Meter testing for ecm work

[Robert W. Hughes]

> Is there a way to test to find out if a analogy (needle) type meter has
> enough impedance to work with say an O2 sensor?.
> 

A meter reads current so a 1ma meter would need 1ma for full scale and
if it had a resistor in series with it such that the meter resistance
plus the resistor was 1000 ohms it would read full scale at 1 volt ( 1
volt / 1000 ohms = 1 ma). The most sensitive meter practical to use is a
50ua movement which would require 20K. The point of this is that an
oxygen sensor is usually said to require a minimum load of 1 to 10
megohms which with a full scale output of 1 volt suggests that the
sensor can deliver .1 to 1 ua. However a simple op-amp circuit connected
as a voltage follower (output tied to neg input, sensor tied to pos
input) should supply a very high impedance for the sensor and the output
could easily drive a 1ma meter.
-- 
Robert W. Hughes (Bob)
BackYard Engineering
29:40.237N, 95:28.726W or perhaps 30:55.265N, 95:20.590W
Houston, Texas "The city with too much Oxygen"
rwhughe at ev1.net
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