I'd like to monitor a 12V boat battery. I don't want to risk damage to the ADC input, what with pumps and things kicking in and out.
Would a 50k resistor from the bat+ and a 10k to ground, with a 0.1uF cap across it be relatively safe? Or is there something better I can do?
If it gets too complicated, I won't bother, it would just be a nice extra.
With the AVR at 5V you could do with 47K + 8K2 and then add a 3V9 Zener diode over the 8K2
with 15V there should only be 2,2V over the 8K2 so well below were the zener should be acting up ( else you can even go to a 4V7 zener )
Adding a 100nF capacitor will also keep spikes from getting through. Just make sure you have shielded the connecting wires good enough.
Guess you are going to use the internal 2,56V reference.
I guess this is essentially the same as for an "automotive" (ie, car) application ?
I'd like to monitor a 12V boat battery
Assuming this is a lead acid battery, in my solar charger I use a 100k/15k .1% resistors with a 100nf cap to the T25 ADC.
it uses the internal 2.56v ADC ref.
Hope that helps!
Jim
Note that if the battery is below deck and/or in an enclosed space, be careful what you put in there - explosive gases may be present. There may be specific legal requirements.
Thanks.
The person I'm doing this job for (the voltage monitor is a mnor extra), has plenty of experience of boat stuff.
He built a whole bunch of stuff for Ellen MacArthur, when she sailed round the world, that I supplied all the firmware for, for example. But thanks to all for the advice.
Hi.
I make some dynamo regulators for boats and need to measure both battery and output voltage for dynamo.
some info:
it is stupid to use a Zener. Zener will leak some current and it will make some error measure when the voltage is high.
if you want to protect input, use a larger resistor from the +Bat like you have written and use some Schottky diode between adc pin and VCC . can use one to gnd too.
if you want a high accuracy, use a voltage reg LM4040 series. work with the highest input voltage to the adc to have the most resolution.
V bat undercharging will be 14V4 for optimal lead battery, can be more. if you set 15V max for 12V you will be good.
I use some transistor to disconnect the ADC resistor to not have leak current and some leak voltage on the cpu too.
Good luck
Thierry
Thanks.
I agree about the zener, butwould have thought the internal clamp diodes would suffice with a 47k resistor.
I don't need extreme accuracy, but apparently the digital video recorders being used can seriously screw up if the supply drops below around 10V.
Instead of a zener, use a TVS & the reverse current can be extremely small (sub microamp). Trade off, it is not optimized for regulation as well "the regulate", so it's less useful for providing a steady reference voltage.
https://www.onsemi.com/pub/Collateral/ESD9X3.3ST5G-D.PDF
According to mfg it is not best to rely on "hammering" the internal clamp diode, unless it is a rare occasion. The cap can soak up most of the overload, assuming it is not steady overvoltage. Sorta depend's on who's chip & the specific part. With that said, I've done it anyway!
