Electrical – Implementing a constant current source circuit that can be switched off by a voltage signal

current-source

How do I implement a constant current source circuit that is able to output 10mA constantly, ideally the load of the circuit would be a 100uF capacitor in series with 100 Ohms.

Is there a way to then have the current source switch off with voltage signal given to it?

Best Answer

Here is a sweet little circuit that when powered on will charge your 100uF capacitor up with a constant current of 10mA up to the voltage as set by the pot. (The pot can be set for anywhere from near 0V up to near 15V). Due to the constant current the voltage ramp will be a linear ramp.

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The U1 opamp acts as a comparator to monitor the capacitor ramp voltage and shut off the current source when capacitor voltage has reached the voltage as set by the R2 pot. The U2 opamp controls the voltage across R5 at 1V and causes Q1 to source a constant current of 10mA at its collector. The comparator opamp, U1, drives the U2 (+) input reference voltage hard against the positive supply when the output capacitor voltage has reached its set level to force off the current source. Note that this circuit works with an opamp like the LT1638 because of its rail-to-rail I/O capabilities. This particular opamp is a dual device so this circuit can built with one SO-8 package, the garden variety PNP transistor and a small handful of resistors. (Note that if the control voltage is set from some other circuit you can remove R1 and R2 (the pot) and connect the voltage directly into the (-) input of U1.

Here is a waveform plot of the circuit showing the capacitor charge up to a 5K ohm setting (~7.5V) of the pot wiper.

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