Automotive ignition noise. - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: June 15, 2017, 01:05:43 am
content: Hi guys,This is driving me nuts and I'd like a fresh set of eyes.A bench apparatus is used for evaluation, testing and demonstration of automotive ignition coils.12-13V power is supplied, either from a lead-acid battery or high-power bench SMPS.An ATmega328P timer generates an appropriate firing waveform with varying desired frequency and duty cycle, for example 10 Hz (600 RPM) with 1 ms on-period (dwell, while the primary current increases).The AREF decoupling capacitor, ISP interface to the AVR, and reset pull-up for the AVR are missing from the schematic - yes, I know they're not shown.A spark gap (fake plug, essentially) throws the spark, and the spark gap is grounded with as small a loop as possible back to the power ground point.An ordinary HD44780 LCD is used - and there are extensive problems with LCD interference, garbled characters, and just complete blanking of the LCD display due to some sort of conducted and/or radiated interference.A metal case doesn't do anything.The LM2940 board supplies 5V to the microcontroller board, and a SMA6T18CAY TVS is also provided for transient protection, along with other components to offer a bit of robust filtering and protection to the 5V supply.12V power is supplied to the LM2940 board, and is also supplied to the ignition coils via a current-sensing shunt and current transimpedance amplifier. Battery voltage is also measured by the AVR.Two different coil ports are provided - one for a two-pin conventional coil using an external low-side IGBT (ISL9V5035). IGBT emitter to the "chassis" ground, and a 1k base resistor is used.A Bosch BIC112 (four-pin with internal IGBT) interface is also provided - the coil takes a TTL IGT signal for its internal IGBT, along with the 12V supply and ground. (Two different ground pins go to the coil - the "power ground" along with the separate small-signal ECU ground.)The 5V regulator board is the only point where the small-signal ground net joins the high-current "chassis" ground net.ZD2 and R11 are included in series with the IGT signal to the Bosch coil - without them IC4 seems to be rapidly destroyed. Not sure why, but it seems to stay intact with a bit of protection.It basically works, kind of works, but there are conducted EMI/transient issues - the AVR intermittently stops producing pulses altogether, it just stops after a little while, but it will come back after a power cycle.And there are extensive LCD issues - it gets garbled, and usually just goes blank. It just can't remain usable for any length of time.A TVS is added on the 12V net, the clean ground net is completely separate from the power ground, extensive filtering is present on the 12V rail and I'm kind of running out of ideas, nothing really works.Any ideas?

author: Unknown
author_rank: Unknown
timestamp: June 15, 2017, 01:18:22 am
content: Could you perhaps work around some of the issues by optoisolating the control circuitry from the firing circuit, maybe on a separate supply?

author: theatrus
author_rank: Frequent Contributor
timestamp: June 15, 2017, 02:22:48 am
content: What is the construction look like? How close is everything?Have you sniffed around with a naked o-scope probe around various components? That could help locate the source or at least the paths its taking. An isolated supply is another thing to try, even if it means just powering the circuit off of batteries as a test.

author: LukeW
author_rank: Frequent Contributor
timestamp: June 15, 2017, 03:52:54 am
content: What would you optically isolate, the IGT signal?

author: Probb
author_rank: Contributor
timestamp: June 15, 2017, 04:38:07 am
content: Is your board multilayer (more than 2) with appropriate power and ground planes with signal traces in the center layers?

author: Probb
author_rank: Contributor
timestamp: June 15, 2017, 04:43:49 am
content: Also, just quickly reviewing your designs, I'm not spotting any kind of common mode rejection circuitry at the supply input. This could potentially create issues with noise, especially if you're sourcing from a SMPS.

author: Unknown
author_rank: Unknown
timestamp: June 15, 2017, 05:54:51 am
content: Quote from: LukeW on June 15, 2017, 03:52:54 amRight; it just seems closest to where the big currents are flowing. Not sure what to do with the voltage sense though, depends what kind of speed you want on that signal? If not much, you could try to low pass filter it rather than solely relying on the TVS diode.

