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Old 20th December 2015, 01:21 PM   #1
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Default current source

Hi

I can see that it's an OPAMP classic current source but I can't figure out what is zener D3 for.
Picture doesn't show but it's a classic triode preamp B+ regulator.
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Old 20th December 2015, 03:00 PM   #2
Mooly is offline Mooly  United Kingdom
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Looks like some kind of clamp to me, perhaps to prevent latchup under certain conditions and/or simply to protect the opamp inputs being pulled above or below the opamp supply voltage.
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Old 20th December 2015, 03:22 PM   #3
Ketje is offline Ketje  Belgium
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D3 is for protection of the U1 inputs. Par ex. C4 is discharged a start-up, pulling the inputs to ground, can destroy U1.
Mona
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Old 20th December 2015, 07:40 PM   #4
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you are both right, thank you.
You put me on the right path. So, I found some explanations about this opamp's behaviour. But the protection recommended is by schottky diodes.
So, why a zener is used here ?
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Old 20th December 2015, 07:52 PM   #5
Mooly is offline Mooly  United Kingdom
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A zener allows the voltage across the diode to reach the value of the zener. Using a standard type diode would clamp the voltage across it at a permanent 0.6volts and not allow any voltage swing. The zener allow swing up to the zener voltage.
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Old 20th December 2015, 09:57 PM   #6
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I get it, it make sense.
I'm a little bit less ignorant now.
Thank's again.
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