OP AMP Wiring help please

[lostintranslation]

I am running an LED that reads capacitor voltage in a circuit. The LED bleeds the cap voltage slowly, but I was trying to get the bleed out, and was told that a LMC662 OP AMP would stop the bleed. I am having a hard time figuring the wiring out, I am better at visual diagrams than interpreting circuits. I had it hooked up as indicated in this wiring diagram, and it only read 9.3v, which I believe is the battery reading that powers the LED and the OP AMP both. Anybody see my mistake in wiring diagram? Please and thank you, any help would be appreciated.

 

[CDRIVE]

I am running an LED that reads capacitor voltage in a circuit.

I must say that your handle is apropos. I really don't understand what you're trying to do.

Chris

 

[lostintranslation]

lol. right? trying to increase the impedance of a volt meter that is bleeding down a capacitor that it is reading. i don't think that is going to happen, I think that the specs on the op amp i have won't allow for the voltage i need to read. pretty sure my only option is to get a digital volt meter with higher impedance. thought I could do it with a voltage follower, but after today, pretty sure it aint going to work out. thanks for chuckle tho, it is appreciated. yes, i am, lost in translation...lol

 

[duke37]

A voltage follower should work to reduce the discharge rate of the capacitor.

The very nice picture you show is incomplete, you need to show all connections

 

[CDRIVE]

<<pretty sure my only option is to get a digital volt meter with higher impedance. thought I could do it with a voltage follower,>>>

Ah, I get it now. You have a moving coil (DeArsonaval) meter movement. Yes, a Voltage Follower in conjunction with a meter multiplier resistor should work.

Chris

EDIT: Actually, your schematic indicates a LED display, not a meter movement. A voltage follower would still work though.

 

[Harald Kapp]

I think AQPAmp A is wired as a voltage follower. There seems to be a wire from the output to the "-" input, although the drawing is rather unconventional with the labeling interupting the line.

However, the inputs of second OpAmp should be tied to a fixed potential to avoid floating inputs. I suggest you connect the second OpAmp as avoltage follower, too, and connect the "+" input to 1/2Vcc (battery pos.) via a simple resistor divider (e.g. 1*1kOhm).

Note that the OpAmp will not stop the capacitor from loosing charge during the measurement. Only it will take more time due to the high impedance of the amplifier.

 

[lostintranslation]

somebody on another board/forum told me that the voltage follower would not work with LMC662 OP AMP I am using, the maximum permitted input voltage is 2 volts below its supply voltage, and the maximum output voltage is slightly less than the power supply voltage. The OP AMp as wired stops the voltage at 9.2 volts as max charge on the Caps, it should be higher, but due to the limits of the spec on the op amp, that is the highest it will go. trying for it to read up to 35 v

 

[Harald Kapp]

Put a resistive divider before the OpAmp, e.g. 1Meg/9Meg divides by 10, then use the voltage follower to buffer the voltage at the divider's tap. Multiply the reading of your instrument by 10 to get the capacitor's voltage.

 

[CDRIVE]

Put a resistive divider before the OpAmp, e.g. 1Meg/9Meg divides by 10, then use the voltage follower to buffer the voltage at the divider's tap. Multiply the reading of your instrument by 10 to get the capacitor's voltage.

Ah, further proof that there's no need to accumulate feedback from multiple forums. There's more than enough expertise right here on EP!

As a side note and because I'm still in love with the old girl, I'd like to mention that to my knowledge the old HP410-B VTVM was the only VTVM produced that provided a whopping 100Meg input impedance.

Chris