A 10-20 ohm injector will draw about 1 to 1.5 amps. No resistor is required for this. Use an N channel MOSFET switching the ground side of the injector. Be sure your MOSFET has a high enough Vds to survive the back emf pulse from the injector. Be sure to use a good gate driver scheme to overcome gate capacitance which will allow fast MOFET switching.
It's correct the way it is across the inductance. When the current is interrupted through the inductor it allows a current path around the inductor and clamps the BEMF voltage spike to the forward voltage drop of the diode. The result is that the energy in the BEMF can dissipate as a low voltage, high current surge rather than a low current, high voltage spike.
You're thinking of an H-bridge
No H-bridge thoughts here. The problem with using the zener as presented, or a Schottkey is that it will slow down the closing of the injector. A properly placed zener of correct value will provide for pretty quick closing and pretty good back emf clipping at the same time. Installed as shown, the coil back emf will be 0.7v and Vz will never enter the picture. Installed the other way around, the 0.7 Vf of the zener will allow 12v to bypass the coil and go to ground through the MOSFET releasing smoke.
The rise time of the injector coil and the fall time of the coil have a large bearing on injector on time. Not much can be done about the rise time, but the fall time is controllable.
Why can't I post a normal reply?
If you feel it's necessary go for it. Just make sure to use non-inductive, surge-resistant resistors and capacitor types. Good common candidates are NON-spiral cut carbon composition resistors and ceramic capacitors. There are of course, more better and much more (and I do mean much more $5-$25 a pop) expensive types like bulk ceramic resistors and polypropylene capacitors specifically designed for the job. And minimizing trace lengths and loop areas for fast acting circuits like this (both snubbers and flyback diodes) is critical if they are to do their job.Back to the heart of the matter, because it is one device connected to one NMOS, I should have a total of 5 clampers/snubbers or just clampers. Because I know the requirements for the spark plug snubber, I'll probably keep that one along with the clamper. It might be best to have both because of the fact that an ignition coil can reach 60kV, and regularly does. Do you think that both are necessary? Speed is very important to me. If you think both are necessary, I could use the same components for both, because the ignition coil snubber/clamper has to slow/clamp a potentially higher voltage (no pun intended). The parts are also pretty cheap.
Sounds like you sorted that out on your own.If this clarifies anything, I was basically asking if I could use 1 snubber/clamper for all 5 NMOS circuits and connect it in parallel. Then I realized I couldn't because of the way it's connected.
Also, what do you think about this. I currently have two NMOS controlling two devices (both fuel injectors) but I want them to be synchronized. Instead of using my microcontroller to synchronize two outputs, could I connect it like this: +12V--->[+](fuel injector 1)[-]--->[+](fuel injector 2)[-]--->[drain]NMOS[source]--->Gnd
/--->[+](fuel injector 1)[-]---\
+12V------ ----->[drain]NMOS[source]--->Gnd
\--->[+](fuel injector 2)[-]---/
.
Also, what do you think about this. I currently have two NMOS controlling two devices (both fuel injectors) but I want them to be synchronized. Instead of using my microcontroller to synchronize two outputs, could I connect it like this: +12V--->[+](fuel injector 1)[-]--->[+](fuel injector 2)[-]--->[drain]NMOS[source]--->Gnd
My only concern is voltage drop across fuel injector 1 and the trace width requirement for handling . I know each will require less than 1 amp, so a total (maximum) of two amps will be flowing. Using an online calculator, I think I should be fine with 1.02mm traces (I know, I have a lot of overhead here), but will voltage drop be a concern. The injector's resistance is between 12 and 20 ohms.
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