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Old 10-12-2010, 07:09 AM   #108 (permalink)
I have to start over?
Join Date: May 2010
Posts: 214

Big inefficient truck - '94 Dodge Ram 2500
90 day: 12.1 mpg (US)

Honda Civic - '84 Honda Civic DX Hatchback
Thanks: 2
Thanked 8 Times in 7 Posts
Originally Posted by PaulPaulJones View Post
I'm new here, so a little patience and any feedback is welcome.

Having said that:
Seems like you want a constant voltage out for any voltage in above X?

An LM317 (3v drop/used) or an LTC3080 (0.3Vdrop/used) should work, possibly with a small input smoothing capacitor.

Basically, feed it up from 3.7V to ~40volts and it gives you whatever smaller voltage you want out down to about 1.2 volts.
The lower the voltage difference from input to output, the better, and the LM317 uses ~3.7V internally. Effectively, if you set the output to 8.3V (12v-3.7v), anything below 12v in would result in no voltage out.
You do need an output load using at least 10ma in general for the lm317 to work correctly, a nice LED should work well here, possibly in parallel with the inj input.
Two resistors are used to set the desired output voltage. And yes, you can use a variable resistor for fine tuning.

Response time on an lm317 averages about 20 usec, and you also get a 65db input ripple rejection. Seems like it would be easy to account for the response time either during calibration, or in code itself.

Heck, you might even be able to use a simple 7805 (5v regulator) and dispense with the resistors. Average 7805 uses/drops 2V internally, so for anything OVER 7V, you get 5V out, anything less and you get 0. An in, an out, and a ground, doesn't get much simpler than that.
Again, a little calibration or code could make up for response time, or whatever smallish time is spent between 0V and the 7V cutoff.

Cost for a 7805last I looked was about $1.30USD at radio shack, lm317 was about $2.19USD.

You can also use an LM317 as a constant current regulator with a single resistor.

Another possibly useful idea is to use a C form coil for a current tap, think of a toroid (donut) shaped piece with a slot cut in one side. Now, wrap wire from one side of the C around and around to the other side, ;eaving the slot open. Use a plastic or rubber form to avoid making a choke (basically, a capacitor, except for current instead of voltage) to avoid ringing artifacts. Hmm...might even be able to use a choke as a very basic 'smoother' for the input spikes.
Now you have a 'slip-on' pickup so no wire cutting, or even splicing.

Anyway, just some ideas to chew on.

Like I said earlier, I'd really like feedback on these ideas.
Thanks for the ideas, I'll put a few neurons together to try to implement... eventually. I'm still looking forward to having an MPG readout! Thing is, it would take me a year to calibrate at the rate I've been driving my jeep lately

As a side note, I don't mind cutting and splicing wires under my hood. The wiring under there is so ugly with so many unused connectors and whatnot, that I don't care what it looks like. Besides, I already have the injector in series with the kill switch.
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