An Ignition Kill That Won't Work

Discussion in 'General Electronics Chat' started by pntrbl, May 21, 2008.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    This Ignition Kill circuit has been giving me a headache for days. This is the umpteenth version and it won't work either, but perhaps someone here will come up an ingenious solution that never would have occurred to the likes of me.

    I need to gnd the ignition after a Briggs/Alternator combo has finished charging a battery. Plan A (subject to massive revision based on what I learn here) is a normally closed reed relay. I'll open it initially with the starter circuit. U1A goes up energizing Q8 and I haven't figured the current limiting resistor yet. :)

    The starter circuit gives up when tach output (U1C) goes down. So we switch to Q6 altho I'm not sure that's the proper application for a PNP. Might be wishful thinking on my part .....

    Now the motor's running, we're charging. The input at U3A will come from a shunt in the charging wire, get amplified, and applied to U1D. The + input at U1D gets over the 5.6v Zener ref depending on adj at the 10K pot. Output goes up. Energizes Q7, plugs Q6 ... but maybe not ... it's a PNP and I'm not real clear on them! LOL!

    As the battery charges and the U3A input drops, U1D will flip shutting off Q7 which will let the relay close and ground the ignition. But unfortunately the motor will still be spinning. That means we'll still have tach output which will pick up where Q7 left off and it'll keep right on running.:mad:

    Somehow I need to get the ignition on until we're up to speed and charging, without having an affect once it shuts off.

    I hope I've explained myself well enough. All thoughts or suggestions are welcome ....

    SP
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    OK, now I have an idea where you're going with this.

    But first, did you make progress on getting the alternators hooked up and the like?

    Oh, go back and re-read the last post I made on the other thread you started this evening. Try the "measuring peak voltage" experiment by pulling R12/R13.

    Now back to your Q...
    Adding the load of energizing the relay to enable spark at the beginning of the charge cycle seems to be quite a chore to keep track of, along with the cost of keeping that relay's coil energized the entire time the generator is running. If using a relay to short the ignition out, seems like it would be easier to simply charge up a capacitor while charging the batteries, and when the batteries are charged, connect the capacitor up to the relay's coil so that the contacts close and the magneto's output gets dumped.

    Basically, you size the capacitor so that it's large enough to hold a sufficient charge to keep the relay energized long enough to make sure the engine is stopped before the cap runs out of charge.

    If you used a MOSFET instead, you could use a much smaller capacitor. You would also need to use a bleeder resistor on the gate of the MOSFET so that it wouldn't still be ON (and shorting out the magneto) the next time the motor was started.
     
    Last edited: May 22, 2008
  3. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    I have started getting the alternators together but not much progress yet. Still no flywheel either.

    SP
     
  4. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    An amazingly simple solution once again. Why is it that I always use 42 times more parts than you do?:D

    When you say connect the capacitor I'm assuming we'll still need U1D to tell when the batteries are charged, and then somehow electronically connect? Transistor? MOSFET instead of transistor?

    SP
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Maybe because I've been doing this a few weeks longer than you have? ;)

    Yep, something like that...

    One big item that still needs to be worked out is the alternator configuration.

    If you're going to use two 12v alternators, one for the low-side batteries, and one for the high-side batteries, you'll really only need to monitor the charging on one side. Since the alternators have internal regulators, you might as well just let them "do their thing".

    If you're powering your board off the "low side" batteries, then that side will need more charging than the high side batteries. Since the built-in regulators will cut down the charging current automatically, you won't have to worry about the high side getting overcharged. The biggest problem is keeping the frame (ground) of the high-side alternator from shorting against the frame of the low-side alternator. That's another reason for having the alternators on opposite sides of the Briggs' flywheel, and why I suggested looking into building the brackets using something like auto motor mounts as insulators; they're basically a solid block of rubber with steel plates on either side. They'll be very rigid and tough in your application.

    Another thing to be resolved is enabling/disabling the rotor(field) current. I haven't read the GM 10-S alternator documentation yet. If you're using two alternators, one will need the field supplied from the high-side batteries, the other will need the field supplied from the low-side batteries - so two MOSFETS will be required. If you're going to use a single 28v alternator, it will just need one supply from the high-side battery.

    Measuring the charging current is gonna be easy :) Copper wire has a known resistance, and it depends upon the gauge. Basically, all we will need to do is monitor the voltage across the wire carrying the charging current, either on the + side or - side, using another wire that has almost no current flowing through it. When the voltage across the high-current wire drops, that means the charging current has dropped. You won't need to buy the shunts; the wire alone will do the trick.

    Using the wire resistance calculator on this page:
    http://www.stealth316.com/2-wire-resistance.htm
    10 feet of 8 gauge wire, with 60A the drop will be 0.384V, with 4A the drop is 0.0256V.

    So, if you know that your load is going to be around 2 amps, and a battery nearly charged will take about a 2 amp charge, that would be time to cut off the motor. You could actually cut it off sooner, say at perhaps 8 amps, as when you're charging at 2 amps you're basically "topping it off", and that could mean running the motor for a much longer time to get it charged that completely. You'll have to do some experiments to find out what works best.

    There's something else; I don't know what the "minimum output" of these alternators is. When they're installed in a vehicle, there's always going to be SOME kind of power drain; like the ignition system and running lights.

    There's yet another consideration - and that is the LM339 is not a "rail to rail" device (neither is the TL082). That means we won't be able to measure voltages that are within roughly 1.5v of a power "rail". The easiest way around this is to use BOTH the high and low side batteries to supply Vcc to the board - but this also means that a number of resistors will need to be changed!

    R1, R4, R5, R6, R7, R8 and R16 will all need to be increased. RLY1 will need to be supplied from the low-side batteries, so that you don't have to buy a different relay.

    Just so that we're on the same page, I've attached the current configuration of the circuit I have in the simulation. There have not been any real changes since the last time I posted it, but it helps to have a "what is" configuration that we know works, before proceeding to a "what will be" configuration.

    You need to make a decision about whether you're going to use two of the 65A 12V alternators in series, or something else. If you decide to go the two 65A alternators in series, I can start making updates in the simulation.
     
    Last edited: May 22, 2008
  6. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    I'm just a slight step behind in that R5 went to a 37K from the 33K I last saw. And I don't have one. :mad: Series a 4.7K in for now? Does the simulator make these decisions for us?

    SP
     
  7. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Hmm. I had visions of leaving the regulators out and sending whatever current was necessary to get 28v@6000 rpm out of both alternators, and then running them in parallel. I was also thinking to tap 12v for the "control board", I like that term :), from the low side of the batteries. Whether that's a wise idea or not ...... I do not know. :rolleyes:

    On the mounting config I'd like to put my machine skills to use and run the alternators inline off of one pulley. Ever since this electronics obsession took over my life I have been seriously neglecting my poor lathe. :( I've spent many happy hours whittling away minute amounts of metal and it's been looking lonely lately. It needs me .....

    I've got a rubber bushinged coupler that will tolerate some amount of mis-alignment, but I'm also absolutely certain I can hold the mis-alignment to under .010". Insulating the frames to run in series would be a new dimension I hadn't considered, but there's plenty of machinable insulator materials. Delrin's nice.

    Who knows? Maybe I'll find 2 more and we'll run 4 on 2 pulleys someday! LOL! What's next? Headers?

    SP
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, I just double-checked it. The difference in performance is so small, it's negligable. 33k is fine. In the simulation, 33k causes the field voltage supply to cut on at 60mS; 37k at 61.8mS. It is not worth fiddling around putting two resistors in series.

    No, it was something that I would've had to have changed. I was trying to get a large hysteresis, and that's basically a function of the ratio of R5 to R7. R7 is about as low as it can safely go right now.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Well, as they were built, they'll generate 65A @ about 14v each, with no fiddling around trying to design a replacement regulator. Trying to run them both together at 28v, 32.5A each creates some "special" problems. First, they weren't designed to generate that much voltage. While that in itself isn't such a huge hurdle, each would require it's own custom-built regulator; and if they weren't perfectly matched voltage-wise, they would "fight" each other.

    Something that isn't obvious is that the current in the rotor winding is constantly changing to keep the rectified 3-phase output at a constant level. This is a tough thing to do at 14v; it requires ramping several amps of current up and down at a pretty high rate of speed in order to maintain a constant output voltage. It would require a good bit more current to double the voltage, but we'd have to ensure that the wattage rating of each alternator was not exceeded. So, the regulators would have to be electrically coupled together. Trying to diagnose problems could be a real chore.

    Theoretically, you could match the alternators in phase output and run them from one current source - but that's theory, and reality would be different. If they were just a fraction of a degree out of phase from each other, they'd be fighting each other and wasting power instead of charging the batteries. Just ordinary manufacturing tolerances for these things pretty much nixes the idea of phase matching.

    I miss having a lathe, and a milling machine, and a horizontal grinder, and... :( No basements in FL - if you had one, it would be full of water.

    Have another look at the military alternator picture. It has two pulleys on it for a reason! Trying to run two alternators on one V-belt isn't going to work very well, I'm afraid. The 130A alternators have a serpentine belt pulley, which has a great deal more torque transfer capability than a simple V-belt.

