Analog input protection for automotive - Page 1
Forum section: electronics / projects

author: dmg
author_rank: Regular Contributor
timestamp: January 11, 2016, 06:40:04 pm
content: Hi,I need to read NTC temperature sensors for an automotive application. It's my first automotive design and I'm a bit clueless about protection circuitery neccesary for such an input. I'm aware of protections required for events such as load dump in power rails and I'm also aware that any input/output coming from the device needs to be protected against short to ground/battery and all sorts of abuse, as the person installing the thing might connect anything to that input. I've also read several posts by free_electron on this forum that talk about the subject, but I'm still lost, seems like there are many possible solutions for protection and everyone is trying to sell theirs, then you pick one, analyze it and when it looks reasonable you find someone who tells you why that is crap with good enough arguments, and back to step one. As I have zero experience in automotive (my field is RF), could you please guide me on the design of the input protection?I'll use the ADC of the microcontroller, buffered by an opamp which will also low-pass filter it, and I thought about using a simple resistor divider for reading the NTC. It's a 24 V system and the design should be fairly low-cost.Thanks in advance.

author: Ian.M
author_rank: Super Contributor
timestamp: January 11, 2016, 06:51:50 pm
content: Isolated sensor or grounded to the chassis or engine block?

author: DutchGert
author_rank: Frequent Contributor
timestamp: January 11, 2016, 08:22:26 pm
content: A (pulse-proof) high value/high power resistor (to limit current flow when connecting it to 24V) with clamp diodes to GND and the positive rail should do the trick. You could add a cap between the resistor and the clamps optionally

author: dmg
author_rank: Regular Contributor
timestamp: January 11, 2016, 09:09:10 pm
content: Quote from: Ian.M on January 11, 2016, 06:51:50 pmIt's isolated, there will be up to 5 of these sensors, to save connector pins I planned on providing a common analog ground pin to all of them and then an input pin for each one. There will be an internal pullup to form a voltage divider and that will be read by the microcontroller's ADC. That pullup will be tied to a 3,3 V or 5V rail which I suppose I'll have to protect too, as it will have to take energy from the clamps without getting upset. There I think I could use a prebiased zener or something like that, but I'm not sure if I'm going over the top or not.The problem I have is that I don't have a clear picture of what do I have to protect against. I know that people will mistakenly connect 24 V to the inputs and I have to clamp it to protect the opamp input and I'll also provide ESD protection (a cap in paralel with the NTC). Also, being something for a 24V system (a bus) I know I'll have high energy transients to deal with... so what-if sittuations arise and I don't feel like adding beefy 600W capable load-dump TVS's on each input/output/whatever. Also, most information I find is for 12V systems...

author: Ian.M
author_rank: Super Contributor
timestamp: January 11, 2016, 09:24:24 pm
content: Consider the scenarios of:* a flashover from an HT lead to your sensor wiring.* a short from a high current 24V line to any of the sensor wires, INCLUDING GROUND.Neither case can be permitted to dump significant amounts of energy into your MCU's supply or ground rails. I'd keep the sensor ground separate, possibly with a polyfuse between it and true ground, and shunt regulate the sensor supply with a beefy Zener, again with a polyfuse between it and the sensors if the sensor supply is exposed at the connector, or if the pullups are low enough value for the current through them if there's a short to 24V to be a problem. The sensor side ground and pullup supply after the polyfuse would be buffered by well protected OPAMPs with large input resistors and used for the ADC + and - references.

author: Dave Hedgehog
author_rank: Newbie
timestamp: January 16, 2016, 10:05:49 pm
content: I repair a lot of 24v automotive stuff. A common input setup I see is:The sensor ground is connected to chassis ground through a 2x2 polyfuse. Usually one polyfuse per device (sharing about 5-8 analogue sensors)Inputs go through 10k resistors, then are decoupled with (IIRC) 1nF down to ground, then through another 10k or so resistance before being clamped with a suitable zener (usually a zener array) before they find their way into the op-amps.The sensor supply is nothing special, just a current limited emitter follower. No TVS clamp or anything.This seems to work well in practice. I've repaired thousands of one particular item over the last few years (universal input/output node for a bus/coach) and maybe have had approx 60 with a blown open polyfuse from serious wiring errors. 15 with a shorted decoupling cap, 30 with a shorted zener array and a few where the 10k resistors had failed open.I've never had to replace an op-amp, which are just jellybean 2902 stuff.

author: max_torque
author_rank: Super Contributor
timestamp: January 17, 2016, 02:21:21 pm
content: First, work out what is the maximum series resistance that you can put between the input and the outside world! If you have a high impedance input buffer and a low frequency signal (like an NTC, which has a 10-90 time in the order of seconds!) you can filter the crap (<<< technical term) out of the input without degrading your signal integrity! And, with a high input impedance, you never need to shunt much energy away from the input in the case of accidental over voltage of the input. Typically, i'll use up to 47k on thermistor inputs, with either zener or diode clamps, depending if the input needs to be rail-to-rail etc. If you use diodes, you'll need a second series resistor to limit current in the inputs internal clamp diodes, typically between 1k and 5k.

author: Smokey
author_rank: Super Contributor
timestamp: March 20, 2023, 04:16:46 am
content: What about for something faster than an NTC? What about the potentiometers in a modern accelerator pedal?

author: Smokey
author_rank: Super Contributor
timestamp: March 20, 2023, 04:33:08 am
content: This Reddit post has some interesting information:https://www.reddit.com/r/AskElectronics/comments/9twh4p/adc_for_automotive_applications_esd_and_emi/