At 01:33 PM 3/12/2004, Roberts II, Charles K. wrote:

>I am working on a PIC based High Voltage module. The schematic of the
>current meter portion of the module is attached. I am measuring the
>voltage drop across a knowm resistor, but to do it I had to use two
>voltage dividers to bring the levels to something I can read. I measure
>about the same current with or without a load. What could be going on?
>Loooking into the devider there are 50Meg Ohms to Ground, Is this what
>is messing up my reading?

I don't think that you have a hope of making this work well.  The problem
is matching the divider resistors accurately.  I see two easy alternatives:
monitor the ground current instead or do it magnetically with a hall effect
current sensor or flux gate current sensor.

Monitoring the ground current is the easiest.  Hall effect current sensors
(LEM modules, etc) aren't very sensitive - they are normally intended for
measuring current in Amperes, not mA or uA.  Flux gate current sensors are
much more sensitive but need to be shielded or have some other mechanism
for ignoring the effects of external magnetic fields.  This makes them
expensive.

dwayne

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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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