recirculation diode question

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recirculation diode question

Postby Keithg » Tue Sep 12, 2017 7:49 am

I am looking for an explanation as to why we do not use a recirculation diode on injectors but do on PWM idle (for example)? The MS1 v3 schematic has D5 and D7 which feed the PWM (Low Z) circuit and the Flyback clamp circuit. If these are omitted (in my build and I guess many High Z injector builds), there is no longer a path to V+ (through the PWM circuit) or gnd (through the active flyback) from the injector but only to ground through the FET internal avalanche diode. The Microsquirt and jbperf boards also do not have a recirculation diode on the injector drives. The VND5N07 is 'avalanche rated' which I understand to mean that when the voltage on the Drain is higher than the avalanche voltage rating, (release an injector coil) the internal avalanche diode conducts allowing current to flow to the Drain (ground).

I am working on a non EFI/Engine Management project and was unable to get solenoids to pull in with an avalanche rated FET unless there was a recirculation diode present and am curious as to what the EE reasoning is for not using them on injectors and trying to figure out why I was unable to get these to pull in with avalanche rated FETs.

Somewhat on topic. I hope this dopes not offend anyone as I am trying to understand why we do not use a recirc diode on injectors but do PWM idle for example.

Keith
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Re: recirculation diode question

Postby jsmcortina » Tue Sep 12, 2017 2:13 pm

A partial answer.
The recirc diode allows the idle valve to "float". However with an injector we want a rapid closing event for precise fuel control.

James
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Re: recirculation diode question

Postby Keithg » Fri Sep 15, 2017 6:17 am

James,

Thanks, that does help. I knew there was something about it I was forgetting. Still puzzled as to why I could not open the solenoid without the recirc diode, though.

Keith
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