Diode Back EMF Clamp Question - Page 1
Forum section: electronics / projects

author: KvnTM
author_rank: Newbie
timestamp: September 27, 2019, 12:38:27 pm
content: Hi guys I'm new to this forum.I'm currently designing a control unit for a car gasoline engine which sits in a boat. Please don't ask why...I need to tap off the signal from the ignition coil and convert it to logic levels. The ignition coil is just a transformer without any circuit attached to it.I'm aware of the massive back emf on these coils which can kill any sensitive component.I came up with the following schematic:The spark gap is just to protect the circuit from massive voltage spikes if something goes wrong. Probably not necessary but nice to have.A Schottky diode to clamp negative spikes and a zener to limit the voltage.The AND gate is just to get a valid signal.I'm not quite sure if this would work. This is my first go on really noisy signals with potentially massive spikes.Any suggestions would be appreciated.Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2019, 02:16:49 pm
content: Mind that, if the spark gap fires regularly, it'll sap voltage from the coil and likely cause misfires.What's D4 for? D5 will conduct in the forward direction as well, and the LED won't mind a volt or two in reverse.If you have 12V available, have you considered this circuit?- Remove SPARK_GAP- Connect 1kV diode (ES1J say?) in series with X1-1 (pointing left)- Connect pullup from +12V to left of R7- Move D5 to the left side of R7- Add capacitor from R7 to GND (left and/or right side) as needed for filtering- Adjust R7 value, and replace D4 with a resistor- Remove OK1; clamp diodes and resistor dividers will suffice to get a clean logic level.Assuming the ignition coil is wired as normal, i.e., one side to +12V (or a current limiting resistor), other side to points or ECU output switch, to ground.If it's ground referenced instead (switched to +12V), flip the input diode, and move the pull-up from +12V to GND (pulling down).Note that ignition is timed to turn-off, coincident with the voltage spike. I suppose you could just as well use a large value resistor divider to sense this directly -- with say a 100kohm resistor, a few hundred volts is only a few mA, easily clamped with diodes.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 27, 2019, 02:51:35 pm
content: Quote from: KvnTM on September 27, 2019, 12:38:27 pmThat sounds like a magneto type ignition which is still common on small engines but hasn't been used on cars in almost a century. If this is the case then the primary waveform will be a negative-going pulse during the portion when a spark is actually delivered by the secondary. The same is true of "Kettering" style ignition, in which the primary is energized by the battery for some period ("dwell") before a spark is delivered at the moment the primary switch turns off (whether that switch is mechanical - points - or electronic).High energy, capacitor discharge and coil-on-plug systems can operate similarly or differently depending on all sorts of things so best to confirm what type of system is being used before going down that rabbit hole.

author: KvnTM
author_rank: Newbie
timestamp: September 27, 2019, 03:03:43 pm
content: I'm sorry if you assumed that I tap of the hv side of the ignition coil. I'm tapping the input of it. Not the hv output.The ignition isn't controlled by the ECU. It is switched mechanically. 80's Oldschool Technology. What MagicSmoker was saying.The coil is directly connected to 12v and the switch goes to ground.I put in D4 to clamp negative spikes. I thought Schottky diodes are faster but if just the zener is enough I'll remove it.I really want to keep the optocoupler in place just to feel safe because my device will control the throttle valve while idling.If something on the input goes wrong there's a higher chance of the avr surviving. At least that's my assumption. Feel free to correct me if I'm wrong.I guess the spark gap won't fire regularly on the 12v side. I just put it there if something goes really wrong. Like a insulation failure in the coil or whatever.I made some changes to the schematic. VIN is +12v-14v from the Battery.R29 pulls the input high and the ignition will pull it down. Correct?Also since it is pulled high with a 10k resistor the zener will clamp the voltage without burning lots of current if it overshoots, correct? Charging voltage is something around 14v and quite noisy.EDIT:I just realized that the pullup has to be lower bc of the led in the coupler. A 390 Ohm pullup would burn 0.5W so this is not an option.UPDATE:I reversed the connections of the coupler and added D4 for reverse voltage protection.

author: KvnTM
author_rank: Newbie
timestamp: September 28, 2019, 10:28:38 am
content: Quote from: T3sl4co1l on September 27, 2019, 02:16:49 pmI applied some of your suggestions into my schematic in the post above. Could you approve this design or is there something else that wouldn't work? Thank you so much!Kevin

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 28, 2019, 10:47:50 pm
content: Quote from: KvnTM on September 28, 2019, 10:28:38 amUnless you are paying Tim for his services you shouldn't expect him (or anyone here) to design your stuff for you, approve your design, etc.