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Old 1st February 2011, 09:41 AM   #31
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Quote:
Originally Posted by runebivrin View Post
Interesting with the 1/10 watt power rating. That indicates a voltage of 316kV. The plot thickens...
All resistors can have a maximum voltage rating and maximum power rating which are separate and cannotbe extrapolated!

The max power is a function of the package dissipation and could not be reached before the maximum voltage rating kicks in (arcing, breakdown etc, even for an infinite resistor.)
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Old 1st February 2011, 10:04 AM   #32
AndrewT is offline AndrewT  Scotland
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That was the problem one of our Members hit.
He used a 50V rated smd resistor in the feedback loop.
The most it would ever see was 40Vpk, but during testing he found an odd distortion. It was the resistor breaking down due to operating at near it's rated voltage.
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Old 1st February 2011, 10:10 AM   #33
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Quote:
Originally Posted by cliffforrest View Post
All resistors can have a maximum voltage rating and maximum power rating which are separate and cannotbe extrapolated!

The max power is a function of the package dissipation and could not be reached before the maximum voltage rating kicks in (arcing, breakdown etc, even for an infinite resistor.)
Well, that was kind of my point. By specifying the power requirement the OP is implying that the voltage present across the resistor will be very high. Otherwise the power rating is quite moot.

Maybe he's going to measure the voltage from a Van Der Graaf generator, who knows?
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Old 1st February 2011, 10:53 AM   #34
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Originally Posted by kavermei View Post
Wow! Do you mean high energy photons or even UV and visible light?
I don't have a conclusive answer, I was the mechanical engineer and am not very familiar with the circuit function. I did do the fabrication and testing though.

While developing the prototype, we saw the output drifting seeming at random. In the lab, under normal florescent lights, we found the op amp output changed as we moved a paper over the circuit and casting a shadow on it. Moving the paper away returned to the original value.

I am only guessing that the input bias current and/or input bias voltage were being affect by the light. Given the really high impedance of the circuit resulted in significant voltage swings (around 0.5 volts as I recall). The negative input had only the 1 gigaohm resistor and one side of the piezo sensor connected to it. So this kind of output could be caused by just a nanoamp shift on the input.

Once we realized that that the circuit needed to be shielded from light, electrically shielded and hermetically sealed, we got the performance we wanted. The electrical shield also provided excellent light shielding, so we didn't analyze the problem further.
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Old 2nd February 2011, 09:32 AM   #35
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Hi all,

& thanks for all your replies ... Surprised at the amount of thoughts and exchange the question has set about

And then I'll have to apologize to you all - I have, for various reasons, decided to let my project go so I won't be using the 1 Gohm resistor ....

And to those of you who have asked what the 1 Gohm was for, I can say that it was for a capacitor microphone design. From the + volt supply to the capsule - to allow for a very low cut-off frequency.

Again thanks for replying!

Jesper
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