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Old 22nd April 2004, 10:13 AM   #21
moamps is offline moamps  Croatia
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Dual LED offset indicator

+ offset red
- offset green
zero orange
set window with R9

Regards
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Old 22nd April 2004, 12:09 PM   #22
boholm is offline boholm  Denmark
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I would prefer the LED not to light at all, when there is no DC. That's more logical to me. Agree or disagree?
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Old 22nd April 2004, 12:15 PM   #23
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If you need an indicator to tell you that you have offset because you cannot hear it...
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Old 22nd April 2004, 12:35 PM   #24
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Quote:
Originally posted by moamps
Dual LED offset indicator

+ offset red
- offset green
zero orange
set window with R9

Regards

That's the idea, but I think in your circuit the window is always +/- .6V, independent of the pot. That's a pretty wide window for offset. I think that if you want to make the window variable (although I see no need for it), you need two pots, each across the diode and feed the fractional diode voltage to the comparator inputs.

You may also want to add a small positive feedback across the comparator to introduce some hysteresis, to avoid oscillations around the switching point.

You may also want to put R7, R9 in series with the comparator outputs to limit the LED current.

Lastly, place diodes across the LEDs in opposite polarity to avoid destroying them with too much reverse voltage.

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Old 22nd April 2004, 12:51 PM   #25
Stocker is offline Stocker  United States
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Quote:
Originally posted by boholm
I would prefer the LED not to light at all, when there is no DC. That's more logical to me. Agree or disagree?
Agree

Quote:
Originally posted by Circlotron
If you need an indicator to tell you that you have offset because you cannot hear it...
haha... somebody found out the secret!
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Old 22nd April 2004, 01:35 PM   #26
moamps is offline moamps  Croatia
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Hi Jan,
thanks for yours correction.

Quote:
Originally posted by janneman
That's the idea, but I think in your circuit the window is always +/- .6V, independent of the pot.
Drawing error. Corrected.

Quote:
....you need two pots, each across the diode and feed the fractional diode voltage to the comparator inputs.
Two pots aren't needed if window is symetrical.

Quote:
You may also want to add a small positive feedback across the comparator to introduce some hysteresis, to avoid oscillations around the switching point.[/B]
OK, but not so important.
Quote:
You may also want to put R7, R9 in series with the comparator outputs to limit the LED current.
Led current is defined only with R7 and R9. In corrected schematic with R7+R10 and R9+R11.
Quote:
Lastly, place diodes across the LEDs in opposite polarity to avoid destroying them with too much reverse voltage.
Done.
Regards
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Old 22nd April 2004, 01:49 PM   #27
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Looks like a winner. Clever, that R9 stuff. There may be some assymmetry if the ref diodes are not matched, but that's acceptable I would think.

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Old 22nd April 2004, 01:52 PM   #28
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I'm sorry, but this won't work (I might be wrong).

1 LP-filter, preferable up to 3th or even 4th order, cut frequency 0.01-1 Hz. Those filters most also have low offset voltages. This is dependent how low DC voltage you want to detect, how advanced the filter should be.

2 Fullwave rectifier or two halfway rectifiers, one for positive and one for negative signals

3 Comparator with HYSTERESES!

4 Indicators

Filtering out a couple of millivolts from several volts or tenths of volts is a tricky business. Don't forget that DC is low frequency AC!
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Old 22nd April 2004, 02:17 PM   #29
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Per-anders,

You may remember that this comparator stuff is preceded by an integrator like the servo loop normally used to null the offset. Please look up the previous posts.

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Old 22nd April 2004, 02:23 PM   #30
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Some amps may also DC offset slightly when producing a larger AC signal.
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