Voltage monitor for car battery

[Acetronics2]

Hi, Passion

we just had forgotten the reverse diode ( 1N400x) from regulator output to its input ...

to be fully "bullet-proof" ... ( see 78L datasheet ...)

But ... , If some LCD (or LEDs ) are used as a display ( we didn't talk about that ...) , that could make you need a 7805 instead of the 78L05 ( in all cases will improve the 5v ref voltage stability ... see output impedance of the 78L series vs. 78 series ... )

Alain

PS: No, it's not the LCD that will draw current .... but it's backlighting !!!

[Pic_User]

I am still playing with Paint.net to draw the occasional schematic for the forum.
Here is the latest from the circuit committee.
Put a fuse in place of the input resistor. Added the wrap around diode.
I am learning to not put values onto the schematic. Then people can discuss different values and the schematic does not change.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1918&stc=1&d=118650657 1">
The entire drawing was drawn in the paint.net (freeware) program, anyone have a better way to go?
-Adam-

[Acetronics2]

Hi, Adam

Did you try that : http://www.diptrace.com/downloads/dipfree_en.exe

...

Alain

[mackrackit]

This had a good write up in QST last month but have not tried it yet myself.
http://tinycad.sourceforge.net/index.html

[Demon]

I've been going through the thread and trying to plug values and names on all the components discussed so far. Keeping detailed information out of the picture is fine and dandy for a discussion, but it doesn't help me when I save the pic in my schematics folder for future reference.

I could use help with D1 and Z1, zener is not enough for a noob like me.

I've used a LD1084 5V 5A 'cause I don't know exactly how much current I will need for the final application (backlighting is a hog). I also have a LD1085 ADJ 3A voltage regulator standing by in case I can get away with a smaller model, both of these are automotive grade and available at DigiKey.

[mackrackit]

For D1 a 1N54xx should work. It is rated at 3 amps.
http://www.onsemi.com/pub_link/Collateral/1N5400-D.PDF
Or if you do have to go with a 5 amp circuit then a MR75x at 6 amp rating.
http://www.onsemi.com/pub_link/Collateral/MR750-D.PDF

For the zener... That is to protect the regulator from a higher voltage than it is designed to handle on the input. If you regulator can handle say 35 volts then a 22 volt zener would work, but what are the chances of the voltage going over 35 in a car? Z1 may not be needed. IMO

[Demon]

Dave, I took the vocational progam through high school so yeah (switched to computers in college), I agree my '92 Jimmy will not generate 35 V, under normal circumstances. But that's the problem, car parts have been known to fail and cause the most unusual effects.

If I can decipher all the sensor signals as planned, I will be spending a lot of time and energy on this project. I've already invested time and money as it is. If a potential problem only has a 0.01% chance of happening and it DOES occur, I'm gonna be one very ******ed off person if I skimped on a $2 safety component. I can afford a zener, it's not like I'm trying to survive on a student budget living in the boondocks (although that doesn't seem that far in the past ).

I also have ZNRs and a few other types of surge/clamping devices on hand that I've ordered for other projects, I'm just not sure what rating I should use. But since the voltage regulator is supposed to handle up to a maximum of 30V, I assume it would be wise to use something that would start clamping at 15-20V. That way I'd stay away from pushing the voltage regulator to the extreme rating.

Robert

[mackrackit]

Here is what you need.
http://www.picbasic.co.uk/forum/show...14&postcount=7
Bullet proof.

[mister_e]

TVS and/or Transorb are often use indeed. I suspect the TVS to be more "accurate" for this application.

[Demon]

I've kept on tweaking the circuit, using only values I need as opposed to going-all-out.

<IMG SRC="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2537&d=1208975010">

- SR503 schottkey, limits surges to 30V 5A to protect voltage regulator.
- 1N5250 zener, limits surges further to 20V, well below LM1084 30V abs max and over 12-15V normal operating condition.
- 1N5246 zener, limits surges to 16V, voltage divider is designed with a 16V max.

I read about condensation in automotive applications. The circuits in my instrument cluster are covered in a clear product, but what about the in-circuit fuses and those in the fusebox? Those aren't covered and yet they don't pop a gasket when I have to scrape the INSIDE of my windshield. Are 5V logic circuits that much more sensitive as opposed to 12V automotive circuits?

Side question: can I use ordinary water-based lacquer spray to protect the assembled circuit? Or do we really have to use the specialized products?

[mackrackit]

I read about condensation in automotive applications. The circuits in my instrument cluster are covered in a clear product, but what about the in-circuit fuses and those in the fusebox? Those aren't covered and yet they don't pop a gasket when I have to scrape the INSIDE of my windshield. Are 5V logic circuits that much more sensitive as opposed to 12V automotive circuits?

Think about condensation. Warm moist air on one side of a barrier, something causing a cooling effect on the other. It matters where the device is placed. Take the radio for example. Placed in the dash. The air around the radio for the most part will be the same temperature as the air inside the radio. The case having "holes" in it to allow for air flow. If the radio was placed against the window or an uninsulated metal panel then you would have a problem.

Side question: can I use ordinary water-based lacquer spray to protect the assembled circuit? Or do we really have to use the specialized products?

As long as it is dry before you power it up it should be fine. Anything non corrosive that will seal the board will work.