Electronics > Beginners
Overly sensitive circuit question
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Chris Wilson:

--- Quote from: T3sl4co1l on May 17, 2018, 10:18:24 pm ---Reduce the 1M to, say, 100k, or even 10k.  This reduces the error amp gain, so that it only crosses the threshold for larger phase errors.

Tim

--- End quote ---

Hi Tim, thanks for helping. At the risk of me sounding completely stupid, are you saying replace the 1 meg pot with a 100k or 10k pot? I don't see why this is any different to reducing the resistance of the existing 1 megohm pot by simple adjustment, as it's got one side connected to the slider... Sorry, but I need to also understand what I am doing in order to learn from this and I am unclear as to whether you mean adjust or change the pot. Thanks.
T3sl4co1l:
Yes, adjust it down.

Alternately, increase the 1k.

You might not have much resolution, say in the first 10% of range, in which case changing the range (either way) might help.  It's not a precision control, as far as I can see.

Tim
Chris Wilson:

--- Quote from: T3sl4co1l on May 18, 2018, 06:35:15 pm ---Yes, adjust it down.

Alternately, increase the 1k.

You might not have much resolution, say in the first 10% of range, in which case changing the range (either way) might help.  It's not a precision control, as far as I can see.

Tim

--- End quote ---

That worked fine, thanks for the explanations, but I have spotted an annoying possible issue. I have fixed up an old, but originally quite expensive current and voltage source and reader. It's a Haven Minical MKII and it gives a positive and negative source with excellent accuracy. My motor drive circuit has a red and green LED to show power is being sent in one direction or the other to the variometer drive motor.  What I am seeing is at  -5.25mV the green LED will light. In theory at +5.25mV the red LED should light, but it does not, it only needs +1.06mV to light. I will check what sort of mV levels come from the bridge in a bit, I can't recall the levels, but where would I look for the inaccuracy, and where do I probe with a DVM , ground wise, to see the first opamp is unbalanced (I can't see how it cannot be, but hey, what do I know!). I am guessing any imbalance is in the circuit around the final two op amps? Diodes or 4.7k resistors? Thanks again :)
T3sl4co1l:
What was the opamp?  RTFDS. ;)

Tim
Chris Wilson:

--- Quote from: T3sl4co1l on May 19, 2018, 02:50:14 am ---What was the opamp?  RTFDS. ;)

Tim

--- End quote ---

Opamp is a TL084  I have added pin numbers to the sketch and the link to the datasheet is here

http://www.ti.com/lit/ds/symlink/tl082.pdf


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