<VV> Thermister Swap

RAMBLERPAT at aol.com RAMBLERPAT at aol.com
Mon Sep 1 19:13:42 EDT 2014


 
J. B., 
 
    Sounds interesting, I will study the situation. I  was mindful of 
keeping the stock appearance of course, I forgot to add  that earlier. So, in so 
many words, the key to utilizing the Rochester unit  appears to find the 
correct capacitor in order to use the stock gauge.  Like I mentioned earlier, I 
initially was searching for an affordable  AC thermister used or otherwise, 
and then I stumbled upon this. My need for  a head temp reading is still my 
priority at this point, however,  I would like to address the gap for 
available and affordable  thermisters for both future and present Spyder and 140 
Corvair owners. It  is too bad AC or someone else never continued 
production, but I can  see a small market for them as a possible reason. However, they 
 seem rather simple in design, and they use a small amount of  materials.   
     
 
Patrick

 
 
In a message dated 9/1/2014 2:51:49 P.M. Eastern Daylight Time,  
jb30343 at windstream.net writes:

Patrick,  Talk with your brother.  This stuff is not  rocket surgery.  I 
have no formal training in electronics but I have  managed to design many 
simple circuits to deal with various needs where  off the shelf solutions 
were either unavailable or expensive.  You  will need to do a little 
algebra but nothing scary.  Parts like op  amps, resistors, small 
capacitors and small transistors are insanely  inexpensive.  You can burn 
a lot of them up without any real impact  on your wallet.  There are some 
good cookbooks and websites that will  give you basic schematics that you 
can adapt to fit your needs.  You  can do this.

The Rochester part you're looking at is just a thermistor  in a proper 
housing.  The thermistor is just a resistor that changes  value with 
temperature.  The problem you're trying to solve is that  the values and 
the curve of the new part is way different than that of the  old part. 
Here's a little of what I remember thinking 15 years ago.   I do not have 
a great memory.  The circuit in the Corvair is just  car's nominal 12 
volts fed through the thermistor and the gauge.  My  thought was to 
essentially put an op amp circuit in place of the gauge and  use that 
circuit to feed the stock gauge.  An op amp is just a tiny  integrated 
circuit amplifier.  Using two or more, working together  and against each 
other it should be possible to bash the output into  something closely 
resembling the stock thermistor.  If I got any  farther than that, I 
don't remember the details.

The Arduino in an  off the shelf microprocessor that is designed for 
hobbyists and  experimenters.  It's like a digital erector set.  To use 
it in  an application like this, you would put an Analog to Digital (A to 
D)  converter in place of the stock gauge.  The converted digital signal  
gets fed into the processor.  You write a program for the processor  that 
would basically be a Look Up Table (LUT)  the processor then  tells the 
Digital to Analog (D to A) convertor what voltage to feed the  stock 
gauge.  The Arduino route would be a lot bulkier and more  expensive than 
a "roll your own" solution made out of a few Op Amps.   It would also be 
more accurate.  The really intriguing part is that  you would then have a 
computer riding around in your Corvair which could  be used for all sorts 
of other applications.   --J.B.

RAMBLERPAT at aol.com wrote:
> J. B., My oldest brother is  the electrical engineer in the family and 
> he is retired in a  different state, on the other hand I am a social 
> worker with little  electrical trouble shooting knowledge. I was 
> thinking that the  Rochester unit is already DC because it can be used 
> in light aircraft  (if that is what you were referring to). Not sure 
> what the voltage  would be in those though. The rest of what you were 
> saying make as  much sense to me as algebra which is close to none. At 
> some point I  may try to purchase one of these Rochester units and see 
> which  direction it leads me, and perhaps learn a few things along the 
way.
>  Thanks!
> Patrick
> In a message dated 9/1/2014 12:29:15 P.M.  Eastern Daylight Time, 
> virtualvairs at corvair.org  writes:
>
>     Sounds like you might want to take  on this project, Patrick.  I
>      started
>     down a similar road about 15 years  ago.  It's a worthwhile project
>      but
>     it wasn't anything I wanted to pursue. Here's  more or less where I
>     left
>   it.  A few things have changed.  There were no bare  thermistors on
>     the
>      market that were close to following the curve of the one used in  the
>     Corvair.  I had not considered replacing  the gauge nor was the
>     Rochester
>   unit you found available when I was looking.  Replacing the  gauge
>     might
>     be a good  option but it will likely mean sacrificing stock
>      appearance.
>     I don't think you'll be able to match  the Rochester sensor to the
>     stock
>   gauge with just a resistor network but you should be able to  get
>     close
>     with an op amp  or two.  That's the direction I was going before I
>   decided not to proceed.  If I were taking on that project  today, I
>     would
>     think  about using an Arduino processor, an A to D converter, a D to A
>   converter and a LUT.  The Arduino is major overkill for a  job like
>     this
>     but once  it's in the car I'm sure you could think of numerous
>   other jobs
>     for it.  If you have  the time, you might enjoy working out a
>      solution.
>     Have fun.   --J.B.
>
>
>
>     Patrick via  VirtualVairs wrote:
>     > To all, I have been  reading the feedback on the  ROCHESTER 3080-38
>      > TEMPERATURE PROBE that I happened upon while trying to save   a
>     few dollars
>     > on  the unfortunate shortage of affordable thermisters 
>      situation. It is true
>     > that the $300 - $400  needed to purchase  an NOS AC thermister is
>      > prohibitive for many Corvair owners, and  the $80 or  so
>     necessary for a used one is a
>   > risk as well. In  addition, I well aware that this is  the way it
>     is in the
>      > old car business  world and hobby. For theses reasons,  and
>     others, I was so
>      > excited when  I discovered the new Rochester temp probe for  $80
>     in the  2015
>      > Spruce Aircraft catalog and I recognized the possibilities that  it
>     > represents.
>      >
>     >      It appears that no  one here knew of  this so the Rochester
>      3080-38 is
>     > untested in a Corvair auto  application as  of today of course.
>     However,  the
>     > good news is that there is VV confirmation  that  this unit it is
>      essentially
>     > the same dimensions including the  thread size  as the AC 
thermister.
>     >  Perhaps, this is half the battle right there. It is  also  my
>     understanding is that
>      > this thermister will work for us if  we match it to a  compatible
>     gauge,
>     >  after all it is designed to measure head  temps in an air  cooled
>     engine. This
>     >  discovery may help some of us save some  dollars and give us  an
>     upgrade to
>     > more  efficient and reliable  equipment too. Though I have not
>   researched any
>     > head temp gauges  yet I  am hopeful the someone out there will
>      try this set up
>     > and let us know how it   works, provide us with the costs, and
>     let us know  if
>     > it was worth the   efforts.
>     >
>     >  Patrick Laus
>     >
>     >  64 Spyder Coupe
>     > Metro Detroit  area
>     >
>      >
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