Question #1. Yes. The Vref is the - input.

OK, just had to confirm - it wouldn't have been working otherwise.

Question #2 Yes. 10 meg. That's my best guess of a positive feedback value for between 4-5 mV of hysteresis.

Yeah, you have to be careful with the hysteresis. If you try to use too small of a hysteresis resistor with a comparator, things get goofy really quick. Better to start out with high values, and then if it proves to be not enough, look first at the input to see if maybe you could remove some noise.

Question #3 Right now the Zener's a 1 watt 1N4733 from the Shack but I got some BZX79C5V6's on the way from Mouser that AudioGuru recommended. They're a 1/2 watt and run on 5 mA compared to the 40 mA I'm putting thru the 4733. No reason to drain the batteries anymore than necessary ...

Absolutely! Good call by AudioGuru. As it is, the current is pretty low for that Zener you're using.

Being the kind of person who can't leave new toys alone I've been running the motor some more. Hard to tell with a DMM but it seems to respond very briskly on the up side. When I throttle down things get a little goofy and slow. Up and down even but again, with a DMM I can't really say.

Don't worry about the down side - that's when the motor will be taking a nap anyway!

Been thinking about "preserving the signal." That's probably a small current on the output of the speed sensor circuit? Am I possibly trying to do too much with it? Could be! Most of my projects attempt to defy the laws of physics some where along the way!

Well, actually - your idea was to short the base of the TIP120 to ground, which WOULD work - however, you would be wasting power across the 1k base resistor for the entire time the motor was running. Not a happy thought with the gas prices of today!

Let's just try to "lock in" what exists at the moment; a snapshot if you will. That way, if things go awry, we can return to this point later. It's a reality check.

Have a look at the attached - the top half is (basically) what you have already, without your proposed modification. The bottom half is what we've been fiddling with the last day or so.

Note that the comparator inputs have the - on top and the + on the bottom. Sorry, I can't change that. It's very counter-intuitive, and may be hard to get used to (it still trips me up)

Anyway, look at it and if you see any glaring errors thus far, let me know.

 

OK, I'm not waiting any longer!

Have a look at the attached. Don't let the fact that I had to tie all the grounds and most of the + supplies together throw you - I was running out of symbols to use; the software version I'm using only allows 50 symbols to be in use at a time, which is a pain. However, it's still pretty useful.

I'm now using the 3rd comparator in your LM339; U1C. Note that I added a 3.6v Zener on the high side reference input, and the buffered output of the filtered "tachometer" is on the low side input. The Zener might not be necessary; it and R14 might be replaced with a 10k trimpot; ends on the rails, center tap to the high side input. But, I wanted to make sure it had a good chance of working the first time around.

With the engine OFF, there won't be any input on the low side of the comparator, so the output will be conducting (the open-collector output transistor will be biased ON.) Note that the output of U1C is connected to the base of a NEW transistor Q2, that has been connected between R10 and the TIP120's base.

This transistor Q2 is getting it's collector current from the R10/R9 network; in effect, it and U1B are creating an AND gate. For the starter relay to engage, the battery voltage must be below the reference voltage established at U1A, AND the engine must be BELOW idle speed as established by the 3.6v reference at the high side input of U1C. Unless both of those conditions are met, the TIP120 will not have any current supplied to it's base!

Note that because there is a Zener on the high-side input, we can't really use a hysteresis feedback. However, the signal has been filtered sufficiently that this really shouldn't pose a problem.

Are you following me?

If you build it, it will run...

 

Looks just wonderful to me. I'll start collecting the parts to finish smoothing the speed sensor output. To my absolute surprise I had all the parts I needed to build out to C1 on hand today. Didn't even need a trip to Radio Shack.

On the R98. If I understood correctly you earlier mentioned it was there because of a missing edit and not really needed? I sure hope so because I suspect the coil's internally grounded thru the case somehow and I don't know how I'd open it up to get 10 ohms in there.

Whoops! Another post!

brb

SP

 

Yeah, I'm following! Mission accepted. I'll build it.

I must admit I'm not understanding the TL082 unity-gain buffer tho. First of all, I don't know what a unity-gain buffer is! LOL!

Google is my friend tho. I'll find out and I can't say thanx enough.

SP

 

OK. A unity-gain buffer is a way of taking the speed sensor output and giving it enough oomph to do more work without changing it. Yeah. That works.

SP

 

Hang on a sec - two slight changes!

