AVR-Chat

Bryan Martin wrote:

> That was it.  When I sourced VCC and ground off the SDK500 the voltage went 
> to 4.95 w/no resistors.  Question though while were are talking about this. 
> Lets say I wanted to run two different power supplies one for part of the 
> controller portion and another for the RF portion (picture 2 different 
> battery banks - trying to isolate noise getting into the RF side) is this 
> possible?  Would this work if I connected the ground planes together?

OK do you know much about electronics ?

If yes then you realise you need a common reference point or your signals will 
appear to float and you will get various problems.

If no, then here is the explanation:

If you have 2 batteries say 1.5V and 9V, if you measure from the positive of 
the 1.5V to the negative of the 1.5V you well get a voltage of 1.5V. The same 
with the 9V battery, measure from the + and - terminals and you will get 9V.

Now if you measure from the 1.5V's - terminal to the 9V + terminal, you will 
see no voltage (in the real world the multimeter will show some stray voltage, 
but this would only be a few milli volts) you will not measure 9V. The reason 
is the current has no where to go to return to the - terminal of the 9V battery.

So if we place a wire between the -terminal of the 1.5V and 9V batteries, and 
then measure between this wire and the + of either battery, we will see 1.5V 
at the + terminal of the 1.5V battery and 9V at the + terminal of the 9V battery.

So why did you measure 5.8V in your RF module:

Well there are 2 main reasons:

#1 the module was being feed from more than +5V

#2 the length of wire between the 0V rails on the different boards (possibly 
through resistors, diodes etc) was sufficient to raise the 0V rail of the RF 
module to 0.8V

I hope this helps.

-- 
Regards,

Kat.
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