Introduction: (written 9-4-06)
Designed to monitor current consumption of various circuit board trays on an atmospheric sounding-rocket payload for experimental study.
Support Module Current Sensors
Devin R Ott
Design - completed in October 2004
The supply currents of each device will be measured by the NT-5 magneto-resistive current sensors from F.W. Bell technologies. These current-to-voltage converters have a nominal current rating of ±5 amps, and a corresponding nominal output of ±2.5 volts. In other words, the currents sensors output: VM=1/2IIN
Now that we have a voltage (VM) proportional to the supply current, we can employ an op-amp to fit our desired voltage range into the 0 to 5 volt window required for the PCM. For example, a 0-100mA measurement range would create a 0-50mV output range, requiring an amplifier gain of 100V/V before going to the PCM.
The circuit below was designed to represent a current range (0 to IMAX) as a DC voltage range (0 to 5 volts) depending upon the value of the feedback resistor R2.
To measure (0 to IMAX) set: R2 =10,000÷IMAX
To calculate IIN from the final output voltage (VOUT): IIN = (VOUT÷5)×IMAX
As you may have noticed, the measured currents of both + and – supplies will be opposing the Positive Current Direction specified on the NT-5 sensor. In other words, the sensor will be measuring negative current, and thus outputting a negative voltage. This was done to allow the amp stage to be an inverting follower, the most stable type of op-amp configuration.
Power Supply Bypassing:
The most crucial stabilizing components are the by-pass capacitors in parallel with the supply voltages. When placed near the op-amp, the capacitance provides extra filtering on the power-lines and decouples noisy AC signals that can cause the amplifier circuits to oscillate.
A high value capacitor (1 µF to 10 µF) is used for low frequency bypassing and is typically an aluminum or tantalum electrolytic. Electrolytic capacitors become ineffective as frequency increases, so a more stable capacitor (mylar, ceramic or mica) is used to decouple the high-end noise.
These high-end bypass caps typically range from 0.01 µF to 0.1 µF, and are most commonly ceramic due to their high availability and low cost.
IMPORTANT: the by-pass capacitors should be mounted as close to the op-amp as possible.
(no farther than 6 inches).
IIN measured supply current ( A )
IMAX the maximum desired measuring current ( A )
VM output voltage from NT-5 current sensor ( V )
VMAX the sensor’s output voltage (VM) when measuring IMAX ( V )
AV the op-amp’s voltage gain ( V/V )
VOUT final output voltage (0 to 5 volts DC) from op-amp ( V )
VM = 1/2IIN
AV = R2/R1 AV = VOUT÷VM
The required op-amp gain: AV = 5÷VMAX AV = 10÷IMAX
AV = R2/R1 = 10÷IMAX
Letting R1=1 kΩ: R2/1000 = 10÷IMAX R2 = 10,000÷IMAX
The measured current: IIN = (VOUT÷5) × IMAX
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© 2006, Devin R. Ott