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Old 03-31-2011, 11:44 PM   #1
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Default wanted: vvt solenoid, operational, for freeeeeeeee

Uhh, local pickup is ideal. more free that way.

this is for the good of the people. the megasquirt powers that be want to see what kind of flyback voltages are generated during coil operation.

I dont think I need a whole valve cover but I do need the solenoid and the cast aluminum housing that the oil line connects to.
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Old 04-01-2011, 12:40 AM   #2
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I can answer flyback voltage questions.

The answer is: it will fly back all the way to the output MOSFETs "avalanche" voltage.

The right approach is to CLAMP the flyback voltage with a "catch" or "freewheeling" diode - a Schottky from the Drain of the MOSFET, to 12V. The whole control system will work better. And no heat generated in the MOSFETs. Then a ~32% (at 14.4V) duty cycle will hold its position. If you need to slew current down, the L/R time constant from the coil resistance is plenty fast enough.

I mounted the diode (SB540), right at the solenoid.
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Old 04-01-2011, 11:13 AM   #3
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Jason,
Here's the active flyback they use. With the VVT holding a constant duty, things go bad. Their hardware designer suggested a 22V zener for D13 but it didn't do anything except lower the duty cycle requirements. (Note that's V V T, not W T on the upper right output)



Right now I have a 1n4001 from VVT to +12 and it "works".
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Old 04-02-2011, 12:22 PM   #4
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1N4001 will work fine - it's just that an SB540 will generate a bit less heat, which I was a bit concerned about because I mounted it in the engine bay.

Re: schematic: moving D3's cathode to 12V will do the same thing.

I highly recommend doing this instead of changing D13. That doesn't really work as you've seen, and

Interestingly the GEMS EM36 (predecessor or cousin of AEM) manual, recommends placing a "flyback" clamp diode when using an output to control a VVT solenoid, just like your 1N4001.
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