    A V-belt needs to have contact with the pulley for a pretty good portion of the radius. The sidewalls of the belt "bulge" as it's bent around the pulley, which helps to grip the pulley tightly and provide a slip-resistant mechanical torque transfer. The higher the percentage of contact, the less chance of slippage. Slippage = wasted power and rapid heating/wearing of the belt drive. If you try to load two alternators on a single belt, you will wind up with more than double the slippage.

    LOL!
    Well, that's an interesting idea; running them on the same centerline. However, you're still going to need two V-belts for reasons I've already mentioned. Trying to design custom coupled regulators is going to be a dicey proposition at best, for more reasons I've already mentioned.

    GM has a large team of electrical engineers that they pay large salaries, to develop these things in well-equipped, well-financed research facilities. It's not very realistic to assume that we could come up with a complex adaptation of their regulator design that would be cost-effective, efficient and reliable between the two of us whom are located a continent apart from each other, on a shoestring budget. The probability of project failure is just too high for my tastes, and is really more effort than I feel that I can expend on it.

    OTOH, running the two alternators in a series configuration means that the alternators are completely stock. If need be, in an emergency you could run to an auto parts store and pick up a replacement with no hassle whatsoever, bolt the thing in, and you would be back up and running with very little downtime.

    The biggest hurdles to overcome is ensuring that the alternator's frames are insulated from each other, proper mechanical alignment, and that they both have solid, non-slip drives. Automatic tensioners would be a capitol idea, and are readily available at your local auto salvage lot from any GM 3800 V6 motor. That would help to keep drive maintenance low, and actually allow you to increase the belt's contact radius beyond 180°.

    I'd really like for your project to be a success. Let's take the road with the least amount of risk.
     
  10. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    You've convinced me Sarge. Series it is. I don't why I always think I'm smarter than the factory engineers. You shoulda seen the self binding shackles I designed for the "Always Loose Pinto!" LOL!

    Actually I thought all the regs did was adjust for engine rpm and since the Briggs is governed at a steady 3600 that it would work at a set current. Phase shift never entered my mind .....

    On the belt I was thinking serpentine might be a necessity too. Had a buddy with VW based dunebuggy that couldn't pull 3rd with his flame throwing off road lights on. Alternators can create a lot of drag. If I have to I'll go to a cogged belt. Automotive cam belts are always 2:1 and I wouldn't mind having some Pinto parts on this thing.

    I've still got my eye out for a 28v military too. These 65A GM's could only be temporary.

    SP
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Well, there's another reason having two 14v alternators in series would be easier, too; and that's monitoring the charging current.

    If you're running 28v alternator(s), then the charging current will be on the rails. If you're running two 14v alternators, you'll have access to the lower + charging current right in between the two 28v rails. Remember, with the IC's you're using, you can't monitor voltage levels that are within about 1.5v of a rail; so we'd have to fiddle around with resistors to pull the voltage away from whichever rail. However, we'll be dealing with very small voltages across the wire that's charging the lower half of the battery bank; trying to use resistive dividers would make the voltage difference even smaller, and more prone to noise.

    While a military 28v generator initally sounded attractive, you'd run into this same charge current monitoring issue with one of those - not to mention that when it breaks, you'll either have to fix it really quickly, or find a replacement. Either option is likely to be expensive and a hassle.

    Also, the military vehicle alternators put out 65A, period. They're all the same design, and all the same rating. This means there is no upgrade path in case you want more charging current. But if you start off using a standard 65A GM alternator, you could go out and get a pair of 90A or 130A alternators, and just bolt them in with a minimum of fuss.

    A cam belt drive is an interesting idea. Alignment would be critical though. If the alternator rotor shafts are not precisely parallel to the crankshaft, the belt will walk right off. This isn't a problem in an engine block, because the crank and camshaft bores are line-bored with great precision. Trying to hold that kind of tolerance in an open-air bracket would be mighty challenging. Thermal expansion/contraction alone could throw the alignment off. If you went that route, you'll likely have to crown the center of one of the pulleys. Still, keeping the doggone belt on will be tough.

    Seems to me that your best choices will be either a V-belt, or the type of serpentine belt they're currently using for engine-driven accessories. I don't know if the standard serpentine belt pulleys can be easily fitted to your alternators if they don't already have that type on them. You'll have to investigate. The serpentine belt will certainly slip less than a V-belt, especially if you use an automatic tensioner and idler pulleys to keep the belt wrapped most of the way around each driving/driven pulley.

    I knew a guy who stuffed a souped-up 302 V8 and traction bars in a Pinto Runabout. He could do wheelstands in the thing.
     
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