1) Had to move R10 from the collector of Q2 to the base, and reconnect the collector to the output of U1B. This gives U1C's collector a clear current path to Vcc, enabling it to control the base of Q2. Sowwy, my mistake! It wouldn't work the way I had it before.
2) Changed D4 from a 3.6v Zener to a 3.3v Zener. A 3.6v Zener would still work, but I wanted the starter to turn off more quickly, while the voltage on C1 was still building up fairly rapidly. This will help to minimize the probability of relay chatter.

Don't forget that the TL082 needs a bypass cap across it's power pins! It should be around 100nF, tantalum or ceramic.

Take a look at the attached. The filtered tachometer output is (A) violet trace, the 3.3v reference input is (B) cyan trace, Q2's base voltage is (C) yellow trace, and Q1's collector (ground side of relay's solenoid) is (D) green trace. Initially, the voltage of battery BA1 is lower than the threshold set via R2, turning off U1A and U1B, so the output of U1B goes high. The output of U1C is high because there is no tach signal yet, so the starter relay is engaged (green trace is low, so Q1 is conducting). As the tachometer output becomes more positive than the 3.3v reference, the output of U1C turns ON, pulling the base of Q2 to ground, turning it off, which cuts off the current source to the base of Q1, turning IT off, and the relay's coil loses it's ground - the relay contacts open, and the starter motor turns off. Are we having fun yet?

Yes, you're correct - R98 actually represents the resistance of the primary coil (L1), and is not a "real" part on it's own. The simulation won't run without it. That's why I edited it out of the first few simulations, because I knew it would be confusing. 10 Ohms is an arbitrary number. L1 really isn't needed in the simulation at all, because the signal generator is what's supplying the input to the simulated tachometer, but I put it in as a visual cue as to where everything connects.

Good onya for finding out what a unity-gain buffer is It's also called a "voltage follower", because the output of a voltage follower circuit follows the voltage at the input. This is a perfect excuse to use a JFET-input op amp. I chose the TL082, because it's available at your RS store - it's the ONLY JFET-input op amp they carry. Note that it is a DUAL op amp! So, to keep the other side from flailing around, you should connect the unused side's output to it's inverting input, and the non-inverting input to the cathode of Zener D1 (between D1 and R4). We're going to make that Zener do double duty

 

Another long day in the mines yesterday Sarge, but I'm loose today and rarin' to go!

I noticed during the R10 switch that R5, the feedback on U1A, dropped from 10 meg to 2 meg. I figger that'll make 25-30mv of hysteresis but I'm not thinking that's a bad thing. RWMoekoe mentioned that hysteresis affects accuracy during a previous post and I believe him. But we're looking for a clean switch. If some stray tiny spike of noise tips U1A over when it's right next to the threshold I say slam it and start the charging cycle. But I've been wrong before.

I'm gonna go poking to find out when I get this wired up but I'm wondering about the output of U1C during "normal-battery charged" conditions. U1B will be sinking all of the current thru R9 which won't leave anything for R10 to pull up on the collector in U1C. I wonder what it'll do ......

None of which matters. As soon as U1B goes high and opens up it'll quit sneaking the current off of the collector of Q2 and U1C will get the clear path it needs to get pulled up on the R9/R10 network, biasing the base on Q2 at the same time, and we're off to the races.

A very ingenious design Sarge. Sneaking the current off of the collector on Q2 is downright nefarious! I never would have thought of that in a million years and I must thank you once again for your help.

I'll be at Radio Shack as soon as they open the doors .....

SP

 

Another long day in the mines yesterday Sarge, but I'm loose today and rarin' to go!

Yee haa!!

I noticed during the R10 switch that R5, the feedback on U1A, dropped from 10 meg to 2 meg. I figger that'll make 25-30mv of hysteresis but I'm not thinking that's a bad thing. RWMoekoe mentioned that hysteresis affects accuracy during a previous post and I believe him. But we're looking for a clean switch. If some stray tiny spike of noise tips U1A over when it's right next to the threshold I say slam it and start the charging cycle. But I've been wrong before.

Actually, I had to reduce R5 in order to get the SPICE simulation to run! It would crank and crank and crank... and finally blow up about 230mS into the simulation. Reducing it caused it to run through like greased lightning in comparison. RWMoekoe was somewhat correct about the accuracy thing - but in this case, you compensate by slightly increasing your R2 setting, and it all balances out.

One thing that hasn't been discussed is "resistor noise" - they do generate electrical noise, much as a nozzle on the end of a garden hose generates audible noise. You can quiet that right down by putting a small capacitor (around 100pF to 1nF) between the comparator's + and - inputs.

