Reducing Oscilloscope False Triggering from 24VDC Automotive Coil Signal Noise

Hello. 

I have a 24VDC signal from a couple switches in series that drives a interposing relay to ignite a spark plug from a large inductor and capacitor (essentially a battery). Long story short the signal is vey noisy.  I have tried several RC circuits to reduce the electrical spike.    The problem is that I'm using this 24DC signal to arm an oscilloscope on the external trigger function.  I always get false triggering. It was originally a 16 Vdc spike with a very short tc. Probably less than 1ms. I have been able to reduce it to approximately 2 Vdc but I occasionally get a spike about my trigger threshold of 2 Vdc.  The scope immedance is 1 M ohm. I have thought about trying a Schmidt trigger or opto isolator. Any help would be appreciated.  If at all possible I wanted something passive.  But at this point desperation is setting it.   I have pics available too  

Thanks in advance. 

Greg. 

You are possibly having a problem with your "grounds". If the 24V DC system is derived from the mains, and the scope is plugged into the mains as well, you can get a large ground loop. If that is not the case, you still have to arrange the grounds carefully, preferably in a star arrangement, or use a hefty piece of copper sheet. That won't help with radiated emfs, which could be significant. If the ground lead on your scope forms a large loop, you need to wrap the scope probe ground around the scope probe tip and keep the loop area as small as possible(if it is long). Some scope probes come with a small sprung ground lead that you put on to the end of the probe when the retractable hook assembly is removed. An opto coupler can be useful for getting rid of the problem of grounding, but you still can get capacitive coupling problems across the barrier. Since it is only for the trigger circuit, the non linear behavior of an opto coupler shouldn't be a problem. Have you got snubbers/diodes on the relay coil to deal with the back emf. The relay contacts would serve the same purpose as points, which normally have a capacitor across them as well, to reduce arcing,
Cheers,
Richard

Hi
Greg Law,

Yes it is the problem of ground as mentioned by Mr Richard Garbic. I would
suggest you to provide a 33 ohms 2 watt resistor in series to the earth input terminal
the other end of the resistor should be connected to the chassis of your osc scope,
this resistor will act as an earth loop circuit and when noise is developed a voltage
is developed across the resistor and and slowly it is grounded to the chassis of the
equipment this prevents the noise from entering into your signal side of your scope.
i have tried this in amplifiers to avoid hum and spikes, since you are using inductive load provide diode and cap to prevent back emf provide a 47 mfd cap of 100v across the rly coil cap voltage is rated high since high ac voltage can occur during make and break of rly. Provide 0.1 cap poly in the in put and out put of your power supply . Hope this will help you.


Regards V.Sambath kumar.