Automotive input protection cost-down - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: May 02, 2015, 02:51:27 am
content: Is this automotive input protection excessive?It would be nice to get the component count down as i have lots of channels.Maybe remove the 220R.Any comments appreciated

author: LabSpokane
author_rank: Super Contributor
timestamp: May 02, 2015, 04:16:37 am
content: What does the 220R get you? I'm only asking because I need to do some input protection as well.

author: digsys
author_rank: Unknown
timestamp: May 02, 2015, 04:24:36 am
content: If you want serious vehicle protection, add a shottky to CPU rail+ (Anode to D1 cathode, cathode to V+) and increase R3 to say 1K0.Vehicle GNDS can do serious bad things, R3 protects if that point goes -VE (via D1). Ground loops are very common.

author: Psi
author_rank: Super Contributor
timestamp: May 02, 2015, 06:29:27 am
content: The reason i added R3 was because the TVS clamping voltage for a 5V TVS is ~8V which is too much for the AVR i/o.The 220R allows the AVR's internal protection diodes to handle this 8V -> 5V.However, to get the TVS to clamp there would need to be a large current through the 33k which seems unlikely

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2015, 06:44:42 am
content: If you use a big enough resistor and capacitor, you don't really have to worry about overvoltage or ESD -- the capacitor absorbs the ESD as well as a TVS or clamp diode (ESD of 8kV and 150pF comes down to 8V into 150nF -- you can assume the high-ohm resistor arcs over during an ESD event). You will want a schmitt trigger input; check if your MCU has this by default, and if the hysteresis is specified well enough for your application. If not, you might as well toss in a 74HC14 -- it's cheap enough.I agree with a resistor (100 ohm to 1k?) after the cap/TVS, to provide some resistance between clampy-bits and MCU-bits. Otherwise, you have two low clamping impedances in parallel (i.e., the MCU clamp diodes plus X), and the MCU gets a ton of pin current.So, the simplest "belt and braces" method would be something like, connector pin--big R--parallel C--series R--logic input.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: May 02, 2015, 06:45:20 am
content: The 220 ohm is to protect your micro-controller in the event that a power on reset temporarily has it as an output set to High,Personally i would loose the C1, if you know your never going to output high into the TVS, you can loose the 220 Ohm, and if its only high / low digital, knock the TVS down to something like 3.3V

author: Niklas
author_rank: Frequent Contributor
timestamp: May 02, 2015, 09:42:21 am
content: Cost wise I would not focus on the passives, but on the TVS. Also the capacitance on C2 could be lowered by a decade or so. I saw an issue about a year ago with 1uF ceramic with 10k series resistors filtering input signals and reference voltages on a automotive PCB. For DC and LF it was ok, but the capacitor had almost no effect at higher frequencies (>10MHz) and cause the EMC tests to fail repeatedly. A 1nF ceramic cap was added in parallel and the test passed immediately. However, the capacitor combination can have issues at frequencies between the individual self resonance frequencies.The 220 ohms resistor can also have two other benefits depending on the location and power plane sectioning. One is to limit the current going into the pin and the risk for latchup. If the power plane is split into several section (Digital 5V, Analog 5V etc) and the signal trace is crossing that gap, then the resistor could be placed as a bridge for added filtering.

author: Psi
author_rank: Super Contributor
timestamp: May 02, 2015, 09:52:46 am
content: Is there a good reason to keep the 22pF when i have the 100nf further on?Thanks Niklas, yeah i plan to lower the 100nF to 10nF on all channels (currently only the speed sensor and RPM input channels have 10nF there, the rest are 100nf).

author: Ian.M
author_rank: Super Contributor
timestamp: May 02, 2015, 10:10:57 am
content: I also believe C1 doesn't do anything useful.It doesn't need a TVS diode as a cheap Zener will be good enough for any current that R1 can pass without burning out. You should keep the 220R resistor (and ideally increase it to 1K) as it is required to steer the bulk of any overload through the Zener or TVS diode rather than through the MCU's internal protection diodes.I don't much like that circuit for a 12V digital input as the threshold for logic '1' will be far too close to 0V and it will be prone to false triggering by stray voltages caused by differing ground potentials or galvanic corrosion. A 22K resistor to ground across C2 would help, forming a potential divider that translates the levels to get better logic thresholds.N.B. Protecting an analog input that exceeds 80% of the MCU's Vdd rail voltage while maintaining linearity up to the Vdd rail is *DIFFICULT* You would need to replace the Zener or TVS diode with a precision clamping circuit with a clamping rail maintained slightly below MCU Vdd (Its a really bad idea to dump fault current into the Vdd rail with simple diode clamps, as if it exceeds the current load on the Vdd rail, the rail voltage will rise and you risk destroying every chip on your board.)

