Voltage monitor for car battery

[Andy Wood]

I feel like this is a circuit designed by a committee.

The idea of the 10 Ohm resistor was to put a “bungee cord” protection for the Zener. Something to absorb a small surge for a short time to soften the blow on the Zener.

I was hoping Melanie would come to my defense about the reverse polarity diode. The idea is to burn out the 10 Ohm “fuse”, if someone installs the instrument with the polarity revered. It works okay on equipment that gets installed once and stays hooked up. Not a good idea on a portable tester that has to be hooked up a lot.

If the series diode is put inline in front of the measurement “tap-off”, it affects the accuracy by the forward voltage drop. Even though this “band-gap” voltage is considered to be steady for many purposes, it does change by temperature and current flow. This is the reason the reverse polarity diode (dead short) is sometimes used.

I put Alain’s “low leakage”, good idea in too. Diodes to shunt the over voltage to plus (Vdd) and the reverse voltage is shunted to (Vss) ground (“wraparound diodes”).

What else do we need? As Dave said it is a “cost spent to cost protected” ratio fine line. But fun to discuss!
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1905&stc=1&d=118609804 3">
-Adam-

Just one more thing to be aware of - check the datasheet of your intended PIC for the ADC source impedance. Most PIC's have a maximum recommended analog source impedance of somewhere in the range 2k to 10k. To stay within these specs you will obviously have to decrease the value of the voltage divider resistances or buffer the divided voltage with an op-amp.

Regards,

Andy

[Pic_User]

You don't need me to defend you Adam *smiles* just asbestos underpants...
... then you've got a potential flame-out in a vehicle... mounted next to the flexible plastic piece of gasoline hose they've just fitted whilst parked at the filling station...

Geee... I think I missed my calling - should be writing disaster movie scripts in Hollywood...

> flame-out in a vehicle... mounted next to the flexible plastic piece of gasoline hose!

Now THAT is a second good reason to keep wearing asbestos underpants!
Does asbestos protect one from whip lashes?
-Adam-

[mackrackit]

Impedance of a DC voltage divider.

If I remember right

R or (Z) = (R1*R2) / (R1+R2)

Using the 3 to 1 example with 10K resistors - Z = 6.6K
Using the 4 to 1 example with 10K resistors - Z = 7.5K

No problem here, Besides that, I use the 3 to 1 example, It does work.

Asbestos <> RoHS Compliance

[Andy Wood]

Impedance of a DC voltage divider.

If I remember right

R or (Z) = (R1*R2) / (R1+R2)

Using the 3 to 1 example with 10K resistors - Z = 6.6K
Using the 4 to 1 example with 10K resistors - Z = 7.5K

No problem here, Besides that, I use the 3 to 1 example, It does work.

Asbestos <> RoHS Compliance

The OP hasn't specified a device. If using the popular 16F88, the maximum recommended analog source impedance is only 2.5k. It is always best to stay within the specifications of the device for reliable results.

Regards,

Andy

[mackrackit]

The OP hasn't specified a device. If using the popular 16F88, the maximum recommended analog source impedance is only 2.5k. It is always best to stay within the specifications of the device for reliable results.

Regards,

Andy

Are you sure you have the correct data sheet. Section 12.1, paragraph one in the data sheet titled PIC16F87/88 clearly states "The maximum recommended impedance for analog sources is 10K."

I do agree one needs to stay within the specifications if you do not want to see the magic smoke

[Andy Wood]

Are you sure you have the correct data sheet. Section 12.1, paragraph one in the data sheet titled PIC16F87/88 clearly states "The maximum recommended impedance for analog sources is 10K."

I do agree one needs to stay within the specifications if you do not want to see the magic smoke

Now there's a trap - the current datasheet from Microchip states 10k:

http://ww1.microchip.com/downloads/e...Doc/30487c.pdf


However, the (older) datasheet on the melabs site states 2.5k:

http://melabs.picbasic.com/devicedata/30487a.pdf


A good lesson - always ensure you have the latest datasheet!!!

Regards,

Andy

[mackrackit]

And to make sure you are not using old parts. MicroChip seems to make improvements every so often. With all of their products it is hard to keep up.

But I have to wonder, impedance being an AC term, does this apply more to an ADC source that is switching? Like an old load cell driver?

But as far as the original question on this thread, I think it has been answered and designed. Thanks to Adam.