I'm gonna go poking to find out when I get this wired up but I'm wondering about the output of U1C during "normal-battery charged" conditions. U1B will be sinking all of the current thru R9 which won't leave anything for R10 to pull up on the collector in U1C. I wonder what it'll do ......

That's fine! As I mentioned before, the addition of Q2 etc was just to build AND gate logic for driving the base of Q1. If the battery voltage is over the threshold set by R2, we don't want the starter running! As long as the output of U1B is low, the base of Q1 will be low since there's nothing to pull it up - and the relay's coil won't be energized.

None of which matters. As soon as U1B goes high and opens up it'll quit sneaking the current off of the collector of Q2 and U1C will get the clear path it needs to get pulled up on the R9/R10 network, biasing the base on Q2 at the same time, and we're off to the races.

Yep, that's about the size of it!

Now if you really want this thing to be power-efficient (and you should!) then after you're through with all the bread-boarding, it would be a really good idea to think about using CMOS comparators instead. CMOS or JFET-input comparators will eliminate the need for the unity gain buffer(s), and you'll be able to use much larger values of resistors. Unfortunately, you can't get them from Radio Shack. However, everything you're doing and learning now will be directly applicable to an updated design using the CMOS comparators.

The end result will not only be more fuel-efficient; it will have fewer parts, too!

 

Remember when I mentioned that we were going to make that 5.6v Zener do double duty?

Here's the next installment of insanity!

Remember me mentioning connecting the 2nd half of the TL082's output to the inverting input, and the non-inverting input to the top of the 5.6v Zener? Well, now you're also going to disconnect U1A's low side from the Zener, and connect it to the output of the TL082 which is now buffering the Zener voltage. This lets us use that 5.6v reference in lots of places, since the TL082 op amp will keep it nice and steady. This will become even more apparent when you get that low-power 5.6v Zener diode in; we wouldn't have been able to supply that reference voltage accurately to several places otherwise.

This also lets us get rid of the 3.3v Zener, and replace it with a 10k pot! Now you can adjust the starter solenoid cutoff level wherever you want - but for now, adjust the pot so that you get about 2.9v to 3.3v on the + input of U1C. Additionally, since we don't have to fight the Zener reference on the high input, we can now use hysteresis by connecting a 220k resistor from the output of U1C to the high input. In the attached image, you can see the hysteresis working; it's the green trace. See it drop at about 146.3mS? That's about 19.5mV, which is about 70% more than the noise the simulation shows will be present on the filtered tachometer output (yellow trace). The blue trace is the collector of Q1.

 

I'm still on Post #27 Sarge and I'm sure it's all my fault! LOL! There's no doubt it's operator error and I'm still looking for the last bug.

The 1st little problem was over the new low power 5.6 Zener. I had a 5.1 in there and with the increase to 5.6v R2 went out of range. Upping R3 to 4.7K solved that easily enough, and that that was the last thing that made any sense at all!

Gawd ... what happened? I do know I had D2 across the CE of Q1 instead of the relay. Before I realized that tho, for some reason I thought D2 was backwards. It was when I reversed D2 that the TIP120 went up in smoke! Yeah, I know they're hard to kill, but that was before that poor thing met me!

Somewhere in there I also realized I had left the collector of Q2 unconnected. Breadboard error. What affect that may have had I don't know.

Back to the Shack for another TIP and a new 25 ohm 3 watt pot. Did I mention I used the wiper for a fuse when I bumped it to gnd? One o' those days ....

So now it's all back together but I must have hurt something else or I've still got it wired wrong. Whats happening now is V+ is dropping like a rock to under 10V as soon as U1A switches. The outputs of the 339 at U1B and U1C only go up to 2.1v instead of 12V. I know at one time I was seeing 12v out at U1B. That's a good spot to monitor. Had the meter on it a buncha times. So I think it's more than likely I've somehow managed to hurt the 339 too. Just call me Killer!

And obviously my breadboard skills leave a lot to be desired. I've never done it before and apparently it ain't as easy as it looks. So I'm thinking from a practice makes perfect point of view I'm gonna rip it all down and just plain start over.

Any other opinions are welcome tho.

SP

 

I'm still on Post #27 Sarge and I'm sure it's all my fault! LOL! There's no doubt it's operator error and I'm still looking for the last bug.

That's OK. I'll just post faster to make up for it.

The 1st little problem was over the new low power 5.6 Zener. I had a 5.1 in there and with the increase to 5.6v R2 went out of range. Upping R3 to 4.7K solved that easily enough, and that that was the last thing that made any sense at all!

