Post by thread:[EE:] Automotive interfacing questions

I have figured out the resistances from the car circuit diagram and I have turned out the first shot at that part of the circuit, I think I have the source impedance right but could someone smarter than I take a look at it and let me know if I'm ok or not... Diagram is available at http://cowshed.8m.com/car/inputs.html Thanks Anthony -- Anthony Toft <spam_OUTtoftatTakeThisOuTcowshed.8m.com> -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

>-----Original Message----- >From: Anthony Toft [.....toftatKILLspam@spam@COWSHED.8M.COM] >Sent: 05 July 2004 05:17 >To: PICLISTKILLspamMITVMA.MIT.EDU >Subject: [EE:] Automotive interfacing questions > > >I have figured out the resistances from the car circuit >diagram and I have turned out the first shot at that part of >the circuit, I think I have the source impedance right but >could someone smarter than I take a look at it and let me know >if I'm ok or not... > >Diagram is available at > >http://cowshed.8m.com/car/inputs.html > I would be tempted to use an extra resitor before the zener to provide better protection, this should survive 12V being fed directly into the circuit with no series resistance. Note the a 5v1 zener could have significantly more than 5.1volts across it at higher currents. The 2k is a compromise, a higher value would provide more protection, but the PIC ADC requires a minimum source impedance to achieve their rated accuracy. If you are using a 16F part than they are mostly specced at 10k, so the 1k resistor could be safely increased to say, 8k2. The 18F parts require a much lower source impedance of 2.5k Regards Mike ----+---- 12v | \ / Steering wheel R \ | 2k +---/\/\---+-------+---- To PIC | | | \ -+- | / 800R /_\ 4V7 --- 0.1uF \ | --- | | | ----+----------+-------+-- 0v ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= Any questions about Bookham's E-Mail service should be directed to .....postmasterKILLspam.....bookham.com. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> requires a minimum source impedance to achieve their rated accuracy. If you > are using a 16F part than they are mostly specced at 10k, so the 1k resistor > could be safely increased to say, 8k2. The 18F parts require a much lower > source impedance of 2.5k This is a 16F876-A with a _maximum_ source impedance of 2.5k, mimimum does not seem to appear in the datasheet. Should I make Vref from the car source as you described in the other post? and if I do, how will this affect the use of the comparators (or should I give that idea up)? {Quote hidden}> ----+---- 12v > | > \ > / Steering wheel R > \ > | 2k > +---/\/\---+-------+---- To PIC > | | | > \ -+- | > / 800R /_\ 4V7 --- 0.1uF > \ | --- > | | | > ----+----------+-------+-- 0v Thanks! -- Anthony Toft <EraseMEtoftatspam_OUTTakeThisOuTcowshed.8m.com> -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Normally, you can't make Vref larger than VCC. So, what you end up doing is scaling your inputs so that they go through only the 0V-VCC range. You should add your own protection diodes, to VCC and ground at the micro, and have enough series resistance from the input so that current into these diodes will be minimal even with 12V on the input. Plan for the worst in automotive applications, you won't be dissapointed. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

To all those who have commented already, thanks, I have made the changes suggested and uploaded the entire circuit diagram as it is right now. I still have a little left to do to it, but it's getting there :) Thanks to all who take the time to help a hobbyist trying to figure this world out. Anthony -- Anthony Toft <toftatspam_OUTcowshed.8m.com> -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads