AVR-Chat

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Tim Gilbert
Sent: Friday, December 14, 2007 9:15 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Re: ADC Vref

 

Steve,
We've done a lot of voltage measurement in the aviation world, which is
similar to your situation.
Several points:
a. I wouldn't worry about the 10K input Z for the ADC for such a low
frequency signal (DC). We use 57.6K and 6.11K resistors (1% or better) and
have been able to hold +/- 0.1 volt reading without problems. Yes, that's
less than 10K but we've also done boards using Meg ohm resistors and it
worked fine. Note that we use an external voltage reference since you can't
count on Atmel's internal one being consistant from chip to chip. We've
built a board that used a Atmel 169 picopower and a bare glass LCD. With the
display on, it used an average of 25 uA (if my memory is correct) and
displayed time and voltage.



-- Thanks for the tip.


b. Isn't there some sort of a master switch that turns off all the power to
this tiny little motor? (I could resist the dig, since we deal with jet
engines.) Couldn't you put your voltage divider on the other side so it's
shut off when the boat is shut off? BTW, even 50 mA is probably much less
than the self discharge rate of the battery.

-- good dig [g].  The voltage I need to monitor is the voltage across the
battery.  I need to do this both when the "small" motor is operating and
when it is not.  The battery can be on charge whether or not the motor is on
or even has the power enabled to it.  Thus I can't tie the divider to the
motor on/off.   The "master switch" you refer to will be the SW200 contactor
that Robert directed me to in earlier posts. 

-- and yes  that's true about self-discharge of a 750 amp-hr battery
bank.it's a good reality check.   Mine is equivalent to about a 150-200 mA
continuous drain.   It's surprising, however, how fast all the little bits
can add up, so that's why I pay attention to them.   But, yes, when push
comes to shove I'll probably just use a permanent divider with a total
resistance of 100 K or so and just forget about its 0.1 mA drain.

c. I haven't seen any mention of surge protection etc. I'd assume that
you'll get a nasty voltage spike with the motor is turned on/off. We always
include a TVS, fuse and diode on the input. We've actually had the power
pulses from avionics knock out the power supply on our boards. Watch your
grounds

>  Are you referring to protection on the power input or signal inputs (or
both)?   I always include exactly the 3 things you refer to on power input
and always put protection diodes on any output driving an inductive load
(typically a solenoid/relay/contactor coil).   

Do you have recommendations for signal input, especially ones coming along
"long" wires (like 10 to 30 ft)?

Regards,

Tim Gilbert
JEM Innovation Inc.
303-926-9053 (office)
303-437-4342 (cell)
720-890-8582 (fax)
www.jeminnovation.com
www.pdksolutions.com






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