excitation) that can be used for resistance measurement with the help of two user-supplied external shunt resistors. For i100 analog input specifications, click here.
Resistance Measurement: Voltage Divider Circuit
Resistance measurement using a voltage divider involves connecting a resistor of unknown value in series with an external user supplied shunt resistor of known value, applying a voltage across the divider circuit, and measuring the voltage across Runknown, as illustrated below. The voltage across Runknown is measured between a pair of instruNet Vin+ and Vin- input terminals while the excitation voltage is supplied by the instruNet Vout terminal. instruNet then calculates the value of Runknown using the following equation, and returns a value in "ohms" units:
Runknown (Ohms) = Rshunt * (Vin+ - Vin-) / ((Vin+ - Vin-) - Vout)
Fig 3.4 - Resistance Measurement - Voltage Divider Method
To do a Resistance measurement using a Voltage Divider, you must:
1. Set the Sensor field in the Hardware settings area to Resistance.
2. Set the Wiring field in the Hardware settings area to Voltage Divider .
3. Set the Rshunt field in the Constants settings area to the value of your external user supplied Rshunt resistor, in ohms units.
,
,
4. Set the Vout field in the Constants settings area to specify the excitation voltage that is to be applied to the divider. In high current cases (e.g. >2mA), it is often helpful to alternate the polarity of the excitation voltages to evenly burden the +/-12V supplies.
5. Set the Voltage Range field in the Hardware settings area to something similar to Vout * (Runknown_Maximum / (Runknown_Maximum + Rshunt)).
6. Wire your voltage source per figure 3.4, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
Resistance Measurement: Bridge Circuit
Resistance measurement using a bridge circuit involves connecting a resistor of unknown value as one leg of a full-bridge circuit, applying a voltage across the bridge, and measuring the voltage across the two intermediate nodes. The intermediate node voltage is measured between a pair of instruNet Vin+ and Vin- input terminals while the bridge excitation voltage is supplied by either the instruNet Vout terminal or an external voltage source. In figure 3.5, Runknown is a resistor whose value is being measured and Ro is a similar valued resistor of known value4. This technique is only accurate if Runknown stays in the range of Ro, +/- 50%. If you need to measure a resistance with more range, please use the Resistance Measurement using a Voltage Divider, described earlier. instruNet calculates the value of Runknown using the following equation, and returns a value in "ohms" units:
Runknown (ohms) = Ro * (Vout - 2.0 * (Vin+ - Vin-)) / (Vout + 2.0*(Vin+ - Vin-))
Fig 3.5 - Resistance Measurement - Bridge Circuit
To do a Resistance measurement using a Bridge circuit you must:
1. Set the Sensor field in the Hardware settings area to Resistance.
2. Set the Wiring field in the Hardware settings area to Bridge .
3. Set the Ro field in the Constants settings area to the value of one Ro bridge completion resistor, in ohms units.
,
,
4. Set the Vout field in the Constants settings area to specify an excitation voltage to be applied to the bridge. If you are applying an external excitation voltage, enter -Ro value in the Ro edit field (e.g. -100 instead of 100ohms) to tell the software that the excitation is external, and then enter the external excitation voltage in the Vout. In high current cases (e.g. >2mA), it is often helpful to alternate the polarity of the excitation voltages to evenly burden the +/-12V supplies.
5. Wire your voltage source per figure 3.5, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
,
|
