Automotive analog meaurement - Page 1
Forum section: electronics / projects

author: Bootalito
author_rank: Unknown
timestamp: January 06, 2017, 11:33:14 pm
content: Hey all. This is my first real post here and I'm looking for a sanity check and perhaps some suggestions. I'm designing a battery powered automotive data logger. I'll be using 4 of the arduino nano's inbuilt ADCs to measure and save different voltages from around a card to an SD card. My questions revolve around input protection. According to many interweb sources (here is one:automotive_circuit_protection.PDF), automotive circuits can see upwards of 120V DC for 500ms! Whoa momma! I've designed the following circuit:the four different channels will have 4, 2 position screw terminals. The negative terminals will all be connected internally through a 100mA PTC fuse. A 20.5Vwm TVSS diode will clamp to 24V max, a Zener diode will then clamp to 20V max(not sure if Zener is really needed, its there to get more range out of the ADC), then a voltage dividor to bring voltage down to around 4.8V when Zener is at 20V., buffered through a jelly bean LM324, then direct to the nano. Below is shown only two "inputs" But the other two will be connected similarly. using the other two op amp inputs.I'm really interested in ensuring I don't destroy the unit on reverse voltage or the voltage spikes outlines in the PDF above. Any comments would be appreciated.

author: vltr
author_rank: Contributor
timestamp: January 07, 2017, 02:53:58 am
content: I'm not knowledgeable enough on the topic to provide you direct input, but I would look at the schematics used by megasquirt for their ECUs:http://www.megamanual.com/ms2/pcb.htmShould help you with some idea of how others have skinned that particular cat.

author: Ian.M
author_rank: Super Contributor
timestamp: January 07, 2017, 04:29:00 am
content: If you are expecting ground potential differences, your design is fairly disastrous as the track between the two terminal blocks will melt off the board if on input's ground pin goes to +V while the other stays grounded! (Quite possible if a factory fitted grounding strap in the engine compartment breaks.) Separate differential input circuits are strongly recommended, or at least a separate polyfuse for each input's ground pin.Zener clamping isn't satisfactory for full range ADC inputs - the soft knee of the Zener characteristic curve will result in a lot of non-linearity at the top end. Instead, you can eliminate the Transorbs and increase your input divider resistors by an order of magnitude. Then the worst case peak dissipation in the 68K resistor during a 200V peak load dump will be 0.55W which a 1/2W metal film resistor will survive undamaged for tens of seconds. The worst case current that would need to be clamped is 26.4mA per input, easily within the capabilities of a diode clamping circuit to a clamping rail maintained at the +5V rail level (seehttp://www.microchip.com/forums/FindPost/634506). However as you need an automotive temperature range you need to avoid using Schottky diodes in that clamping circuit, substituting low leakage Silicon junction ones, and increase the resistor between the clamp and the output, so the OPAMP's input protection can handle the slight remaining overvoltage.Also I'd recommend using something better than a LM324, as you need a 5V single supply OPAMP with true rail-to-rail inputs and outputs. The LM324's common mode input range limit of +V -1.5V, is likely to cause you major problems.How are you powering the logger? Protecting a 5V regulator against ISO-16750-2 load dump pulses on the vehicle +V bus is one of the most challenging parts of any automotive application that needs more than a few mA on its +5V rail.

author: max_torque
author_rank: Super Contributor
timestamp: January 07, 2017, 04:05:56 pm
content: Firstly, how many of these devices are you making, and are they commercial (ie do they need to make a profit?).If it's a one off or hobby device, then you can throw more expensive / robust protection at it, protection that might not be justifiable in a commercial product (depending on the target of that product, ie MIL/motorsport /aftermarket etc etc)The second most important rule is the "be the last module to buckle" rule. This means that most modules on the vehicles supply will include some form of load dump protection in them. When they get to their limiting input voltage, they will either attempt to block that voltage, or shunt it to ground. The first module to shunt, gets the lions share of the load, and hence is most likely to fail! (or needs the largest power handling capability).Luckily, a datalogger does not need a high current supply from the vehicle, so you can probably do the "block the supply" method up to a very high voltage, before you need to buckle and then shunt that voltage to ground. A simple high voltage (say 100Vdc) transistor acting as a pre-regulator can help enouirmously to extend the operating range of even automotive qualified Linear regs (typically ~45V max Vin).it's also important to look at the supply in conjunction with your EMC mitigation requirements (if any) as the EMC input filtering can be used to help take the edge off load dump transients etcThe actual analogue inputs can be protected by significant series resistance as long as they are terminated into a suitably high impedance buffer before the actual ADC. There are some very nice single chip solutions for high common mode instrumentation amplifiers ( INA149 for example) now available, that whilst being expensive, make things very easy / simple! Some of those choices are made depending on the input range, scaling and polarity (uni or bipolar etc) requirements of the analogue measurement. I'd suggest for a "universal" logger, you allow for bipolar signals with some common mode offset and protect, at a minimum for continuous connection of any input to around three times the nominal supply voltage (ie 12v systems withstand 36V continuous, 24V system withstand 72V)The other big issue is temperature stability over the full automotive range, especially for voltage references etc. All my automotive modules include onboard temperature measurement, that can be used for thermal corrections etc