> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Robert Mann
> Sent: 08 April 2009 20:51
> To: piclist@mit.edu
> Subject: Automotive sending unit / gauge mismatch.
> 
> I am trying to figure out the best way to design a PIC based unit that
can
> convert mismatches between sending unit and gauges.
> 
> 
> Looking a digital pots I see ones that have say 5000 OHM and 256 steps
> using
> something like that would not give many steps in the 0 - 1000 ohm
range.

Digital pots will be of absolutely no use in this application, they have
no where near the current handling capability required.

This is actually quite a complex task, a sensor could be a high side or
low side device, so a universal circuit will have to work equally well
in both situations.

You can use a MOSFET as a voltage controlled resistance, but there are
some obstacles to overcome:
1) The devices transfer function is temperature dependant and varies
from device to device.  You would need to implement voltage and current
feedback to set a specific resistance (or characterise each transistor
used over voltage/temperature).
2) To bias an N channel transistor into conduction on high side
applications, you will need to generate a bias voltage higher than the
battery voltage.  Conversely if you use a P channel device you will need
a negative voltage to bias it on low side applications.
Lots of information on Google about voltage control resistors however.

Also consider how you are going to cope with the wide range of sensor
resistances.  Presumably you want to convert the sensor resistance into
a voltage and sample it with an ADC, to do this you either need to make
the sensor part of the potentiometer or feed it a constant current.  To
make full use of the ADC resolution, this resistor/current will have to
be changed according to the sensor resistance e.g. what gives good
resolution on a 1000 Ohm sensor would give ten times less resolution for
a 100 Ohm sensor.  When you only have a 10 bit ADC, you could be down to
a handfull or LSbits very easily.

Regards

Mike

=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist