On Mon, 11 Sep 2006 02:04:46 GMT, "Farticus"
wrote:


"John Fields" wrote in message
.. .

Assuming that you're using an "N" channel MOSFET for the switch and
ignoring the 1 megohm resistor for the moment, is this what your
circuit looks like?


+V---------+-----------+------+--------+
| | | |
| | | [2k2]
[LOAD] | | |
| | +---|--[1M]--+
| | | | |
+-----[1K]--|--+--|+\ |
| | | ------+-- TO µC
D [5K]---|-/
Vc-------G | |
S | |
| | |
-V---------+-----------+------+-----------GND


--
John Fields
Professional Circuit Designer

Yes thats it John.


---
OK.

If you've got a well-regulated supply capable of handling the load
current, there's _no way_ the reference voltage can change, so
something else must be happening.

That's not a very good way to sense the current through the load
because you're using the MOSFET's channel resistance as the sensing
resistor, and it can/will vary, so I suspect that's where the
problem lies.

Assuming that you want the output of the comparator to go high when
the current through the load increases past a certain point, a
better way to do it would be like this:

+V---------+-----------+------+--------+
| | | |
[LOAD] | | |
| | | [2K2]
D | | |
Vc-------G | | |
S | +---|--[1M]--+
| | | | |
+---[1k]----|--+--|+\ |
| | | ------+-- TO µC
| [5K]---|-/
[Rs] | |
| | |
-V---------+-----------+------+-----------GND

Where the drop across Rs at the trigger current is greater than the
the voltage on the comparator's - input.



1. What's the supply voltage?

2. What's making the current in the load change? That is, is the
load resistance itself changing (Are you using the MOSFET like a
switch) or are you varying Vc to make the MOSFET look like a
variable resistor?

3. What's the range of current through the load?

4. Anything else you can think of :-)


--
John Fields
Professional Circuit Designer