Motive

Measuring a peak current, the peak current detector output was range over. Enoki would like to lower gain to measure larger current. Input impedance was 0.1Ω.

Summary

Simply adding a shunt resistor to feedback resistor of OP amplifier, Gain was lowered.

Method

Feedback resistor of the peak current detector was 100 kΩ, adding shunt resistor 68 kΩ as shown in Photo1.

Measurement devices
Input current: 1A ammeter of self made loader indicator and DMM 10A range

Photo1 Shunt resistor

OP amplifier output Vout : DSO CH1 [500mV/div, 10ms/div]

Detector power supply voltage: CH2 [1V/div]

Current source: DM-310MV through 2W 3Ω resistor (0.7A max)

Photo2 shows the method.

Photo2 Gain measurement

When MV-310MV powered off, Vout was -14mV therefore offset.

Result

Figure1 shows the result.

The formula shows linear relation Vout with input in Figure1. Ignoring the little offset,

Figure1 Detector I - V characteristics

Coefficient = 1/0.00404 = 227 [mA/Vout]

Conclusion

As the pervious coefficient was nearly 100, so one could read current direct. One has to calculate converted value this time.

Soldering the shunt resistor lead off, one can restore the original coefficient.