Maker Pro
Maker Pro

OP AMP Wiring help please

lostintranslation

Nov 6, 2013
26
Joined
Nov 6, 2013
Messages
26
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.



weldernewOPAMP8DIM.jpg
 

lostintranslation

Nov 6, 2013
26
Joined
Nov 6, 2013
Messages
26
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

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
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

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Joined
May 8, 2012
Messages
4,960
<<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.
 
Last edited:

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
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

Nov 6, 2013
26
Joined
Nov 6, 2013
Messages
26
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
 
Last edited:

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
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

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Joined
May 8, 2012
Messages
4,960
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
 
Top