Hobbyist seeking help with automotive ESD/transient/overvoltage input protection - Page 1
Forum section: electronics / projects

author: aabill
author_rank: Contributor
timestamp: September 13, 2023, 09:59:53 am
content: HelloI would much appreciate constructive criticism of my first attempt at input protection in an automotive environment. It is the product of much Googling and my own more general knowledge gained over the past few decades.Project is a device to take inputs from various vehicle sensors...switched raw 12v (e.g. door open)switched regulated 12v and/or 5v (sensors added to vehicle by me e.g. screenwash fluid low)analog from resistive sensors 5v and/or 12v reg (e.g. coolant temp)5v pulse trains from ignition controller (RPM) and hall effect vehicle speed sensorand send as serial data stream via bluetooth to DIY instrument panel (the latter is complete, pic attached for interest)Device inputs are Arduino Nano GPIO and CD74HC4067 module. Output from the latter will be divided down to 3.3v and fed to a Nano analog pin.Vehicle is a composite monocoque (pic attached for interest) so heightened ESD risks? It is petrol-engined with no steel bulkhead to screen ignition noise from passenger compartment. The device will be grounded direct to battery negative and powered as shown in the attached block diagram. Earth loops will be minimised but can not be completely eliminated, e.g. oil pressure low switch is grounded to engine which will be above ground when cranking. Also attached is my input pins protection schema showing a unidirectional TVS diode (with current / power limiting resistor) followed by a resistive divider (values according to each sensor's supply voltage and impedance) and a capacitor to form a low pass filter with a zener to protect against over-voltage from user error such as applying 12v to an input sized to receive 5v. EDIT: and to protect from TVS diode clamp voltage when activated by +ve transients. My understanding wrt -ve transients is that the TVS diode will clamp before the Arduino and mux input protection diodes?I hope the above isn't too garbled. Am I on the right track? Does my component selection look sensible? What might improve my project's likelihood of survival in such a hostile environment?Thank you and best regardsBill

author: jfiresto
author_rank: Frequent Contributor
timestamp: September 13, 2023, 10:39:50 am
content: The license plate identifies the car as 1987 Quantum coupe according to a site that gives the car's MOT History. You might want to obscure the plate if you do not want to spread that information.Another site states the Quantum is based on the contemporary Ford Fiesta Mark I. I don't think Ford used a central load dump clamp back then so you may have more to do to protect the electronics against that possibility. Somewhere, I may have a copy of a later Ford electrical requirements standard you can design against.

author: jfiresto
author_rank: Frequent Contributor
timestamp: September 13, 2023, 10:49:06 am
content: I foundan EMC standard to start withfrom 2009. Section D.3, "Test Pulses G1 and G2", p. 105, show what I wrote about. You may want or need to protect against Pulse G1.

author: aabill
author_rank: Contributor
timestamp: September 13, 2023, 12:05:38 pm
content: John thanks for your useful posts.The car is in the public domain so not worried about it being seen here.Although ostensibly based on a Fiesta Mk2 mine has a Zetec powerplant and the alternator is a modern unit, but I guess probably still not fitted with EMC reduction measures. I will ensure that the car is only ever fitted with an as-new battery (no chance of dead/high resistance cells) and I will not be jump-starting it nor disconnecting it with the engine running. Might possibly my schema as it stands be sufficient? I will look at the standards though...ThanksBill

author: aabill
author_rank: Contributor
timestamp: September 13, 2023, 02:26:00 pm
content: Hi JohnThanks for the link to that document. It looks a useful guide as to what levels of protection to aim for.I see that "Pulse G1" is to simulate disconnection of the vehicle battery while the engine is running. I am happy to protect my device against that risk by ensuring that battery connecting cables and terminations are well maintained and of course by refraining from manually disconnecting the battery while the engine is running!!I'd be grateful though to have comment from someone whose area of expertise this is, especially regarding component selection and sizing, before picking up the soldering iron...RegardsBill

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 13, 2023, 02:44:06 pm
content: jfiresto already mentioned it briefly, but have a look into "Load Dump" in an automotive context.