author: Rerouter
author_rank: Super Contributor
timestamp: May 02, 2015, 12:30:53 pm
content: speed and RPM in a vehicle are generally hall effect signals, so either full on or full off, but depending on the flexibility of his device it could be required to get a RPM signal off an alternator, which is +-150V with a very narrow duty cycle, or on the flip side, an inductive signal which is +-1V if your lucky, some can be a s low as +-350mV,I've been perfecting my own automotive frequency counter / generator for a little over 2 years and have a universal counter input already done, but it would be bigger than what you have there, (about 6-7 components)

author: LabSpokane
author_rank: Super Contributor
timestamp: May 02, 2015, 02:07:09 pm
content: Quote from: T3sl4co1l on May 02, 2015, 06:44:42 amI'm reading data sheets for the 74HC14 and it *seems* like the different manufacturers have very different implementation with respect to ESD. NXP bills their device as being robust to ESD while Fairchild states their device is very susceptible to ESD.Is that a fair assessment, or is it just a case of legacy "Danger Will Robinson" language percolating up?

author: Circlotron
author_rank: Super Contributor
timestamp: May 02, 2015, 02:10:30 pm
content: I'd ditch the 5V1 TVS for a 4V7 zener. If you go above rail and make the MCU input protection diodes conduct a little, even though there will be no damage, the diode current may flow to unpredictable places on the die and make the MCU malfunction.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 02, 2015, 04:46:38 pm
content: Quote from: LabSpokane on May 02, 2015, 02:07:09 pmIt depends on the manufacturer and intended application. Some devices are ruggedized against ESDs and others are not, so you need to be mindful of exactly what parts you are ordering. The ESD warnings on most chips and boards is rarely just for fun.All electronics is potentially vulnerable to ESD, especially in loose parts form. It mostly depends on how it gets delivered.

author: Marco
author_rank: Super Contributor
timestamp: May 02, 2015, 05:18:16 pm
content: What's wrong with two resistors with two 1n4148 clamp diodes to the rails in between?Handle the capacitance/TVS parts on the rails ... where they don't add a per input cost.PS. if you're really worried about someone putting enough energy on the input to significantly raise the rail voltage right through a 100K range resistor and with all the decoupling capacitance spread across the board then use a separate rail for it (the battery input works) or use a regulator which can sink current. You still avoid per input cost.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2015, 05:34:26 pm
content: Quote from: LabSpokane on May 02, 2015, 02:07:09 pmI mean, to bring hysteresis. Still with all the RC(D) stuff. Although I suppose if you want to go with an integrated solution, there ought to be an ESD rated line receiver out there with reasonable ratings (RS-232 comes to mind, but without the negative input range required).Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2015, 05:35:50 pm
content: Quote from: Circlotron on May 02, 2015, 02:10:30 pmYou don't gain much by using lower voltage zeners or TVSs. They're limited by the same physics.Note that a "3.3V" TVS has almost exactly the same peak voltage drop of a 5V part. Lower voltage (true zener) types have much softer knees and/or higher ESR.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2015, 05:46:10 pm
content: Quote from: Marco on May 02, 2015, 05:18:16 pm(Or better yet, a BAV99 or BAT54S, or an array.)But that doesn't solve the capacitance, if you need pin filtering too.Can actually be worse, because the clamp diodes should ideally have bypass caps on each (or every few), but if you skimp on that, you get a lot more overshoot on the clamping.The nice thing about really slow pins is, you can use the capacitance as a TVS, because of its sheer size. It won't do jack to surges or improperly connected signals (the "plug it into the wall" test, say?), but if those chances are very remote, it's quite a valid method.Tim

