ADCIN car battery monitoring

[mitchf14]

Thanks for your replies again guys, you've all been very helpful, what would the 5.1v zener do i know its for protection, it will start conducting around 3.3 v only when D1 as blown what is the purpose of that. For my voltage divider i want to build it with a 10k var in series with a 2.5K, and set the 10k at 5k, this will give me accuracy to tweak out the 5% tolerances. With a peak voltage of 15v which the battery should never get that high the out of my voltage divider will be 5 v. This will give me a wattage of 20 mW for the 5k and 10 mW for the 2.5 k now all this to ask my resistors should not get very hot but would this be enough to change resistance values as they get hotter, so as my resistors get hotter i would lose more and more accuracy or is the wattage so low i should not worry about it. Also is their a way to protect my ADCIN if someone would ever use two batteries and would connect them in series which would give me 24 v and 8 v at the output of my ADCIN, so for that to never happen, how can have no more then 5 v output into my ADCIN to protect it i'm trying to make it as dummy proof as possible. thanks

[Acetronics2]

Hi, Mitch

Just missing the reverse diode between 7805 In and Out ...

otherwise schematic is fine for use.

Alain

[amgen]

voltage divider,
if you consider full-scale voltage to be 25.6 , a 10 to 1 resister ratio, then,
A) the voltage reading is directly from 8-bit acd value...... 125 would be 12.5 volts
B) and the applied voltage to PIC is about 2.5 volts

don

[mitchf14]

Thanks again guys, why would i need to put a reverse diode between the in and out of my reg, every time i add a diode i lose 0.7 v is it really needed also, what is the purpose of that 5.1 volt zener in their on my opinion that no good my reg needs minimum 6.5v to work.

Can someone please clarify this for me.

thanks

[Acetronics2]

Hi, Mitch

see 7805 Datasheet for that : it's for avoiding reverse current flowing through the regulator when input voltage is < output voltage ( typically @ power shutdown ...)

Note Anode is @ reg output and Cathode @ reg input .... !!!

Alain

[Andy Wood]

Thanks again guys, why would i need to put a reverse diode between the in and out of my reg, every time i add a diode i lose 0.7 v is it really needed also, what is the purpose of that 5.1 volt zener in their on my opinion that no good my reg needs minimum 6.5v to work.

Can someone please clarify this for me.

thanks

The value of the zener diode is incorrect. For a 12 volt system, a value of 18volts would be more appropriate. I would use a Transorb, not a zener for this task.

A 12volt automotive system can contain many voltage spikes much higher than the 12 volt battery. The transorb is there to protect the input of the regulator from these spikes.

If there is a possibility of the input being connected to a 24volt system, I would use a 33v transorb. Also ensure the capacitors on the input of the regulator have a voltage rating of at least 35volts.

The 10ohm resistor could be replaced with a resettable polyfuse. This way, if the transorb conducts due to overvoltage, the polyfuse will trip and go open circuit. When it cools down again, operation will resume as normal.

Andy