nhromyak

I may end up leaving my 7730s behind and just go with a 7747 (this is speed density, right?).

But, I want to run the four Holley injectors, even though I fear they are too small for my motor.

I understand the 7747 can NOT drive all four of them. What can I do?

I could build a box, if it is relatively simple, and I can find all the parts and part numbers.

Or if I must, and probably be easier/quicker would be to buy one. But I don't want it to be much more than what the 7747 will cost.
Where to get one?

Thanks!

jwscab

you'll need an external injector driver board. You can build one if you have some soldering skill, using the following parts:

LM1949
appropriate transistors, as recommended in the data sheet.
some resistors, caps, etc.

this is a national semiconductor part, which is the same controller thats actually used in the 7747, peak and hold controller. available from www.digikey.com i think it might cost you about 20-25 bucks total for the parts.

you can find the data sheet at www.national.com , follow the design suggestions, and you'll do just fine. there are examples in the data sheet.

one thing you need to decide is how you want to approach the design. I made one that had 4 injector drivers, in case I wanted to drive 8 low impedance injectors, but you can use just two for 4 injectors, by sizing the current sense resistors and transistors.

to interface to the 7747(or any ecm), I had a 1K ohm resistor tied to the driver output pin, and to +12V, to provide the necessary pullup to get the waveform(its like a 'fake' injector). I put this through an inverter, then used the inverter output to the input pin of the external driver.

hope that helps. If you get stuck, I can try and dig up some schematics or something to give you a better picture.

you can also search on the boards, there is some info here and there(probably an old post of mine too).

good luck!

nhromyak

Thanks for the reply.

A few questions, and a confirmation or two...

I see digikey sells each lm1949 for $4.00 plus some caps and resistors as outlined on NATIONALs datasheet, correct?

I can read it, hopefully, I can build it, this is where I need some caps and resistors on their data sheet - correct?


I am not an E.E. How would I size the current sense resistors and transistors for my four injectors?
Perhaps I should simply build four drivers, and have the one of the 7747 outputs go to two of these drivers, then the other one 7747 injector output go to another two drivers?

I can do that.


Huh? an inverter? Why? What does it do? :hail:

yellow73bb

When I get home from work, I'll send you schematics for one that I built a couple years ago. It's pretty simple. I'll also include the board layout files in case you want to get a board made.

jwscab

OK...gotta step through this so bear with me if i miss something.


yep, about 4 bucks each, plus the extrnal parts as well, if i remember correctly, its like 2 resistors, and 2 caps, but its been awhile.

yes, these are the parts i was talking about, each driver needs the external parts. you could use some perf board and wire everything up. I used a type of board that isn't easy to get, but its just like perfboard.

yes, each 7747 driver can drive two other drivers, thats how I wired up my board. this maintains the alternate firing for a pair of injectors. The data sheet spells out how to select the values, it makes the sense resistor measure 400mv across it at peak, then 100mv at hold currents, so the very first typical application is for a 4/1 amp peak hold circuit, which is exactly what you need. the resistor is a .1ohm resistor, high watt. the transistor will be fine with the one in the drawing. read the application hints and it shows you a very good technical detail of how the injectors operate.

ok, so, the reason for the pull-up is that the ECM output is a current output (peak n hold), and 'grounds' the injector to make is shoot fuel, so you need a resistance there to generate a voltage swing. So you wire up a nominal resistance that draws only a small current. The problem with doing this is that this signal is now inverted compared to what the input needs on the extrnal controller chip(when you want the injector 'on' the voltage needs to be high, or +5 volts.) I can explain more, but i hope I didn't lose you so far. but basically, the inverter flips the signal over so its right. oh, the inverter is a 74hc04, hex inverter(6 in a package)

hmm.....now that i think about this, I actually tied the output to a 5 volt pull-up, then into an inverter. you need to supply the LM1949 and 74hc04 with a 5 volt supply. I used an lm7805 to do this, which is a 5 volt regulator. this uses +12v from the battery.

ok, so i probably lost you in all this. if not, great! I do have some schematics for what I did but i will have to find them. if you can wait a couple of days, I can send em to you. PM me or shoot me an email at jwscab@comcast.net, and I can send em to you. I'll have to find them, then scan them, so again, it might be till monday.

i've been trying to make up some boards, but its a back burner, plus, I haven't even gotten the one i built in a running car yet.

jwscab

whoops, I see yellow73bb beat me to a reply. he's the guy who actually had some real boards built. If you still need more info after he sends you his stuff, jut let me know.

jwscab

didn't think i was crazy....I did post my schematics awhile back in this thread:

https://www.thirdgen.org/techbb2/sho...ghlight=lm1949

nhromyak

Kewl!

Thanks John, and JWSCAB!

I wonder where I can find someone to make a PCB for me now... hmm...

Or someone to build me one.

yellow73bb

I've used apcircuits
and
futurlec with good luck. Not too expensive. Just bare boards though.