Protect PIC's input against "high" voltage using AD

[flotulopex]

Hello,

I'm making a battery tester. It is meant to measure 2V, 6V and 12V lead-acid batteries and store the values for further processing.

How can I protect my PIC's input pins against voltages above 12V whithout compromising the AD's accuracy?

The problem is, if I accidentally put my probes on the wrong measuring points, I could i.e. apply voltages up to 24V or (far) higher to the PIC.

Currently, I have two voltage-dividers (resistors) when I measure 6V & 12V batts I do select manually to keep highest possible accuracy. Would be nice to make an "automatic" voltage-divider...

[Jerson]

Automatic scaling down of input? Sure. Think of a potential divider with R1 fixed. R2 values will be switched in by transistors starting with the lowest (highest measurement range) and progressively going lower till you get the best reading. Think autoranging multimeter and you'll get my drift.

[Ioannis]

and if you use logic and low Rds mosfets (Look at Vishay) then your error will be minimum.

Ioannis

[Archangel]

Hi Roger,
A side note: You may be interested, Dale Wheat did a project similar to this for Circuit Cellar May 2009 issue using AVR AT Tiny and Charlieplexed LEDs to display voltage, he used C but some pointers may be had by looking at it, He also sells kits on his Web Site.
http://www.dalewheat.com/slameter.html
ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2009/226

How about a crowbar circuit with a fuse inline with your sense line ?

[flotulopex]

Thanks for the automatic scaling ideas. At this time, this feature is a "nice to have" so I will first focus on "protection".

My current problem is to ensure I'm not going to blowup my PIC if I do a mistake.

Just see an example of batteries farms I take care of. There are batteries in three different models: 2V 1750Ah, 6V 160Ah and 12V 4,5Ah.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=3427&stc=1&d=124361125 9">

If one of my probes falls off my hand and touches any contact somewhere on any other battery while I'm doing the measurements, it can kill my PIC whithin a blink of an eye.

Basically, I need to find a way to protect my PIC (from voltages over 12V) whithout loosing accuracy during the measurement process.

The upper voltage limit is 580VDC, but it is almost impossible to be that inattentive and make such a mistake (well, I'm human so everything is possible)!

[sougata]

Hi,

I did a battery analyzer once. I would like to know the PIC your are using. BTW I have used zener on the inputs.

[Archangel]

How about a crowbar circuit with a fuse inline with your sense line ?

An inline fuse, an scr across the inputs, a zener to drive the scr gate, voltage goes too high, scr shorts inputs and fuse blows.
http://www.radio-electronics.com/inf...ge_crowbar.php
http://www.uoguelph.ca/~antoon/circ/.../SCR-Ovp2.html
http://www.electronics-radio.com/art...protection.php
http://www.ibiblio.org/kuphaldt/socr...ut/thyrcir.pdf

[flotulopex]

I'll try with a SCR then - good links, thanks Joe.

I'm using either a 16F88 or a 16F690 (I would prefer to use a 16F690 because it's cheaper than the 16F88 here in Switzerland).