[SOLVED] Can the current frim the output of a single supply op amp flow in both directions?

[JohnJohn20]

Just venturing into the world of op amps.

I would like to use a non inverting unity gain single supply op amp circuit (voltage follower??) to charge a capacitor (so I can sample an input voltage to compare it with another sample taken at a later time). Op amp I have now are TLC27M2.

1. The capacitor will charge up to Vin. Then, if Vin drops, will the capacitor be able to discharge through the op amp?

2. Can I control when the capacitor is charging from Vin by simply turning the op amp +ve supply off via a PIC? That is, what governs how long it takes for an op amp to "turn on" and stabilize?

3. If the op amp is turned off (i.e. has no +ve supply), will the capacitor be able to discharge through the op amp output?

Thanks.

 

[crutschow]

What an op amp output does when the power is removed is undefined. If you don't want the capacitor to discharge, then you can place a diode in series with the op amp output to make a peak detector circuit. The diode is placed inside the feedback loop so its forward drop does not affect the final voltage.

You can add a CMOS switch, such as the CD4066, at the op amp input to control when you want the circuit to detect the peak value. You can also add a CMOS switch to discharge the capacitor before the next sample is made. Both can be controlled by the PIC.

How fast do you need to acquire the peak sample?

 

[JohnJohn20]

Hi crutschow. Sample rate of 10Hz would probably be OK but 100 Hz would certainly be more than enough.

I think I could use the PIC to discharge the capacitor before the charge cycle, but still need something to switch the buffer output to the capacitor.

If the Vout of the buffer is less than the Vcapacitor, will the capacitor be able to discharge through the op amp?

 

[Venkadesh_M]

That is depend on internal circuit of your opamp basically it wont through the OPAMP but through the other circuit elements like feedback resistor load and so on....

If you want it to discharge means put a parallel resistor.....

But in a live circuit The opamp will allow the reverse current as the same as the positive current (It just want the resultant Voltage in the O/p pin).....

 

[crutschow]

...............................
I think I could use the PIC to discharge the capacitor before the charge cycle, but still need something to switch the buffer output to the capacitor.

If the Vout of the buffer is less than the Vcapacitor, will the capacitor be able to discharge through the op amp?

I don't understand. :???: Do you want the capacitor to discharge through the op amp? If so what is the purpose of the capacitor?

Certainly, without the diode in the circuit, the op amp will charge and discharge the capacitor in response to the Vout of the buffer.

 

[JohnJohn20]

Sorry. I was thinking that if I charge the capacitor through the op amp, process the voltage level, and then if the next sample was lower voltage, would the capacitor need to be discharged (through the PIC say) before charging to the second sample level or would it simply discharge through the op amp to the output level of the op amp buffer.

Looks like the CMOS switch is the way. Thanks.