microsquirt tach input from negative side of coil

[jkeyzer]

Hi there,

I am setting up a Microsquirt fuel-only system on a car that has a Mallory optical pickup module driving the negative side of an ignition coil. For my tach signal, can I connect the OPTOIN+ directly to the negative side of the coil, and OPTOIN- to ground? Do I need to add any special diodes or resistors? I saw that the V3 board has notes about a Zener diode and changing some caps for coil pickups, does Microsquirt need this modification or can I use it as-is?

I was not able to find much documentation for this setup online as it seems that most people are using trigger wheels and noone is using a Kettering style setup for fuel-only anymore!

Thank you!

[SQLGUY]

There's another discussion currently going on about just this topic: http://www.microsquirt.com/viewtopic.php?f=87&t=23033

Cheers,
Paul

[jkeyzer]

Thanks for the pointer to the other thread.

In my case the Microsquirt is actually being damaged by the coil. I have had R20 (the 680 ohm resistor in series with the OPTOIN+ line) fail open twice now while trying to start the car. The car actually did run for a few seconds before it died, and wouldn't restart. The symptom is that instead of measuring 680 ohms acoss R20, I see 40k-1Meg Ohms. I replaced the resistor once and it failed almost immediately after trying to start the car again.

Is Microsquirt not designed to be connected directly to a coil? Should I be adding a protection circuit, or is there a cheap box I can add to filter the coil tach signal before it connects to the Microsquirt?

It's possible that the coil is bad and is sending high voltage spikes back to the OPTOIN+ line - this is the only way I can imagine a surface mount resistor could fail.

Does anyone have any experience with failure of the tach input circuit, and how did they work around this?

Thanks!

[SQLGUY]

If the ignition is on but the engine is not running, and you're running the switched side of the coil to OPTIN +, with OPTIN - to ground, you have the optocoupler permananently on. That's dropping about 1.2V across the opto and about 12V across the resistor. 12V across a 680 ohm resistor is .212W. I don't know whether that's a 1/4W or an 1/8W resistor, but even if it's 1/4W you're kind of pushing it at that point.

The setup in the GIF at the the bottom of the other thread "What I suggested to pommie" will only drop 12V across the resistor when the coil is charging, which should be considerably less than 50% of the time, thus reducing the power dissipation of the resistor to at most a tenth of a watt.

[jkeyzer]

That's a good point!!!

Doesn't this apply to other ignition inputs on OPTOIN+ as well, such as hall effect or optical triggers? (Unless you get lucky and the trigger is closed, which would heat up the coil instead.)

I just checked to see what would happen if you exceed the power rating of a 680 ohm 1/8W resistor, and I was not able to get one to fail, even at 50mA current, at which point the resistor started smoking, but held it's resistance value. R20 has no visible damage on the Microsquirt when it fails. So I don't think this is the same failure mode, unless the flyback pulses on the coil are sending huge surge currents through the resistor and blowing it without visible damage. But frankly I'm surprised that would blow the resistor before the optocoupler. In this case the optocoupler and the series diode are ok, it's the resistor that fails each time!

[newtyres1]

Please don't bust this thread up into other variations like "is Dave capacitor ok for Microsquirt ign trigger?" How about just keep it under the present title for all future uS users doing fuel only as a single reference point.

Ian.

[jkeyzer]

Ian,

Sorry about that, but I thought that the Dave capacitor question was a totally separate topic and didn't belong in this thread.

This thread is about my problems using the existing OPTOIN circuit on the negative side of a coil and having R20 fail open circuit.

The other thread is a question about whether the Dave capacitor will affect the trigger accuracy of OPTOIN for fuel+spark ((http://microsquirt.com/viewtopic.php?f=89&t=23039)

I think these are two completely separate questions/issues, is it ok for me to have two threads?

Jeff

[newtyres1]

Sorry mate, you can have as many threads as you like, I just thought the current title would cover it all. Searching does not always get the results you want...

The pulse from the ignition coil negative terminal when the plug fires is around 300V. Hopefully more experienced people will chime in and answer both questions.

Ian.