quench diodes across low impedance injectors?

[mlmccauley]

I'm hooking a 2.2 system to a GM TBI unit with two low impedance injectors.

After reading all the stuff about controlling inductive kick (flyback board, resistors), it came to me that maybe a good thing to do would be to connect a fairly hefty diode across each coil, physically close to the coil, cathode to +12.

This is a simple, cheap technique that is often used on inductive loads to control ringing.

Is this a good/bad idea? Anybody ever try it?

Would this eliminate the need for resistors or the add-on flyback board? Seems like it would, and even eliminate the need for PWM as well, given the rating of the IRFIZ34N.

Thanks in advance for the help!

Mike

[Philip Lochner]

The short answer is to rather get the flyback board and use it or go with a V3 board and use the PWM.

There's also those peak-and-hold cirtcuits someone is selling...

[IridiumGti]

That is how its done for relays, but you would need very hefty diodes to dissipate the heat because of the current involved. Also switching times may affect the operation of the PWM.
It looks like MS uses a transistor to quench the back emf, allowing the zener to have an easier time. When the back EMF overcomes the zener voltage, the transistor switches on and absorbs the energy.
No doubt, using beefier components on the original board should work better than adding another. For example why not use the tougher TIP32C ( as used in the add-on flyback board )for the quenching transistor instead of the TIP42? The IRFIz34 can handle 20-29A continuous, 100A peak but can only stand 55V. So if the quenching tranny blows, so will they. Can anyone come up with a good reason for not just replacing the quenching transistor with a better one???
Interestingly, the add on flyback board circuit
http://www.megamanual.com/v22manual/minj.htm#fb
seems to have an error. How can the collector of the darlington go to +12V? This should be grounded for a PNP transistor. Also the darlington is what switches the injector on, but it is only rated at 5A! The TIP32C doesn't drive anything, it just absorbs the back EMF from the solenoid.
Perhaps someone who knows electronics better can explain this.

[Heribert]

If you use a diode with a sufficient W-rating it will work.
However, the diode will increase the injector closing time wit up to one millisecond ( or even more) which will lead to problems if you need short short injector opening times .

Use a series resistor to effectively convert the injector to HiZ and std MS flyback control

Or use TWO diodes to replace the MS flyback. One 1N4002 with the cathode connected to +12 V injector feed. Then you wire a 2 W 35 V zener so that the anodes of the two diodes are connected together and the Zener cathode to the junction between the injectors and the MOSFET. Supersimple , superfast closing of the injector and the flyback current is not routed out on the 12 V supply.

But it wil only work on Hi Z or LoZ+series resistor config.

If you run loZ in PWM mode you have got to have the full MS flyback kit

Heribert

[Philip Lochner]

Use a series resistor to effectively convert the injector to HiZ and std MS flyback control

Although an engine will run fine using this approach, I do not recommend this unless the application is a racing car or very high performance car where economy is no issue. My reason is as follows:

Fitting a series resistor and using HiZ switching results in the opening times of the injectors increasing. This is not a major issue as such but the important thing is that the DIFFERENCES between opening times of the different injectors also increase. At small injector PWs (just a a little more than the actual opening time) , this results in great differences in AFR between the different cylinders which could cause rough idle, and the inability to get leanish AFRs at low MAPs (<50kpa). This effect also gets worse the more frequent injection scheme you use (eg using 4/simultaneous as opposed to 4/alternating).

[john.p.clegg]

I do not wish to hi-jack this thread but it seems relative and pertinent,along with the combined knowledge of those involved maybe I will be enlightened.....

I am at this point wiring the injectors,ISCV, wasted spark coil and fuel pump (ford connectors) and fitting in the connectors 12v LED's to act as tell tales....
Now this seems like a good idea to me,especially to help with fault finding...
But

Will doing this affect the operation of said components?


Many thanks
John

[john.p.clegg]

Dear All

Just to answer my own question,the LEDs were a good idea and they lasted a couple of seconds,suppose the back-emf was too much for them....ah well back to the drawing board....

John