Date: Sat, 6 Mar 2010 10:26:36 -0600
Reply-To: Tom Hargrave <thargrav@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Hargrave <thargrav@HIWAAY.NET>
Subject: Re: AIR FLOW METER
In-Reply-To: <4b927ed4.48c3f10a.50e5.21b8@mx.google.com>
Content-Type: text/plain; charset="us-ascii"
OK, now we are getting into my long term manufacturing history. We used to
manufacture thick film hybrids for GTE and the resistors on the hybrids were
made the exact same way - with printed on ink, then we would trim the
resistors up to value with a laser.
We used ink manufactured to different values, for example 100 ohm ink was
100 ohm when printed in a square regardless of the size of the square. But
if you were to print a rectangle twice as long as it was wide the value
would be 200 ohm. Now let me explain.
A square of 100 ohm resistor ink 1cm X 1cm will measure 100 ohms from one
connection edge to another.
Now put two of these resistors in series, or print a patch 1cm wide by 2cm
long, and the resistance is now 200 ohms from connection edge to connection
edge.
Now put two of these in parallel, or print a patch 2cm wide by 1cm long, and
the resistance is now 50 ohms from connection edge to connection edge.
Now put four of these resistors in a series parallel circuit, or print a
patch 2cm wide by 2cm long, and the resistance is back to 100 ohms from
connection edge to connection edge.
Now apply this to the carbon resistor in the AFM. The track worn across the
resistor disconnects part of the parallel resistance from the circuit and
the readings have to change!
Thanks,
Tom Hargrave
256-656-1924
Our Web Sites:
www.kegkits.com
www.stir-plate.com
www.andyshotsauce.com
-----Original Message-----
From: David Beierl [mailto:dbeierl@gmail.com] On Behalf Of David Beierl
Sent: Saturday, March 06, 2010 10:12 AM
To: Tom Hargrave
Cc: vanagon@GERRY.VANAGON.COM
Subject: RE: AIR FLOW METER
At 10:40 AM 3/6/2010, Tom Hargrave wrote:
>The fact that the carbon resistor is laser trimmed is what I was eluding to
>when I stated that relocating the wiper isn't a 100% fix. The problem is
>that the missing carbon track across the carbon resistor changes the value
>all the way across the sweep even when you relocate the wiper.
Dear Tom,
I'd like to see that tested. There has to be some reason that they
made the thing so elaborate, and I strongly suspect that it's to
reduce the sensitivity of the device to the precise tracking radius
of the wiper. Remember that it's acting as a potentiometer and its
output is a smoothly varying voltage; but if you measure
wiper-to-ground resistance while moving the vane it varies all over
the place. I wish I had one to do some measurements on. My
impression is that the large carbon surface contributes comparatively
little to the resistance.
Yours,
David
No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 9.0.733 / Virus Database: 271.1.1/2725 - Release Date: 03/06/10
01:39:00
