# Help with wiring diagram for voltage follower please

Discussion in 'The Projects Forum' started by lostintranslation, Nov 11, 2013.

1. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
In need of help please to construct a voltage follower into a circuit to compensate for a LED display with less than 10Mohm impedance, to compensate for a slow voltage bleed of capacitors, and I am having a hard time figuring out the wiring of this. The easiest solution would be find an LED volt meter with those specs, so far no joy on that avenue. I was told that it needed a unity gain op amp voltage follower before the meter. Input bias current ranges from picoamps to femtoamps, depending on what part is used (input impedance from about 250 meg ohms to the gig ohms). This is the part I found after asking the supplier based on the specs given, now I am just trying to figure out how to implement and wire it into the circuit. Since I am a rookie, and don't really have a grasp of the technical side of this yet, I was hoping somebody might be able to please help me with the wiring diagram. As always thank you in advance.

2. ### ronv AAC Fanatic!

Nov 12, 2008
3,401
1,459
I may have lost something in the translation, but I think what you are looking for is a way to measure the capacitor voltage with the meter shown without discharging the capacitors much? If that's it the circuit on the left will work with Vin from the cap and Vout to the meter.

3. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
yep, that's it, want to measure the voltage with the LED. I can't find a volt LED with impedance of greater than 10Mohm. I have the LED to the cap pos wire now, but it is bleeding the cap, about .x (tenth a volt) a second, which is slight. the voltage bleed from the caps is very slight, but constant, about a tenth of a volt per second or so. I am looking for a way to not bleep the caps, so i was told what I posted above, trying to figure out how to wire in the opamp so that the caps don't bleed out at all?

Last edited: Nov 11, 2013
4. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
how do i tell which side of the OPAMP pictured is the "in", and which side is the "out"?

5. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
does this look right? thanks in advance

6. ### ronv AAC Fanatic!

Nov 12, 2008
3,401
1,459
Yep, but you need power to the op amp.

7. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
i think this might be it? dunno, i'm not sure

8. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
ok, i think this powers it?

9. ### ronv AAC Fanatic!

Nov 12, 2008
3,401
1,459
I was afraid that was how you were going to power it. That will still discharge the caps as the op amp will now draw it's power from them. What voltage charges the caps to start with?

10. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
3 in parallel now, rated 50v 1000uf each. the idea being I added a DPDT switch to allow switching from 1000 to 3000uf. the LED is on it's own power supply, so would it be better to run the power for the AMP off of the 9 volt powering the led?

11. ### lostintranslation Thread Starter Member

Nov 6, 2013
37
0
how bout this one, the 9v powers it up on it's own circuit? better?

Last edited: Nov 11, 2013

Nov 6, 2013
37
0