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Old 5th Jul 2020, 11:04 pm   #61
turretslug
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Default Re: Reverse engineering Lumenition optical switch

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Originally Posted by Radio Wrangler View Post
Inside an ordinary coil is a bar made up of iron wires or iron laminations. A solid iron bar would lose energy in eddy currents, so dividing it up is sensible, just like the lams in a transformer.... Hey that coil IS a transformer, so why the crappy single bar-shaped core?
Why not a full loop?

Cast your mind back to the induction coil they had at school in the physics lab. A single bar shaped core, some primary winding turns on it, a heck of a lot of secondary turns on it and at the end, the armature that drives a pair of contacts turning the battery, the primary, the core and the contacts into a whacking great buzzer.

It's always been like this, ever since Heinrich Ruhmrorff built the first one. No-one's ever questioned it. He even got name-checked by Jules Verne in Journey to the Centre of the Earth. You can't argue against something with those credentials.

So these things turned into trembler ignitions. An improvement on hot-tubes and blowlamps, just. Then Kettering came along and decided to link the contacts to the rotation of the engine. Brilliant idea, but he was following on from the idea of the magneto, a generator with a pair of contacts to synchronise a high voltage pulse to the rotation of the camshaft or crankshaft. Anyway, Kettering must have looked at the trad induction coil and thought 'If it ain't broke, don't fix it' and consequently everybody got a little induction coil under their bonnet. Sometimes multiple ones.

Tell me again why it's a single bar and not a complete magnetic circuit, like a little proper transformer? Think of a line output transformer in a TV. It wouldn't be very effective on a straight ferrite rod rather than the usual gapped U and I cores. The straight core gives less inductance and needs a lot more turns. Think of the mag flux getting from one end back to the other as a damned great gap. A smaller gap would be better.

The first proper core I saw was on the first Vauxhall Cavalier, the rear wheel drive one with an Opel engine... well, it was an Opel with the badges swapped. An ignition coil built as a U and I transformer core out in the open. The windings were moulded up in plastic, but the iron lams were there for all to see.

It looked wrong. My eyes were used to the ouvres of Robert Bosch and Joe Lucas. But when I stoped to think about it, it made sense. It had a proper magnetic circuit instead of just relying on hope. It ought to be significantly more efficient.

The only drawback was that the straight rod core ones were a handy shape to dunk in a canister filled with transformer oil, to keep out the nasty environment, and to dissipate the heat from its lousy efficiency. This may be why I haven't seen a full core coil like it since. Think of trying to operate a line output transformer in the same compartment as a glowing turbocharger with puddles getting splashed up in season. You're trying to make the same sorts of voltages.

So can the trad ignition coil be improved? Oh yes!

Maybe you'd want to make a transformer shaped one, but you'd need an odd shaped can to seal it in with its oil. And that would look wrong to car type people. THey're easily frightened.

David
Thankyou David- this is a minor revelation. Just about since I first looked into what makes cars go, I'd wondered why car ignition coils were the way they were, why weren't they using a completed magnetic loop, like mains transformers. LOPTs etc? Oh well, the car people know what they're doing, and they've been doing it a long time, who am I to question?.... Then, as you say, the tell-tale exposed lam-stack types appeared. Maybe "the car people" had been doing what they were doing a bit too long!

Colin
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Old 5th Jul 2020, 11:52 pm   #62
Oldcodger
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Default Re: Reverse engineering Lumenition optical switch

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Originally Posted by duncanlowe View Post
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Originally Posted by Radio Wrangler View Post

As an alternative, there are power transistors of the necessary ratings in pachkages similar to that. The problem comes in insulating the collector-connected tab. You need to pass heat efficiently, but electricity not at all.

David
With care, I would think a heatsink isolation mount kit comprising an insulating but heat conducting 'tab' and an insulator for the screw hole, after drilling and tapping the heatsink. I seem to remember there are even self adhesive versions around.
Might be re inventing the wheel, but the older idea was to use body( of semi conductor) as heatsink and fit devices where the collector was at earth potential . (Vague to comply with request on #2)
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Old 7th Jul 2020, 9:23 pm   #63
chris.oates
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Default Re: Reverse engineering Lumenition optical switch

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Originally Posted by Uncle Bulgaria View Post
Chris.oates - here's the final circuit with the parts I used. ..
Many thanks, I have stored your sketch away along with all the rest of the new learning from this thread. What a resource this forum is.

Chris
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