AVR-Chat

--- In AVR-Chat@yahoogroups.com, "David VanHorn" <microbrix@...> wrote:

> For this business, you really need
> some software, and some hardware,
> specifically ANALOG!

Agreed.  So, shall we do it?  It seems like an interesting problem and 
it isn't someone's homework.

I would use a P-channel enhancement MOSFET with its source connected to 
the supply being monitored and its drain feeding the top of the voltage 
divider.  The gate would have a resistor to the source to keep the 
MOSFET normally off.  To turn it on, we need an N-channel MOSFET or an 
NPN transistor with source/emitter at ground and drain/collector 
connected to the P-channel MOSFET's gate.  To turn the whole thing on, 
we apply a high level to the drain of the N-channel MOSFET (or switch a 
current into the base of the NPN transistor).  If we pick the right 
parts for low leakage, nothing draws current when it is off.  
Unfortunately, the whole thing may have to be replicated for each 
supply to be measured, but if they are similar in voltage it may be 
possible to share the N-channel/NPN part.

I'd still like to hear from those advocating op-amp solutions how the 
current draw would be less than a resistive voltage divider with, say, 
a 10 Mohm resistor.  We are all assuming the OP doesn't need a heck of 
a lot of accuracy and I hope that's right.

Graham.