SymTech Laboratories wrote:dontz125 wrote:What would the clamp consist of - simply a 1N4007 pointing from the source to the drain? Or combined with a 30-ish-volt Zener fitted opposite to the diode?
To clamp the reverse EMF spike to ~12V, install a 1N400x diode between the VND5N07's drain and +12V, with the diode's cathode connected to +12V.
Understood, and understood why it's not great - a closing time measured in minutes (kidding) does bad things to your idle.
To clamp it to a higher voltage, install a 3W (or larger) Zener diode between the VND5N07's drain and ground, with the diode's anode connected to ground. You can add a forward biased 1N400x diode in series with the Zener diode to make a makeshift TVS and protect against negative voltages.
Vaguely understood, but confusing. The Zener and 1N400x diode are mounted parallel to the VND, with the 1N400x pointed south (assuming the injector is north of the VND and ground is south) and the Zener pointed north, correct? I've been sitting here scratching my head, doing some rummaging on Google, and even doing some scribbles with my daughter's crayons - given the north-bound body diode of the VND, are these diodes actually doing anything? A flyback pulse wants to go north, right? What's stopping the pulse from simply going through the body diode and bypassing the Zener and 1N400x?
I found
this page showing snubber designs and mosfets. I realise we aren't interested in snubbers per se, but the Zener + 1N400x arrangement shown here across the coil made all kinds of sense to me.
Am I misunderstanding the flyback pulse? Is the pulse actually going "outward" from the injector / drain connection, not just "north" towards the +12v? If so, then the arrangement described by Symtech suddenly makes more sense ...
Edited to add the link mentioned in the 2nd-last para.