Hello everyone,
I am trying to create a voltage source that initially increases in a ramp like manner and then stays stable.
To do this I am using an buffer opamp with an RC circuit circuit on the non-inverting input (picture attached). The idea is to feed an input voltage (Vin 3 V) into an RC circuit (R1 and C1), and then take the voltage across the capacitor and feed it into then non-inverting input - this will create a slowly rising voltage on the non-inverting input with a slope determined by the RC time constant (R1 and C1 give a tau of 4.7 s). To kick-start all of this I close switch 1 (picture).
Because I have opamp's Vout attached directly (no resistors, voltage dividers, etc) to the inverting input , I believe the voltage at the inverting input should match the voltage at the non-inverting input. I can then use this voltage to power an LED. Overall, when S1 is closed, the voltage across the LED should light up to full brightness after about 5 seconds (determined by RC,C1 time constant).
I tried this with the V+ and Vin being from separate sources and it worked well (I used a battery pack for the Vin and a power source for the V+). But, when I try having both the V+ and Vin from the same source, e.g. both from a power supply set at 3 V, the circuit does not work at all. Can anyone tell me why this is? I feel like there is something about opamps that I do not understand here (I am a beginner) - without knowing what happens in an opamp I thought thevoltage source (V+) should not affect the input voltage (Vin).
Tim
p.s. Switch 2 is just there to drain C1 when after use.