__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: V_{M}=^{1}/_{2}I_{IN}

Now that we have a voltage (V_{M}) 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 100^{V}/_{V} before going to the PCM.

The circuit below was designed to represent
a current range (0 to I_{MAX}) as a DC voltage range (0 to 5 volts)
depending upon the value of the feedback resistor R2.

To measure (0 to I_{MAX}) set:
**R2 =10,000÷I _{MAX}**

To calculate I_{IN} from the final
output voltage (V_{OUT}): **I _{IN
}= (V_{OUT}÷5)×I_{MAX}**

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). **

** **

__Calculations__:

I_{IN} measured supply
current ( A )

I_{MAX} the maximum desired measuring
current ( A )

V_{M} output voltage from
NT-5 current sensor ( V )

V_{MAX} the sensor’s output voltage (V_{M})
when measuring I_{MAX }( V )

A_{V} the op-amp’s
voltage gain ( ^{V}/_{V })

V_{OUT} final output voltage (0 to 5
volts DC) from op-amp ( V )

V_{M }= ^{1}/_{2}I_{IN}

A_{V }= ^{R2}/_{R1}
A_{V }= V_{OUT}**÷**V_{M}_{
}

The required op-amp
gain: A_{V }= 5**÷**V_{MAX} A_{V
}= 10**÷**I_{MAX}

A_{V }= ^{R2}/_{R1 }= 10**÷**I_{MAX}

Letting R1=1 kΩ: ^{R2}/_{1000
}= 10**÷**I_{MAX}
R2 = 10,000÷I_{MAX}

The measured current: I_{IN }= (V_{OUT}÷5)
× I_{MAX}

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© 2006,
Devin R. Ott