Benching Ecms

goflo at pacbell.net goflo at pacbell.net
Thu Jul 30 06:46:13 GMT 1998


Bruce Plecan wrote:

> Slowly I'm starting to get this,
> How can one tell what a devise can source?.

Short answer is you look at the spec sheet. Not trying to be a 
smart ass :) Usually, but not invariably, transducers are not
designed to source significant current. The output of the sensor
is typically fed to a high impedance input amplifier which
level shifts and buffers the transducer output to suit the app.
For "high impedance input" read "high resistance input", not
quite the same thing but good enough for this discussion - The
higher the resistance (impedance) the less current flow. As a 
practical matter if you present a low impedance load to an output
which can't source much current the linearity of the output will
go to hell.

> I got such cunfusin measurements from a MAP I broke it apart,
> and found some dodads in it so it wasn't a resistive devise.  

Many pressure sensors are resistive devices of a special kind
called a strain gauge. These guys change resistance as they are 
deformed by a mechanical load, in this case perhaps a diaphragm
deformed by pressure. Typically this variable resistor forms an
element of a bridge circuit, such as a Wheatstone bridge. The 
output of the bridge circuit, typically a few tens of millivolts,
is then amplified, temp compensated, and so on. Some pressure 
sensors include this "signal conditioning" in the sensor package.
I have a GM map sensor around here somewhere and it has a 0-5V 
output, so it's such a critter.

But,
> the TPS was and as I recall had like 6K resistance, so I used a 5K
> pot.  If I do the same with 1K+5K, and 10K+50K and get the same
> outputs would using the higher be better?.

I'm afraid I confused matters with my suggestion. The resistance of
the potentiometer drops the voltage output of the sensor to ground.
If the source is 5V and the pot is 1K the current is 5 ma. If the
pot is 5K the current is 1 ma. The size of the pot is determined by 
the current the sensor can source and remain linear. Also the less
current through a pot the better, as a rule, because the wiper 
connection deteriorates.
The voltage at the pot wiper is
determined by the wipers position on the resistive element. If it's
half-way between source and ground the voltage is 2.5V. The closer
to the ground side of the resistor the lower the voltage, and 
vice versa.

Regards, Jack




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