CCS C Software and Maintenance Offers
FAQFAQ   FAQForum Help   FAQOfficial CCS Support   SearchSearch  RegisterRegister 

ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

CCS does not monitor this forum on a regular basis.

Please do not post bug reports on this forum. Send them to CCS Technical Support

Analogue Pin over voltage protection

 
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion
View previous topic :: View next topic  
Author Message
Blob



Joined: 02 Jan 2006
Posts: 75
Location: Neeroeteren, Limburg, Belgium

View user's profile Send private message

Analogue Pin over voltage protection
PostPosted: Tue Mar 23, 2010 8:34 am     Reply with quote

Hello,

I am working on a system that monitors the output voltage of a regulator on a MX bike.

Normally the regulator will give an output of 15V max.

So I use a voltage divider with 2 resistors to translate it to 0 to 5 V.

In some cases the regulator on the bike breaks, causing an overload on the divider input. and thus on my analogue input.

What is the standard or simplest way to protect your analogue pins from being blow?

thanks in advance,

Blob
Ttelmah



Joined: 11 Mar 2010
Posts: 19553

View user's profile Send private message

PostPosted: Tue Mar 23, 2010 8:58 am     Reply with quote

Provided the resistances in the voltage divider are reasonably large, the internal protection diodes are all you need.
If you use (say) 5KR, and 10KR, then you are comfortably below the recommended source impedance for most chips, and the existing protection _provided there is sufficient load on the 5v supply to 'sink' 20mA_, would happily handle over 200v...
If you allready have resistors in this sort of scale, and are having problems, think 'outside the box' slightly:
Are you sure your 5v supply will remain properly regulated?. What will handle HF spikes?. How much load is there on the 5v rail?. What causes the regulator to blow?. What is the nature of the generator output when the regulator blows? (in many cases, you effectively have a magnetic circuit, with energy stored in it, and nowhere for the energy to go - behaves then like a 'flyback' transformer, and voltages can go through the roof - they will rise, _till_ something dumps the energy, either distructively or safely....).

Best Wishes
Blob



Joined: 02 Jan 2006
Posts: 75
Location: Neeroeteren, Limburg, Belgium

View user's profile Send private message

PostPosted: Tue Mar 23, 2010 9:11 am     Reply with quote

Hello Ttelmah,

thanks, I now use 22K and 10K in series.
So that will be ok.

Our company is specialised in motor electronics, we see a lot of broken regulators, stators, flywheels,...

And I just want my new PCB to survive the "garage handyman's magical touch" (thats what causes the problem in most cases)

Thanks,

Blob
SherpaDoug



Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA

View user's profile Send private message

PostPosted: Tue Mar 23, 2010 11:24 am     Reply with quote

_provided there is sufficient load on the 5v supply to 'sink' 20mA_

This is an important part of Ttelmah's reply. Make sure there is something to prevent the 5V rail from rising like maybe a 5.3V zener or enough capacitance to eat the current for the length of the bad input pulse.
_________________
The search for better is endless. Instead simply find very good and get the job done.
languer



Joined: 09 Jan 2004
Posts: 144
Location: USA

View user's profile Send private message

PostPosted: Tue Mar 23, 2010 1:09 pm     Reply with quote

Keep in mind that the ADC acquisition time is proportional to the source impedance.
bkamen



Joined: 07 Jan 2004
Posts: 1615
Location: Central Illinois, USA

View user's profile Send private message

PostPosted: Tue Mar 23, 2010 10:30 pm     Reply with quote

languer wrote:
Keep in mind that the ADC acquisition time is proportional to the source impedance.


Yea, I was going to mention that too...

the higher source impedance, the longer you have the charge the internal cap that's part of the PIC's Sample/Hold.

If you don't mind that, then use the big value divider.

If not, use a divider on the front of an op-amp that's being a buffer for the ADC input.

-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D
Blob



Joined: 02 Jan 2006
Posts: 75
Location: Neeroeteren, Limburg, Belgium

View user's profile Send private message

PostPosted: Thu Mar 25, 2010 3:01 am     Reply with quote

thanks guys,

I use a decent voltage regulator to get my 5V supply. (LM2936MP)

I need this value only at low rpm, so the acquisition time is not that important.

best regards,
Blob
Display posts from previous:   
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion All times are GMT - 6 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group