Current Calibrator

159

01:54:47

Switchabble (10/50/100 uA) Current Calibrator with LED-indicator for good current.
2.5 V precission reference (LM385).
LM358 OpAmp.
9V supply.

published 11 years ago

hurz

11 years ago

You are using an effect which is extremly little. the voltage at negative feedback of 2. opamp is in theorie exactly 2.5V but in real it is not. (actually in theorie i also not) This is because any opamp has a free running gain or open loop gain. In EC its per default 100kV/V. Like a propotional regulator there will be always a little delta between plus and minus input. And this little difference are you using to drive the LED? Change the open loop gain which has anyway in reality not a fixed value. It probably vary over temperatur and is different from chip to chip. Keep it up.

HamSjelw

11 years ago

Thank you for your feed-back hurz. Always welcome, and I will keep your points in mind :-) The idea of the LED-amp is as an indicator of the (calibration) current being good (i.e. near to the preset value). Preset current is characterized by the reference-voltage (2.5 volt) being stable over the preset-reseistors. The LED-reference is kept slightly below (at 2.49 or so). When ever the voltage-drop over the preset-resistors dips slightly below 2.5 V, the LED should turn off, indicating 'bad' current. I am building the circuit now. I will drop a note about real-world functionality.

HamSjelw

11 years ago

I have now built the circuit, and it works precisely as designed and intended :-) Calibration was a bit hard, since I dont have any instruments with suficient precision in the micro-amp range. I used a 22K 0.1% resistor, and a good 5-digit voltmeter for calibration.

GasBot

11 years ago

Great to hear it worked as designed, that must have been a fantastic moment.

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