PSN-L Email List Message

Subject: Re: Gain problem
From: ChrisAtUpw@.......
Date: Wed, 28 Feb 2007 07:06:50 EST


In a message dated 2007/02/28, jonfr@......... writes:

> My issue is that I am not sure how I can turn the gain up properly, so I
> can have my system more sensitive to earthquakes. Since I don't have
> geophones are low gains. I don't know what direction to turn this to get
> the gain up, but not to get it too high and since this turns full circle,
> it is hard for me to tell when I am at minimum and when I am at fairly
> high gain.

Hi Jon,

       The pre-set potentiometers may be 10, 15, 18 or 22 turns. If you wind 
them continually in one direction, they stiffen up when you reach the end of 
the track and any further rotation produces an audible click. This is a safety 
mechanism which prevents damage to the mechanism by 'over winding'. You can 
then wind them in the opposite direction, counting turns, until you hear another 
click - or simply ask Larry what potentiometers he uses?
       If you have a digital multimeter with a resistance range, you can 
measure / setup  the gain. After switching OFF the power, you need to locate the 
opamp connected to the gain set potentiometer and note it's -ve input, output 
and Earth pins. If you read the type number, say LF412, LM324, LF444 etc and 
type this into Google, you should be able to download a data sheet with the pin 
connections. Alternatively, go to the manufacturer's website. 
       You measure the resistance between the -ve inverting input and earth, 
say Rb Ohms. You then measure the resistance between the -ve input and the 
output, say Ra Ohms. The gain of the stage is then = 1 + Ra/Rb. You can then turn 
the set screw on the potentiometer to give the value of the resistance / gain 
that you require. Rb will have a fixed value. Ra will have minimum and 
maximum values which you can measure after winding the potentiometer to both end 
stops. Note the turn direction, clockwise / counter clockwise which increases the 
resistance.
       Look up http://jclahr.com/science/psn/cochrane/index.html and opamp 
U5b. Rb = R9 + R27 = 1 + 2.2 = 3.2 K Ohms. Ra = R26 + R7 = 1 K + (0 to 50) K 
Ohms preset. So the minimum gain = 1 + 1/3.2 = 1.31 and the max gain = 1 + 51/3.2 
= 16.9 This gives a gain range of 1 to 12.9. Sorry that I don't have Larry's 
current circuit.
       If you just want to set up equal gains on your three opamps, set the 
resistance between the output and the -ve input to be the same on all three.
       Hope that this helps?

       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/02/28, jonfr@......... writes:

My issue is that I am not sure=20= how I can turn the gain up properly, so I
can have my system more sensitive to earthquakes. Since I don't have
geophones are low gains. I don't know what direction to turn this to get
the gain up, but not to get it too high and since this turns full circle, it is hard for me to tell when I am at minimum and when I am at fairly
high gain.


Hi Jon,

       The pre-set potentiometers may be 10, 1= 5, 18 or 22 turns. If you wind them continually in one direction, they stiff= en up when you reach the end of the track and any further rotation produces=20= an audible click. This is a safety mechanism which prevents damage to the me= chanism by 'over winding'. You can then wind them in the opposite direction,= counting turns, until you hear another click - or simply ask Larry what pot= entiometers he uses?
       If you have a digital multimeter with a= resistance range, you can measure / setup  the gain. After switching O= FF the power, you need to locate the opamp connected to the gain set potenti= ometer and note it's -ve input, output and Earth pins. If you read the type=20= number, say LF412, LM324, LF444 etc and type this into Google, you should be= able to download a data sheet with the pin connections. Alternatively, go t= o the manufacturer's website.
       You measure the resistance between the=20= -ve inverting input and earth, say Rb Ohms. You then measure the resistance=20= between the -ve input and the output, say Ra Ohms. The gain of the stage is=20= then =3D 1 + Ra/Rb. You can then turn the set screw on the potentiometer to=20= give the value of the resistance / gain that you require. Rb will have a fix= ed value. Ra will have minimum and maximum values which you can measure afte= r winding the potentiometer to both end stops. Note the turn direction, cloc= kwise / counter clockwise which increases the resistance.
       Look up http://jclahr.com/science/psn/c= ochrane/index.html and opamp U5b. Rb =3D R9 + R27 =3D 1 + 2.2 =3D 3.2 K Ohms= .. Ra =3D R26 + R7 =3D 1 K + (0 to 50) K Ohms preset. So the minimum gain=20= =3D 1 + 1/3.2 =3D 1.31 and the max gain =3D 1 + 51/3.2 =3D 16.9 This gives a= gain range of 1 to 12.9. Sorry that I don't have Larry's current circuit.        If you just want to set up equal gains=20= on your three opamps, set the resistance between the output and the -ve inpu= t to be the same on all three.
       Hope that this helps?

       Regards,

       Chris Chapman
=20

[ Top ] [ Back ] [ Home Page ]