That's odd! Measure the voltage across the new 5.6v Zener. I have a sneaking suspicion that it's higher than 5.6v! What resistance value are you using for R4 now?

Gawd ... what happened? I do know I had D2 across the CE of Q1 instead of the relay. Before I realized that tho, for some reason I thought D2 was backwards. It was when I reversed D2 that the TIP120 went up in smoke! Yeah, I know they're hard to kill, but that was before that poor thing met me!

Well, into each budding EE's life must pour a bit of smoke and flame! It may not be the first such incident, and it sure won't be the last, either.

Somewhere in there I also realized I had left the collector of Q2 unconnected. Breadboard error. What affect that may have had I don't know.

Well, the TIP120 wouldn't have been able to turn on - unless you had the base of it mis-connected to something else besides just the emitter of Q2, and the base of Q2 connected to one side of R10 and the output of U1C.

Back to the Shack for another TIP and a new 25 ohm 3 watt pot. Did I mention I used the wiper for a fuse when I bumped it to gnd? One o' those days ....

LOL! Ahh, is the 25 Ohm 3W pot for this project?

So now it's all back together but I must have hurt something else or I've still got it wired wrong. Whats happening now is V+ is dropping like a rock to under 10V as soon as U1A switches. The outputs of the 339 at U1B and U1C only go up to 2.1v instead of 12V. I know at one time I was seeing 12v out at U1B. That's a good spot to monitor. Had the meter on it a buncha times. So I think it's more than likely I've somehow managed to hurt the 339 too. Just call me Killer!

LOL! Actually, if you disconnect the collector of Q2, you should see the output of U1B pop right back up to 12v.

Remember, R10 used to be between the output of U1B and the base of Q1. If you really wanted to, you could put another 1k resistor between the output of U1B and the collector of Q2. However, the only purpose would be so that you could actually see 12V (well, around 7v actually) on the output of U1B!

Right now, if you refer to the schematic again, looking at the output of U1B - if Q2 is ON and Q1 is also ON, there are three forward-biased PN junctions between U1B and ground; the emitter of Q2, and the two emitters of the TIP120. If each PN junction is dropping it's typical 0.7v, you would have three 0.7v drops, or 2.1v - and the resistor supplying the current that flows across these three junctions is R9!

Q2 is still getting biased on; it doesn't need much current. Right now, when both it and Q1 are biased on, Q2 gets 0.7v/R10, or 0.7/1000 = 0.7mA current in it's base. Since a 2N3904's hFE (current gain) is in the neighborhood of 200, this is actually more than enough current to cause the transistor to pass all of the current that's available through R9, or around 9mA, which is plenty to turn on the TIP120.

And obviously my breadboard skills leave a lot to be desired. I've never done it before and apparently it ain't as easy as it looks. So I'm thinking from a practice makes perfect point of view I'm gonna rip it all down and just plain start over.

That's not a bad idea. It's easy to make mistakes, REALLY easy.

OK, I'm attaching another view of the circuit, blown up as big as I can make it, and the pin numbers are turned on too.

If you have Microsoft Paint installed (or another graphics program) save it on your computer, and print it out as large as you can on a single sheet.

As you connect things up, use a pencil and make a diagonal tick mark across each component's connection point to indicate you've connected it. Double-check each connection - and turn the tick mark into a check mark.

Note that in this crazy simulator, I have to connect EACH individual segment of an IC up to power and ground (eg, U1A, U1B, U2B, etc) - so there's extra wiring shown on the schematic that's duplicated. So, start off by wiring power and ground to both U1A and U1B, and then you get to check off ALL of the power and ground wires for both IC's!
LM339:
3 = Vcc (+12)
12 = GND
TL082:
4 = GND
8 = Vcc (+12)

Don't be afraid to leave several blank rows between the two IC's. I usually leave a half-dozen empties. That gives you a very handy jumpering area if you have some IC pins that are really "busy".

You could also use such an area to plug in LED's and their current limiting resistors as indicators to tell you when events are going on, answering nagging questions like "Is U2B really putting out 5.6v?" or "Is U1C's output really low now?" But to do something like that at the moment would be overcomplicating things, and you'd need to have some low-current LED's, which RS doesn't stock.

If you feel yourself getting frustrated or tired, just put it up for an evening. Tired + frustrated = mistakes + breakage!

