ADCIN car battery monitoring

[mitchf14]

Hi guys i want to build a circuit for my second year school project i was hoping someone could give me some more insight on this circuit i found on this forum.

I touched it up the original circuit a little bit in visio and saved it as a pdf, i added some values that where missing, can someone tell me if they are accurate. And if you would be so kind to give me a explanation on how it works. I Have a general idea already how it works and i seems to be a nice little circuit just a couple of parts i have no idea why they are their.

[amgen]

tell how much you understand already..... as a starting point.
there probably are some unnecessary parts in there.
don

[mitchf14]

Well from i can see here D1 is just for protection in case someone would ever reverse the polarity's, and the 10 ohm in front is soften the blow of the D1. I'm guessing the 47uF cap is to block some ac, i was going to use a 50k var to get a more precise 3:1 ratio, The D3 and D4 is for protecting my pic for reverse polarity's also. If this is all correct, i just can't see what all those extra capacitors do.

looking forward to your explanation

[Archangel]

Well from i can see here D1 is just for protection in case someone would ever reverse the polarity's, and the 10 ohm in front is soften the blow of the D1.

Or give somplace to scrub off some current should the voltage cause the zener to fire.

I'm guessing the 47uF cap is to block some ac,

It does that, it also act as a quick voltage source when the load outstrips the power supply

i just can't see what all those extra capacitors do.

They provide a short circuit path to different frequency noise.

[Charles Linquis]

You shouldn't use a 50K pot as a voltage divider. The source impedance into the A/D shouldn't exceed 5K or so. I like to keep it below 2K. A 5K pot would be a better choice.

[precision]

Resistance should be 30K and 20K for full adc resolution.

12V
+
|
|
/
\ 30K
/
\
|
|------- V 0ut 4.8V
\
/ 20K
\
|
|
--
GND

According to Attachment.

.

[mitchf14]

Thanks for all your reply's guys, i keep seeing people putting a small capacitor in parallel with what is the purpose of that my teacher in school never mentioned anything about that is it important, in this circuit it shows 2 of them the 0.1 uF should do the trick.

[Archangel]

Thanks for all your reply's guys, i keep seeing people putting a small capacitor in parallel with what is the purpose of that my teacher in school never mentioned anything about that is it important, in this circuit it shows 2 of them the 0.1 uF should do the trick.

DC flows through the wires, traces etc . . . R/F sorta crawls along the outside, and the .1µF strips it away

[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