Voltage monitor for car battery

[chuck]

Passion1

here is web link with values and op-amp

http://www.winpicprog.co.uk/pic_tuto...ogue_board.htm

Has for the zener diode you just put the block end to say PORTA.0 and the ohter to ground.

hope this helps,

chuck

[BobP]

Hi,

As per your request please find attached a quick and basic suggestion for values I would use.
As you have 10k resistors I used these.

Bob

[passion1]

Bob , Chuck and other

Thank you all for the advice. I really appreciate it!

[Melanie]

> Has for the zener diode you just put the block end to say PORTA.0 and the ohter to ground.

No!

A Zener starts to draw current long before it gets to it's stated voltage... it doesn't just suddenly 'turn-on' at it's rated voltage. As soon as the Zener starts to draw current (a 5V6 Zener might start as low as 3v!!!) then it will throw your Resistor Voltage Divider out and you will no longer have the ratio voltage drop you expect!!!

A Zener is good for protecting Digital ON/OFF inputs but not for precise measurement ADC or Comparator inputs.

[Acetronics2]

Just need to paste µChip built-in input protection by adding two low leakage diodes ( BAW62 i.e.) to clamp input voltage to the Pic supply rails ...

1N4148 also will fit for the here-required specs ...

Alain

[passion1]

Bob

I just noticed that circuit CarVoltage.jpg specifies 10nf caps while you previously said I should use a 10uf cap? Which one is it?
Thank you again.

[passion1]

> Has for the zener diode you just put the block end to say PORTA.0 and the ohter to ground.

No!

A Zener starts to draw current long before it gets to it's stated voltage... it doesn't just suddenly 'turn-on' at it's rated voltage. As soon as the Zener starts to draw current (a 5V6 Zener might start as low as 3v!!!) then it will throw your Resistor Voltage Divider out and you will no longer have the ratio voltage drop you expect!!!

A Zener is good for protecting Digital ON/OFF inputs but not for precise measurement ADC or Comparator inputs.

Melanie

How should I modify Bob's circuit CarVoltage.jpg if I should not use a Zener?

[BobP]

Thanks Melanie,

As always you are correct (damm! I would love to be a male chauvinist. But I always need a woman to keep me out of trouble!).

For cheap and effective protection I willingly sacrifice a bit of accuracy (or compensate in software). The resistor divider values can be reduced and a better choice of zenner to minimise the effect.

Alternative is to suppress the supply line and bit more capacitance on the analog input and miss out the zenner and ensure the resistor divider will NEVER give more than 5v. It all depends on the accuracy and speed you need from the circuit?

An extreme example I once had an alternator go on a company car and it pushed 18volts around. Don't know how long it was like that as it was only noticed when it managed to destroy a lot of the electronics on the car.

Bob

[Pic_User]

Excellent divide by three idea (10k 10k 10k) given by Dave (mackrackit).
How about this....? So far...
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1900&stc=1&d=118606737 9" />