author: ConKbot
author_rank: Super Contributor
timestamp: June 15, 2017, 06:48:07 am
content: What is your output lead made of? Spark plug wires are usually resistive, or inductive. Some use carbon coated fiberglass, some use spiral wound steel or copper, some use a discreet resistor. Some spark plugs have a resistor too, so even if it's a coil-on-plug pack, it might be expecting a resistor spark plug, so EMI might be exceptionally harsh without.If you've taken measures to make sure your high dI/dT current paths (including return paths) don't get anywhere near your other components then it may be time to start whacking in some capacitance on lines that will handle it. Even a few pF can help with higher frequency inference.

author: Niklas
author_rank: Frequent Contributor
timestamp: June 15, 2017, 07:29:53 am
content: Can you provide some pictures of both the PCB layout and also the complete setup? It would help us to try to find the coupling path.

author: Circlotron
author_rank: Super Contributor
timestamp: June 15, 2017, 01:24:12 pm
content: Quote from: LukeW on June 15, 2017, 01:05:43 amMany coils have one end of the secondary terminated at the +12V primary terminal. If you make a spark to ground as you describe, the spark current has to find its way from ground, through the power source, through the positive wiring and whatever is connected to it until it finally finds its way back to the +12V coil primary terminal. The least worst thing you could do is put a 1uF polypropylene cap between coil positive and ground near where the spark strikes. Make sure the cap has as short leads as possible. A better solution is to take the spark to a wire from the coil positive terminal. That keeps the spark current away from everything. Other coils like GM LSx varieties for example have a dedicated connection for grounding one end of the secondary direct to the engine metal.

author: tszaboo
author_rank: Super Contributor
timestamp: June 15, 2017, 01:32:01 pm
content: Put the device in a metal box, the spark gap on the outside, cut a hole for the LCD. Maybe test it first, if it makes a difference. I would also use separate wiringof the power for the spark gap and the microcontroller part.

author: Theobald
author_rank: Contributor
timestamp: June 15, 2017, 02:58:46 pm
content: Hi,Did you connect a capacitor on the RESET pin ? What's the value of the pullup ? Must be a low impedance or the µC will reset with sparks.What's the unused µC pins state: output @ low level ? Very often the default state is input, not the best choice for EMC, if they're unconnected.Did you try ferrite clamps on the both side of the coil control cable ?By the way, spark is the best EMC generator: short time, high voltage, high current -> high energy wideband spectrum, Paul Dirac is watching you...Theo

author: HighVoltage
author_rank: Super Contributor
timestamp: June 15, 2017, 04:24:56 pm
content: Quote from: LukeW on June 15, 2017, 01:05:43 am1. Use a spark plug with high internal resistance of 10 to 15 kOhm2. Use a spark plug wire with either a high resistance carbon conductor or even better a high resistance spiral core conductor that has also some inductance3. Make sure you have no additional gaps or micro gaps anywhere on the high voltage side4. Keep the parasitic capacitance on the secondary side as low as possible, (Spark plug wire does not touch anything else)5. Choose a coil with a very slow discharge time6. Add a 10 uF capacitor between the plus 12V supply at the coil and ground7. Add a 0.1 uF capacitor (1000 V) between the plus and the negative terminal of the coilThis will most likely solve all your problems.

author: floobydust
author_rank: Super Contributor
timestamp: June 16, 2017, 08:12:06 pm
content: I'm not sure what load you are using on the coil secondary- an open-air arc with solid wire radiates a lot of EMI, including the ground return. Best is SAE (J816?) load of 1kV TVS string with 100R series resistor.The IGBT collector is a very noisy trace with 350V radiating RF. It should be far away from everything and not possible to breakdown to PCB with poorly spaced ground pour.MCU crystal traces critical and should be short with good ground pour shielding, and far away from the IGBT.Recommend putting a RC snubber across the coil primary 10nF/1kV+330R 1/2W to lower the EMI frequency and protect the coil against high dV/dT which can arc internally.Coil +12V line could use HF bypass like 10-100nF to a "noisy" ground.ZD2 at 3.9V looks too low to give the IGBT sufficient drive and overloads IC4. IGBT would really like 5v. Suggest 5.1VI have done several ignitor designs. They are not easy.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2017, 09:37:22 pm
content: Riddle:When is the schematiccorrect? During design? In simulation? When constructed?Answer:Only simulation.For all other cases, layout is critical, and the schematic is nearly useless, with respect to RFI. So let's see that layout.Tim