Sensor Wiring

[Darrell Norquay]

At 05:22 PM 8/30/96 -0400, John Alsina wrote:
> I want to use an LM34 three-terminal temperature sensor to measure engine 
>coolant temperature.  

The LM34 is kind of an odd bird.  If you need to measure negative
temperatures with it, you either have to have a bipolar supply or bias the
sensor above ground.  It doesn't like capacitive loads, since it is a
micropower circuit.
You can make a reasonably good 2 wire circuit that is relatively immune to
noise, but it won't measure below zero.  If you already have some of these,
use 'em, but there are better alternatives.  Check out the National
databook, there's a few applications in the data sheet.  They explain what
to do about capacitive loads and describe some 2 wire and negative temp
circuits, and do it better than I can.  If you don't have a databook, check
out the National Semiconductor webpages.  They have online datasheets, but
you need Adobe Acrobat to read them.

A better choice (IMHO) is the AD590 from Analog Devices.  It is a current
output device, and outputs 1uA per degree K (Kelvin, 0K = -273C), so at 0C
it puts out 273uA, and at 100C it puts out 373uA.  Use a 2 wire shielded
cable, carrying +5V on one wire and the current return from the sensor on
the other wire.  Run the return current through a resistor to ground, and
measure the voltage across the resistor.  Select the resistor to give you
10mV/deg C (10K ohms) if you run it on 5V.  If you run it off 12V, you can
increase the output voltage a bit, the sensor doesn't care if it has a
regulated supply or not, but, your offset increases as well and may be more
difficult to deal with.  Put a cap across the resistor if you need more
noise immunity, but the current loop style output is relatively immune to
noise anyway.

In either case, you'll need some gain and offset before the A/D converter to
bring the signal from the sensor to, say, 0-5V.  A differential input would
be useful to eliminate some of the noise, but not essential, and input
protection from accidental overvoltage is also wise.  I recommend an LM10
opamp/reference chip, these are extremely useful for this type of application. 

Use shielded twisted pair cable to connect to the sensor, ground the shield
ONLY at the engine block end.  DO NOT connect circuit ground to engine
ground.  Stuff the sensor (whatever you choose) and some of the cable into
some of that new heatshrink with the glue inside, this will shrink and seal
the whole thing so it's guaranteed waterproof. Put this into a small copper,
brass, or SS tube with one end sealed.  Use standard tube fittings to seal
the tube into a plug that will fit the manifold.

A third choice now exists, and I recommend it highly.  Dallas Semiconductor
has a really nifty series of serial temperature sensor chips.  Talk to them
directly by flapping a uP port bit, they get power right off the pullup
resistor on the port bit, Have 9 or 10 bit accuracy, and have virtually no
problem with noise.



regards
dn
dnorquay at awinc.com