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post #4 of (permalink) Old Jan 10th, 03, 07:29 PM
dnult
Senior Tech
 
Join Date: Mar 2002
Location: Austin, TX, USA
Posts: 5,277
I'm not sure what you're planning to use for guages - that will make a big difference in your circuitry. Most all meter movements are 50 micro-amp full scale and about 2000 ohms coil resistance. On the other hand, some meters have resitive circuitry already inside and have a full scale rating in the volts range. The later probably will not work very well since their internal impeadance is often too low for the sensor.

You'll have to figure out how to combine discreate resistance values to give you the range of meter movement you need. As you can imagine it depends highly on the sensor you use. For example, an oil pressure guage will vary from near 0 ohms full scale to about 250 ohms with no pressure. A temperature sensor will vary from 0 ohms at full scale (~ 300 degrees) to about 2000 ohms at room temperature. At 100 degrees, they are about 1000 ohms. The problem with temperature sensors is that they are non-linear. As a result, you'll notice the meter faces are non-linear as well.

As I said, you'll have to build a resistive circuit to give you the display range you need. You should avoid letting more than about 60 micro amps of current flow through the meter at any time lest it fry. And you must know your sensor's characteristics. A simple little resitive network can be ugly when you get down the the details.

-dnult

[This message has been edited by dnult (edited 01-10-2003).]
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