author: max_torque
author_rank: Super Contributor
timestamp: May 02, 2015, 05:58:55 pm
content: First, you need to consider two major facts which are application specific:1) What signal bandwidth do i need to support, and how does that translate into a maximum input impedance for that signal (you need to know what you're measuring, and how fast you intend to sample that signal. You also need to understand the impedance of the micro's input circuitry.2) What sort of faults can occur, that could couple malicious voltages to your input circuitry. In an automotive environment, it's not a bad ideal to protect ALL external inputs against continuous accidental connection to Vbatt etc but if you signal has a specific higher requirement (load dump, ESD, or things like ignition coil flyback voltages etc) then you need to protect for those as wellIn short, there is no "correct" answer. The more protection, the more expensive the solution, both in terms of cost and pcb 'real estate' and in terms of easily supporting high signal bandwidths etc

author: Niklas
author_rank: Frequent Contributor
timestamp: May 02, 2015, 06:28:32 pm
content: Quote from: Marco on May 02, 2015, 05:18:16 pmRF immunity might suffer from that kind of solution, especially if the input is analog. The diodes can act as RF receivers, rectify RF energy and add it as an offset on the input signal. The distributed capacitance is also separated by a lot of small inductors and resistors in form of copper traces on the PCB. The goal is to limit the noise energy as close as possible to the connector in a balanced way for both positive and negative terminals of power supply and sensor signals. BCI has a strange tendency to find loop holes...

author: Marco
author_rank: Super Contributor
timestamp: May 02, 2015, 07:12:06 pm
content: Quote from: T3sl4co1l on May 02, 2015, 05:46:10 pmBAV99 is a more convenient package, but Schottky diodes have scary max. leakage currents at high temperatures. Max is about 10x typical, ending up with 200 uA at 100 degrees Celsius for the BAT54S, the max is probably conservative and they are probably monolithic so they will probably mostly cancel out once the voltage gets a bit away from the rails ... but relying on probablies seems a bad idea.QuoteThe internal clamp diodes are close to a bypass cap, they can deal with it (same as with the non Schottky diode forward voltage peak, after another high value resistor and with some parasitic capacitance things will have calmed down by then).

author: Marco
author_rank: Super Contributor
timestamp: May 02, 2015, 07:15:37 pm
content: Quote from: Niklas on May 02, 2015, 06:28:32 pmOnly when close to the rails, but the TVS can do the same thing near the ground rail if it's not bidirectional.

author: LabSpokane
author_rank: Super Contributor
timestamp: May 02, 2015, 07:32:54 pm
content: Quote from: T3sl4co1l on May 02, 2015, 05:34:26 pmGotcha. The NXP data sheet I pulled up states 2000V of ESD protection, which I was surprised to see.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2015, 10:25:42 pm
content: Quote from: Marco on May 02, 2015, 07:12:06 pmYeah, hence the choice; schottky are good for logic level, where leakage doesn't matter much. They're usually inappropriate for analog signals.Sounds like one or both are in the OP's application, so it's good to have the option.Tim

author: Marco
author_rank: Super Contributor
timestamp: May 02, 2015, 11:20:37 pm
content: 200 uA is too much with a 33K input resistor, logic or analog. Hell, it's too much with a 3K input resistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2015, 12:47:32 am
content: Yeah, out of 33k it would be kind of a lot. Maybe OK to marginal for the "high voltage" (12V, etc.) connections.Think I usually go for 1k + 1n or 10n for that sort of thing, but that's also on things that don't need/want millisecond filtering, either.Of course, that's also at high temperatures, which might not be a consideration. Or if this is to proper automotive spec, it could easily see the nasty end of 150C or more!Tim

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 03:39:13 am
content: Quote from: Psi on May 02, 2015, 02:51:27 amIf this is for personal use then IMO yes. I used a simple 1m resistor (actually 2 470k) instead of all that in a 1986 Fiero 2 seater and it performed perfectly on a daily basis for 21 years until the car was retired as an antique. It still works for the twice a year warmup. Same resistor only circuit currently used for 80v (hundreds transient) magneto and couple other sensors in a GY6 2 wheel vehicle. Goody-goody naysayers will probably have something else to say about this.If this is for commercial use then your circuit is probably inadequate from a certification viewpoint. The main issue being one of liability and standards as discussed endlessly in the automotive megathread. IIRC record largest thread ever in this forum.

