AVR-Chat

Ok, OP back again.  I was away for most of yesterday.

 

Yes, my application is (sort of) automotive.  Specifically it is a boat
environment, which is 12 VDC.  My guess is that in some respects it's not
quite as harsh as automotive, but in others, eg, high power RF transmissions
and need to shield magnetic compasses from EM fields, it is far worse.     I
have many uses for uC's but the specific one that the post was about is that
I have put in a "bow thruster", which is a huge 5.0 KW, 12 VDC motor that
draws about 400 A (which is another story).   This needs to be run from its
own local 12 V battery (an Optima AGM starting type) because to run it from
the main boat's bank in the aft part of the boat, the voltage drops even in
large 4/0 cable would kill a lot of the motor's thrust.   Even so, the local
battery, if the thruster is run too long (like 5-7 minutes), can get drawn
down to the point where its voltage - I'll call it Vb here -- gets so low
the solenoids in its circuitry start to chatter and within a few seconds
fuse together and bingo, motor stuck on and battery completely drained (ie,
major problem).

 

So my intent is to use a uC to monitor the thruster battery voltage, Vb,
when under load and when it gets too low, shut everything down.   One
feature of this is that the uC power supply voltage (Vcc) will come from the
boat's main bank but the voltage to be monitored is the separate local
battery (in other words I'm not powering the uC from the voltage to be
monitored).

 

True I could do this simple task totally in hardware, with a voltage
comparator, but there are other reasons which require using a uC so the
easiest solution is to just have the uC do the voltage measurement.
Obviously not a whole lot of accuracy is required.

 

Regarding voltage divider current drains, my assumption is that from the
data sheets the analog ADC input likes to see 10 Kohms maximum impedance.
For a maximum input to the ADC of Vcc = 3V this implies a current drain
through the divider, regardless of amount of dividing, of 0.3 mA.   Yeah,
heck, compared to 400 A it is quite negligible, but overall I have so many
of these mA to 10 mA to 50 mA drains that they all add up to a significant
24/7 drain on the batteries, and so I have a general philosophy of trying to
minimize these "background" drains if at all possible.

 

I have used VP3203 enhancement mode P-channel mosfets (actually DMOS FET's,
I just loosely call them mosfets) on PCB's in the past and I see no reason
why one of these, with its gate driven directly by a uC output pin, could
not be used to turn on/off the voltage to the divider.  Their threshold
voltage is only 1 V so a 3 V signal from the uC will do the trick.
Leakage current when off should be less than 10 uA.  Connect the gate with a
pull-up to Vb, source to Vb and drain to the top of the divider.   I always
but TVS's and big input capacitors on the power input to any PCB I build and
(so far, knock on wood) haven't had any problem with transients.

 

In some other potential applications, not this particular one, I will have
need to measure 2 or 3 voltages, all well above Vcc.  In this case I would
simply add one of these mosfets to each divider but drive them all  with the
same gate signal from the uC.

 

Am I missing something here?

 

Steve

 

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Graham Davies
Sent: Friday, December 14, 2007 5:43 AM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Re: ADC Vref

 

--- In AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> , "Roy
E. Burrage" <RBurrage@...> 
wrote:

> Why not just stick with MOSFETs, Graham?

Sure. An NPN transistor is just an alternative.

> He might also want to use some sort
> of gate/source protection scheme ...

That would depend on the application, which we don't know.

> Why would he not be able to expect
> pretty fair accuracy from your scheme?

He would. As you say, the on resistance of the MOSFET would be of no 
consequence, so it's just a matter of using the appropriate resistors 
in the divider and a good reference for the ADC.

Some of the other suggestions, though, did not strive for accuracy. 
Again, it may not be necessary in this application. We don't know 
what the OP hasn't told us. Recall that his original post didn't 
mention the need to draw almost no power, it was just about what the 
data sheet said about the ADC reference input.

Graham.

 

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