Injector Driver Module

[Jose Rodriguez]

Orin,

  I also agree that reducing the power supply voltage would help alleviate the heat dissipation, but there are some challenges there too:

  a) If the voltage applied to the injector's coil is reduced, say, to 3.3V, then the power dissipation is reduced by a factor of at least 4. That is the good news.
  b) The bad news, is that with 3.3V we will not have enough current to quickly turn on the injector coil.
  c) So, if we want to have the best of both worlds, then we need to supply Battery voltage for a brief moment, to quickly turn on the injector, and then supply 3.3V for the hold current.
  d) So the injector driver module would require to supply 3.3V (worst case 8 amps!), and briefly supply Battery voltage (4 amps per injector). Not an easy task. I calculated that the total power dissipation using this scheme would be around 22watts.
  e) So, thermally speaking, this seems like a feasible solution, although complicated and expensive.

   I can see now why most of OEM injector module designers would want to use saturated injectors, and not peak and hold!

>>> <Don.F.Broadus at ucm.com> 08/11/98 07:58AM >>>
Orin, You are absolutely correct on both counts. What do you think about
using a switch mode power supply PWM chip ?
         Some of those current mode control chips  unitrode UC3842  might
work. Thanks for the good info. 
 
Don. 

	-----Original Message-----
	From:	Orin Eman [SMTP:orin at wolfenet.com] 
	Sent:	Tuesday, August 11, 1998 3:58 AM
	To:	diy_efi at efi332.eng.ohio-state.edu 
	Subject:	Re: Injector Driver Module


	> Orin,  According to  Haynes techbook, fuel injection 1986-1996
(10220) "The
	> duty cycle is the ratio of injector on- time to total on-and- off
time. In
	> other words, it's a variable % of one complete injector operating
cycle" .

	Isn't that what I said?

	> know what rpm and what pulse width the on-off times will be. I
still believe
	> the switching transistors voltage drop (Vsat)  Frequency of
operation  and
	> current through the transistor will give you the heat that needs
to be heat
	> sinked. 

	No doubt about it without using PWM... which then has the EFI
problems.
	Once you get to the 'use an inductor internal to the unit' ideas,
	you are getting close to making a switch mode power supply... you
	might consider making one to generate a voltage which would minimise
	the dissipation in the transistor.

	Orin.