author: Psi
author_rank: Super Contributor
timestamp: May 03, 2015, 04:17:26 am
content: Thanks guys,The channels also handle 5V analog signals, so 4.7V zeners are a no go.5.1V zeners also cause too much current flow at 5V and limit the analog max to ~4.8V.I would have to use a ~7V zener.That would probably be fine as the internal clamping diodes can handle the difference through the 1k resistor (220R in picture).

author: nuno
author_rank: Frequent Contributor
timestamp: May 03, 2015, 11:36:10 am
content: I don't remember ever seeing this spec is on their datasheets, but an ATMEL appnote mentions that you want to keep current through internal pin protection diodes below 1mA.Quote from: T3sl4co1l on May 02, 2015, 05:35:50 pmSo that's the difference I saw in 3 zeners I once traced the curves. I thought it was because some were very old types, but the ones with the softer curve are in fact < 5V while the 5V1 or 5V6 in the set is comparatively much more sharp.

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 03:46:40 pm
content: The actual reason was because higher voltage "zeners" are not really zeners at all but "avalanche" diodes. A more significant difference is they have positive instead of negative temperature coefficient. At the dividing line we have best of both worlds: sharp knee and zero TC which is why 5.6v parts (6.2v with the back-to-back diode) are used in most precision references.Anyway it will be interesting to see just how much more complicated and expensive we can manage in this "cost down" effort Compared to replacing the whole mess with a scientifically optimum simple resistor that is.

author: nuno
author_rank: Frequent Contributor
timestamp: May 03, 2015, 04:52:14 pm
content: Some VI curves below just for the record.When using that protection for an ADC input, beware that as the series resistor raises, it can reach the point where the internal sampling capacitor can no longer be charged up to input voltage, on varying signals.

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 05:13:24 pm
content: Quote from: nuno on May 03, 2015, 04:52:14 pmMhz analog speeds are rare so a small cap at the input will fix those ADC charge problems. In most cases digital inputs don't even need this because the few pf inherent in input pins will filter HF noise. For low frequency interference etc software debounce/noise routines suffice.ps. Much bigger automotive issues deal with external high voltages and pulse coupling. Large physical size (high wattage) resistors and a good grounded metal case works wonders here which is required anyway for dirt and environmental protection.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2015, 06:46:32 pm
content: Quote from: paulie on May 03, 2015, 05:13:24 pmNo, he's talking about the zener going partially conductive and loading down the signal. DC effect.Quote from: paulie on May 03, 2015, 03:46:40 pmBut a single resistor won't protect against ESD.That's actually a partial lie; one can purchase 1206 size chip resistorsrated for 10kV.Needless to say, they can only be used invacuum sealed potting compound,on thoroughly cleaned boards. If your board is getting potted anyway, that's a very real possibility -- but if not, it's not much of a "cost-down" option, and you'll need to clamp the ESD somehow.Tim

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 11:55:24 pm
content: Pin capacitance and built-in clamp diodes do an excellent job on that. Assuming you don't allow long wires entering too far into the case. As far as the resistors expensive special purpose chip type would not be a consideration for me over 2 cent half watts or 5 cent one watters.I am a little disappointed there hasn't been more uproar against my methods. No personal attacks or thread reports for safety violation. Guess the old days of low post count are over.

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 11:59:52 pm
content: Quote from: T3sl4co1l on May 03, 2015, 06:46:32 pmThat's probably true. As I mentioned in my first comments here at least two or more in series would be recommended due to potential for high potentials floating around in the engine compartment.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 04, 2015, 07:35:51 am
content: One problem with larger SMD packages is the mismatch in thermal expansion between the component and the PCB laminate itself. Tests have shown that a 2512 sized resistor and its soldering will not be able to survive a full lifetime of the car, even when placed in the driver's compartment and not under the hood. It does not have anything to do with leaded or lead free solder, both alloys failed, and with several types of plating on the PCB (ENIG, HASL...). The solution is to split up the resistance into several smaller packages, typically 0805 or 1206.By only relying on increased series resistance, you will also make the signal more sensitive to component tolerances, leakage currents etc. It might be ok for a low frequency, low resolution signal, but the accuracy of a sensor will suffer and can go out out spec.