 

Ah. The light went on. When I didn't have the collector of Q2 connected is when I was seeing the 12v .... and I'm sure glad to hear that! I just re-did the whole thing with a new 339 and sure enough, it's still only 2.1v!

But ya know. V+ ain't taking a dump no more. The original 339's gonna take a quick trip into the trashcan. I don't wanna fool with it .......

Proceeding with caution. I have met the enemy and he is me.

SP

 

Ah. The light went on. When I didn't have the collector of Q2 connected is when I was seeing the 12v .... and I'm sure glad to hear that! I just re-did the whole thing with a new 339 and sure enough, it's still only 2.1v!

That's a good sign! Now when U1C's output goes low, it'll turn off Q2 by shorting it's base to ground, and then you'll see somewhere in the vicinity of 5 to 6v on the output of U1B, as then R9 and R10 will be a voltage divider of whatever's on the Vcc. Until the alternator starts outputting juice, that is... then U1A will switch off, causing U1B to switch on...

But ya know. V+ ain't taking a dump no more. The original 339's gonna take a quick trip into the trashcan. I don't wanna fool with it .......

Actually, it's most likely that you had a couple of component leads shorting together. Very easy to have happen on a breadboard, particularly if you're not trimming the leads and insulating as you're going along. I usually don't bother insulating, but I generally trim at least one end of a component's leads so that it occupies a small "footprint" in the blue sky above the board. Or sometimes smoky-haze filled sky.

Proceeding with caution. I have met the enemy and he is me.

Walt Kelly fan, eh? Is you an Okefenokeean?

 

Well we got rock and roll hoochie-coo goin' on now! It works! Post #27's in the bag.

I'm still waiting on a flywheel with teeth on it to engage the starter, so what I have to do for now is crank the pot to simulate the battery drop and fire the starter motor. Then I walk over and pull the rope on the Briggs. I swear that starter shuts off on the first pop. It's quick.

The 2nd attempt at a decently laid out breadboard was way better too. That 1st one was a regular rats nest. There was a small forest around the 339 and I'll bet you were right about some rubbing in there causing the Vcc drop I was seeing.

On to Post#29 ... or is it back?

SP

 

LOL! Back to the Future!

Here we go!
You're going to like the mod with the 2nd half of the TL082 driving the reference voltage for everything. It's going to enable you to use just that one 5.6v reference everywhere, have all your voltage levels tweak-able, and even free up a comparator for other duties - because by the time we're close to finishing, we will need all four comparators actively doing comparator-type things:
1) When the battery gets low, turn on the starter. (Done!)
2) When the engine starts, turn off the starter. (Done!)
3) When the engine gets up to speed, turn on the alternator. (To do...)
4) When the battery is charged up, turn off the motor. (To do...)

It would be a shame to have to add yet another comparator IC if we can simply free up one we're currently using for something else.

But, we're going to take this a little bit at a time, right?

Oh, tell me - what's the value you're using for R4 now? It was 1.1k, but you probably changed it when you changed D1 from the 5.1 Zener to the new 5.6 Zener.

 

I got that R4 value balled up when I drew that schematic Sarge. I was intending to use the low power 5.6v so I calced and drew 1.1K for 5mA. Up until today I actually had a 1N4733 5.1v from Radio Shack with R4 being 180 ohms for a 40mA draw.

Today I put the low power zener in and since I didn't have a 1.1K I used a 1.2K. That's what's on R4 now.

Now that I think about it that was probably why I had to bump R3 to 4.7K.

I'm right with ya on using U1D to energize the alternator. I saw a TIP3055 that looked like a candidate for that kind of current but I'm open for options.

For shutting it off we'll have to ground the tach input for a period of time. Once the spark goes away the Briggs continues to spin down and if the spark comes back it'll refire. I was thinking of a pulse of several seconds duration fired by a shunt type device on the charging line. How exactly to do that is the $64 question!

I've never used a 555 but they'll do something like that won't they? Maybe use the shunt to keep a transistor saturated until the charging current is down to a few amps. Transistor shuts off and then raises a trigger on a 555? Then the 555 saturates a transistor at R11 for however long the Briggs needs to completely stop. Might work.

As usual tho, I'm open for whatever's best.

SP

 

OK, I updated my schematic so that R3 is now 4.7k and R4 is 1.2k. I don't have that particular Zener diode in my parts library, so I'll just have to use what I gots! It should be close enough for what we're doing though.

The simulation is showing 5.625v at the top of the Zener; you're probably right around that voltage.

How are you doing on the #29 (#31) update? (they're both the same schematic, really). Are you getting the same voltage at the output of the 2nd half of the TL082 as you read on top of the Zener?

