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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am trying to completely isolate the capacitor tank voltage from my circuit. Whether or not this is total necessary I am not sure.
The two chips being used with regard to this question are the LM6172 high speed opamp
and the FOD3180 optocoupler
I see the optocoupler uses an LED to drive the signal on the other side. I have my capacitor voltage, after being clamped, going into the op amp. One input goes to V+; the other goes to V- with the ground floating. Now I would think this would result in getting an output that could drive the LED of the optocoupler. The optocoupler LED input would be connected to the opamp output and the floating ground. I would then have the optocoupler output completely separate from the input.
This did not work. It seems the output of the opamp went between -15v and 0. I solved the problem by connecting the floating ground to the same ground of the optocoupler. You can see the point of question where I made the circle.
So, how do I completely isolate the capacitor voltage signal from my circuit? The floating ground did not work out too well.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Isolated means not common to either the input or output of the isolator. I see common ground and common +15V on both the input and output side of the optoisolator on your schematic.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
With any electrical circuit, you must have a continuous path for current.
The point about an optoisolator is that you have two indenpendant circuits.
Although a circuit will typically have a ground, it doesn't need one. It *is* convenient to label one node as 0v, and it is even more convenient if that node is the one that's bonded to all of the exposed metal.
Now when you use an opto in a circuit, you have two grounds. OMG, do you connect them, if so where, or leave them isolated, if so how?
The ground symbol you have circled should be isolated from the ground symbol on the ouput side of the opto. It is the opto that is doing the isolating, not the opamp. The opamp, and clamp diodes, and the capacitor sense, and everything else on the opto input side, should use the same ground.
If you feel a need to connect the two ground symbols together than a) do you really need an opto, b) do you understand whether your grounds should be isolated or not?
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Thanks for the replies, although they didn't quite get to what I was looking for...
None the less, I solved the problem by using two separate step-down transformers which gave me two 15v sources. I used one -15v-0-15v to power the opamp, using the ground as the return for the optocoupler. I used the second 15v supply to power the optocoupler and circuitry. It worked fine.
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Issues with Optocoupler