author: aabill
author_rank: Contributor
timestamp: September 13, 2023, 03:53:26 pm
content: Apology - I should have mentioned in my original post that my device is not powered directly from the vehicle battery, but via a switch-mode PSU rated for railway locomotive use and which I believe has adequate transient protection. My concern is to protect IC inputs. Nevertheless thank you for your reply - I copied your last 6 words into Google and the first hit gave me this thread containing interesting and educational links:https://www.reddit.com/r/electronics/comments/rwcm4/how_much_of_a_real_problem_are_load_dumps_in_a/

author: jonpaul
author_rank: Super Contributor
timestamp: September 13, 2023, 07:48:30 pm
content: Alternator load dump transient is 75V many Joules 100 mS.Use large die TAZ specially designed for this application.In 1980s, Motorola developed the MR2525 and MR2535 alt load dump diodes.Modern TH and SMD equivalents are available.https://www.mouser.com/datasheet/2/308/MR2535L-D-1811773.pdfThe other solutions are either more complex, less likey to suceed or suseptible to failure.Bon Chance,Jon

author: Smokey
author_rank: Super Contributor
timestamp: September 14, 2023, 12:12:21 am
content: why the 10k, 39k ratio? Are you expecting 5V signals on that input?

author: Smokey
author_rank: Super Contributor
timestamp: September 14, 2023, 12:15:00 am
content: I know you said you already have a power supply, but here is a reference for that anywayhttps://www.ti.com/tool/TIDA-01179

author: aabill
author_rank: Contributor
timestamp: September 14, 2023, 06:46:56 am
content: @Smokey those are just arbitrary values. As said in my original post they will be sized according to the voltage from and impedance of the source which will be different for each input.

author: aabill
author_rank: Contributor
timestamp: September 14, 2023, 06:55:37 am
content: @Jonpaul is it not the case that load dumps of this magnitude will only occur if the battery is suddenly disconnected while the alternator is charging at high amps? I can ensure that that will never happen.

author: aabill
author_rank: Contributor
timestamp: September 14, 2023, 07:06:24 am
content: Aside from protection against load dumps, I am disappointed that no-one has commented on my circuit layout and choice of components. Does that imply that it is good? I would very much appreciate some input.Thanks.

author: jfiresto
author_rank: Frequent Contributor
timestamp: September 14, 2023, 05:08:47 pm
content: What do you consider "good"? Good for me is a design that solves a problem, simply, economically and permanently. I would probably use half as many parts on each input, but for me to comment, I would need to know the problems. After the surprise disclosures about the modified car and the SMPS for railway locomotive use, it is probably wise to assume that I am ignorant. Others, however, may have little or no trouble inferring problems from bits of schematic, and vice-versa.

author: Gyro
author_rank: Super Contributor
timestamp: September 14, 2023, 09:26:49 pm
content: Quote from: aabill on September 14, 2023, 07:06:24 amI would always put a series resistor between the Zener/C1 and the CMOS logic input. This gives its input ESD protection diodes a fighting chance of diverting any spikes that make it that far. Cheap added protection.

author: aabill
author_rank: Contributor
timestamp: September 15, 2023, 05:35:57 pm
content: I can't see any reason not to do that. Thanks. What are your thoughts on resistor values? The abs max input diode current for the HC4067 is 20mA. That for the Arduino is not specified in the ATmega328 datasheet but is said to be 1mA. Need to bear in mind that for some of the Arduino digital inputs I intend to use with internal pullup reistor enabled which will limit total series resistance if I am to achieve logic 0 at the Atmega input.

author: aabill
author_rank: Contributor
timestamp: September 16, 2023, 10:11:55 am
content: JohnThank you for your comment and apologies for the slow reply - I have been driving back from a holiday. Let me re-state:ContextI have a data transfer device containing an HC4067 analogue mux and an Arduino. It will be mounted in or nearby the engine bay of a petrol engined self-built composite monocoque vehicle.ProblemI know that if I were to connect their inputs directly to the various sensors and switches in and around the engine bay they would be damaged by transients radiated directly and indirectly into the connecting wires by the ignition system (LT and HT), switching of high current motors and similar transients or by ESD.Solution1) Read a lot:standards revealing the levels of "interference" vehicle manufacturers design against (similar to the Ford doc you kindly pointed me tocomponent data sheetsforum discussionsetcI note that standards call for protection from battery disconnection, reverse polarity connection, jump-starting using 24v. I am happy to protect against such things by not doing them!!2) Combine the above with my own knowledge gained from experience3) To the best of my limited ability design protection circuity for those inputs4) Ask for appraisal of my circuit, hoping that someone whose field this is responds constructively.I didn't want to pre-empt responses so wasn't more explicit in my original post but here are some concerns:Have I selected the best inexpensive thro-hole TVS diode for the task?During the time it takes it to start conducting is arcing across R2 possible/likely?Is it ok to rely on it to suppress -ve going spikes or should I add a Schottky?The abs max protection diode current for the HC4067 is 20mA. that for the Arduino apparently 1mA. The final series resistor suggested by Gyro - value?C1 - what type of capacitor is best here?Over to you?Best rgdsBill