Did you manage to get R14, the 10k pot wired up to U1C's input and set at about 2.9v to 3.3v?
That might prove to be a bit too touchy of an adjustment; we might need to substitute a lower-value pot, and put some fixed resistors on either side of it. I was trying to keep the parts count down.
Lemme know where you are, so we can go to the next step!

 

I'm right with ya on using U1D to energize the alternator. I saw a TIP3055 that looked like a candidate for that kind of current but I'm open for options.

Well, the TIP3055 is an NPN transistor. While we might make an NPN work with some fooling around, what we really need is a PNP transistor, because we're going to be connecting the + side of the battery to the field input of the alternator. Now, they DO carry the TIP42G (which they say is actually an MJE34, but I can't find a modern datasheet for it) and it's a PNP that has an Ic of 10A, but a pretty low hFE - so, we're gonna have to build a Darlington pair.

Did you buy their assortments of 15 2N3906-type PNP and 15 NPN transistors? Probably their best buys in small signal transistors, even though the documentation on the packages is all wrong. In the package of 15 PNP transistors, you'll probably find several 4403 (2N4403) transistors. These have a pretty good hFE for what we're going to have to do. See if you can root around and find 'em. Might as well sort them all out now, and put them in Ziploc baggies. I write on the bag with a Sharpie, and then put a piece of clear tape over it.

For shutting it off we'll have to ground the tach input for a period of time. Once the spark goes away the Briggs continues to spin down and if the spark comes back it'll refire. I was thinking of a pulse of several seconds duration fired by a shunt type device on the charging line. How exactly to do that is the $64 question!

Yep! Well, that's coming up, but we'll need to free up a comparator first.

I've never used a 555 but they'll do something like that won't they? Maybe use the shunt to keep a transistor saturated until the charging current is down to a few amps. Transistor shuts off and then raises a trigger on a 555? Then the 555 saturates a transistor at R11 for however long the Briggs needs to completely stop. Might work.

A 555 in a monostable configuration might work, but might also complicate things!
First, let's look at what the minimum amount of logic needs to be in order to determine if it's time to kill the engine:
1) The rate of charge (charging current) has fallen low enough to assure that the battery is charged up.
2) The alternator must have been turned ON long enough to have been providing a charging current.
The 2nd part isn't immediately obvious, but when you think about it, the alternator takes some time to stabilize after it's turned on. If you just supplied current to it, and instantly checked the charging current - there wouldn't BE any! So, we'll have to put a time delay in there before the comparator starts monitoring the charging current.

Are ya with me?

 

OK, here's a preview of the next step - getting power to the alternator!

Depending on your alternator, there are probably three connections:
1) The BATT terminal - this goes right to your battery. It's where the power comes out of the alternator.

2) A discharge/charge indicator line. If the alternator isn't charging, this will have ground on it. If the alternator is only charging part-time (like if there is a bad bridge rectifier) it will have a rectangular-shaped signal on it. If it is charging OK, it will have around 13.5 volts on it. These are generally hooked up to one side of a light bulb in the dashboard of your car, the other side of the light bulb is connected to the battery + line.

3) The "Field" connection. This is a term left over from generator days. It supplies current to the built-in regulator, which supplies current to the rotor winding.

Now this is a "preliminary" version, just showing the basic hook-up. This particular section is going to get a bit dicey, because when power is applied to the field, there is going to be a sharp momentary load put on the battery until the alternator can actually start putting out some power. There are a couple of problems that need to be resolved before you actually wire this thing up.
1) The sudden large power load means that you'll need a LARGE hysteresis effect on U1D; otherwise the power to the alternator will be turned on and off very rapidly. This would be a very un-good thing!
2) The current required to turn the IRF9Z34 on is more than the U1D can sink in a short period of time. This means adding a driver transistor to dump the current. The driver transistor is going to need current limiting resistors so that it doesn't get overloaded, as well.

The reason I decided to go with an IRF9Z34 is that it's capable of handling a lot of current at a pretty low resistance, and it's in my parts library (the latter figured in heavily!) It won't get as warm as a BJT would (bipolar junction transistor) so it will help as far as the efficiency thing goes - after all, gas ain't gettin' any cheaper! Every bit of efficiency we can squeeze out of this circuit will save $$$ at the pump for as long as you're using it.

Ya with me?

 

How are you doing on the #29 (#31) update? (

My customers were after me again Sarge but I'll be putting the other side of that TL082 to work today.

SP