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nitromaxx98
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PostSubject: Do you know?   Thu Mar 24, 2011 10:20 pm

Does an LS1 or similar single cylinder coil pack fire off with a square wave pulse, or does it require gradual charge by sawtooth? Is it a positive or negative pulse? What can be the expected current draw? Is it a TTL level or would it require possibly the current capabilities of a 4 amp MOSFET?
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Bighead
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PostSubject: Re: Do you know?   Fri Mar 25, 2011 1:55 am

Dunno. What are you building?
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nitromaxx98
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PostSubject: Re: Do you know?   Fri Mar 25, 2011 9:49 am

Solar fence charger. Adjustable for wire length and ground moisture.
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Bighead
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PostSubject: Re: Do you know?   Fri Mar 25, 2011 6:32 pm

Well I don't know much about LS1 engines- and even less about fence chargers. But I'll tell you what I know.

Ignition coils all work pretty much the same- at least the ones I've messed with. You have a primary winding surrounding the secondary winding. The primary winding has a lower resistance; the secondary winding has a relatively high resistance with very fine wire and lots of turns. To spark the coil, first you excite the primary windings by completing that circuit, which allows that primary winding to build up a magnetic field. Then when you disconnect the circuit for the primary winding, that magnetic field collapses, crossing the secondary windings and inducing a hell of a lot of voltage- with very little current. So yeah, a square wave will fire a coil. But ignition modules generally actuate the coil with a 'rectangle' wave- with more dwell time allowing the magnetic field to build, and just a quick 'off' to trigger it (doesn't take nearly as long to collapse). At higher RPM's (or just a higher actuation frequency in your case), you'll get a hotter spark with more dwell time. But as long as you keep the frequency reasonably low (maybe less than 20Hz), a square wave should work fine. You DON'T want a saw-tooth wave- you want to the magnetic field to collapse quickly to induce voltage.

Every ignition system I've ever worked on sent the spark by opening the circuit to the primary coil on the ground side. But I don't really see any reason why it matters which side of the circuit is actuated.

As for MOSFETs and TTL setups, that's a little outta my league. I'm more a mechanical guy than an electronical guy. But if the MOSFET can produce a clean square wave, then I think it would work. Don't TTL setups usually run on 5 volts? I'm assuming you want a 12V system here. How do you plan to generate the signal to actuate the MOSFET or whatever? Personally if I was doing it, I'd probably just rig up something with an automotive ignition module. Some are surprisingly forgiving about what kind of input will fire them- any ol' sinusoidal wave around a volt or three will probably do it... though the coil will be fired more or less effectively depending on the wave and voltage. I've tested ignition modules just using a little permanent magnet motor for a signal.

I don't know exactly how many amps that coil will pull, but I think a 4 amp MOSFET would probably do the job. Here's how you find out for sure: Seeings how the primary coil is an inductor, it will draw its maximum current initially, and current will drop off somewhat as the field builds. So if you just check the resistance of the primary coil with an ohm meter and apply ohm's law, that'll give you the maximum current that it will draw.

(current)=(voltage)/(resistance)

With a 12V system, as long as your primary coil's resistance is greater than 3 ohms (and I'm pretty sure it will be), current will be less than 4 amps.

Just outta curiosity- how much power does your solar charger supply?

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nitromaxx98
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 12:03 am

In preliminary testing with a standard ignition coil, the current was high enough to smoke a 16 amp transistor, but I'm chalking that up to transient voltage. The MOSFET should take care of that. I am hoping that a modern coil pack will be more efficient, therefore increasing battery life and reliability being that it is solid state.

The solar panel will be for battery maintainance.
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Bighead
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 11:31 am

I guess you could try a zener diode to protect against the transient voltage... not sure how that'll affect the firing of the coil though (might interfere with a clean collapse of the magnetic field?).

Like I said earlier- the simplest solution would probably be an automotive ignition module. It will already have the current capacity and transient voltage protection built in. Personally I'd prefer a module from GM HEI distributor- which will be on pretty much any GM vehicle from the mid 70's through the late 80's. You could probably make just about any ignition module work, but the HEI setups are among the simplest and most common. They're small and durable, the wiring is simple, and they're a dime a dozen.

How many watts is your solar panel? Just curious about whether it'll be able to keep up with the current draw.
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Ratzilla
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 5:11 pm

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Bighead
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 5:26 pm

That's a good article. I didn't know what sort of frequency you'd want a fence charger to run at... but their suggestion is 1Hz. And if that's the case, then I can suggest and even cheaper/simpler alternative to their circuit.

Their suggestion to use a relay would definitely work... and you wouldn't have to worry about excess current being a problem- standard relays are usually rated for 30 amps, which is more than enough. The only reservation I'd have about using a relay is durability. They aren't really designed to be switched continuously every second 24/7. Might be worth a try, but I'm not sure how long a standard relay would last.

But if we're talking about switching the coil maybe once a second, maybe you could use an automotive blinker relay. I'd go for one of the solid-state ones because they don't draw nearly as much current, and they're probably more durable since the don't use a bi-metal strip and contacts. That would be your simplest plug & play option I think.
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nitromaxx98
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 7:02 pm

I read that article earlier today, Ratz. I agree with bighead on the relay reliability issue which is why I want to go solid state.

I've already got the timing control circuit complete using dual 555 timers. One is astable to set pulse timing, the other monostable to set pulse width. I may eventually go with a DIP selectable PIC to ensure proper consistent timing.

The key is, I want the lowest current draw possible. A stand alone car battery powered fence charger will last about 6 to 8 weeks. I want to double that.

Looks like I'll have to get a coil pack and do some real world testing.

Thanks for the input!
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Ratzilla
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 8:01 pm

I gotta ask. Why the electric fence?
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Justoo
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 8:19 pm

Ratzilla wrote:
I gotta ask. Why the electric fence?

Kids.

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nitromaxx98
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PostSubject: Re: Do you know?   Sat Mar 26, 2011 11:19 pm

Justoo wrote:
Ratzilla wrote:
I gotta ask. Why the electric fence?

Kids.

"If you build it, they will say damn that hurts."
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Ratzilla
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PostSubject: Re: Do you know?   Sun Mar 27, 2011 3:28 am

I'll just take it that pests covers it.
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Justoo
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PostSubject: Re: Do you know?   Sun Mar 27, 2011 5:31 am

If building an electric fence in town, please check local ordinances.

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Ratzilla
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PostSubject: Re: Do you know?   Sun Mar 27, 2011 10:24 am

Good point justoo. It'd suck to be successfully sued by some trespassing jackass who got his nads buzzed and have Hace PD write you up a ticket on top of it.
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PostSubject: Re: Do you know?   Sun Mar 27, 2011 12:57 pm

Not for in town. Farm use only.
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