author: jfiresto
author_rank: Frequent Contributor
timestamp: September 17, 2023, 12:44:30 pm
content: I am sure you will take care not to cause harmful, electrical transients, but there is only so much you can do. Things can go wrong in a 30+ year old car that is well beyond its design life, even while in your care. Since I mentioned load dumps, fuses and fusible links can fail after extended use. Are you sure you can prevent that and what would happen if the wrong one did? You are free to suppose that you can anticipate or handle every eventuality, if it brings greater happiness, but it is often the neglect of another, by itself, minor issue that leads to the loss of a ship.I only trouble you with this because it is very likely, straightforward to protect the inputs mentioned above, against Ford's catalog of automotive, electrical horrors. Just imagine each worst case transient at the input(s) and verify it will not cause grief. Why not do that, be safe, and free the mind to think about other, more interesting challenges?For example, you will likely find that with some simple changes to the leftmost two components, you can make an input tolerate all three, longer-lasting transients that Ford specifies in section "20.0 Immunity to Voltage Overstress: CI 270" – and save some space and money. You point to this when you ask about the TVS diode. With a little effort, you could get a favorable answer and be more sure of your design. Similarly, I bet you could eliminate D2 if you were to imagine applying the test pulses from ISO 7637-2 to an input, while the 3V3 supply is under minimum draw.But so far nothing suggests that is happening. You know the car and its signals better than we do, and are in a much better position to verify your design. (We can notice if something looks off.) This may be why – other than some, terse, reflexive answers – all you are hearing is crickets.My apologies if this is an unwanted or unwarranted pep talk, but I thought it would help you more than my going silent.

author: aabill
author_rank: Contributor
timestamp: September 18, 2023, 06:39:46 am
content: JohnThanks for your comments which are neither "unwanted or unwarranted" and I will consider. My 27 yr old vehicle was re-constructed from scratch 8 yrs ago and is continuously maintained in as close to "as-new" condition as I can so although of course not infallible I choose to regard age-related faults as unlikely. I have seen Spice simulations of ISO test pulse generators connected to protection circuits but of course I can not know how similar my worst cases are to those presented in the standards. I am encouraged that no-one has said "that looks off" and I am going to proceed to construction. I will retain D2 for it's protection against an installation error - attaching a 12v source to an input whose R2/R3 are sized for 5v.Thank you for your thoughtsBill

author: Rx7man
author_rank: Regular Contributor
timestamp: September 18, 2023, 07:49:03 am
content: Some people can break a bulldozer with a rubber hammer... There's a point where it's going to be so much effort to add more input protection it may not be worth it for the cost of the processor you're trying to protect (if something gets really screwed up just put another Arduino brain in it)If you're looking to process a digital signal you probably won't want the capacitor on the input, though it's good for an analog oneIf you're expecting a 5v analog signal, I'd probably just divide it by 2 with a pair of 5k or 10k resistors, followed by a 3.3v zener diode and capacitor and that would provide enough protection.. I don't know the specs on the ADC you're using, but since it looks like you're just using it as a monitor, even a 10 bit ADC is going to give you 20mv resolution on 0-5V, and it'll be pretty safe.I've had the Arduino Mega installed in my old truck (controlling a turbo) for about 5 years now, no special input protection, and I've jump started it, disconnected the battery with it running, and it hasn't missed a beat.. If it dies, it's $10 to put another one in

author: Smokey
author_rank: Super Contributor
timestamp: September 18, 2023, 09:07:33 am
content: The cap is fine if you size the RC filter to pass whatever frequency you are targeting.http://sim.okawa-denshi.jp/en/CRlowkeisan.htm