IGN Module

[Daryl]

The bigest problem iv been having is with the Wipac coil in a 40+ killing the module. The Villiers isnt quite as bad.

[Top Cat]

I discovered this circuit about 12 months ago on an american site and have used it before I got my lonley seagull without problems on larger twin cylinder outboards.I have always used much bigger heat sinks than your bit of ali.
RE MOSFET it is possible to use a mosfet but you need to "find" 10v to pull the gate up. this is not a problem on small outboards with so called lighting coils/or an electric start motor but is more of a chalenge on a Seagull.
Re Zenner diodes...I think daral you could solve your over heating problem with 4 x 56v diodes in series 2 facing one way and two the other ( how do you post a diagram on this site ??? ) this would still give you the 100v protection for the transistor....

OR why not use two of your darlingtons in series so you get 200v ..you have a NPN clomping on and off so will not be a problem to switch both simultaneously...may need some diodes to ensure the voltage across each is equal......

let me know what you think......

[Ian Malcolm]

What sort of test jig are you using to develop this? I guess the simplest would be a seagull in a tank with the points and coil LT connections brought out the drain holes in the baseplate so you can run the engine using an external coil and battery in a Kettering ignition circuit (don't forget a condenser) while testing your circuit on the magneto with a spare plug as the load. This would let you check waveforms on a scope or even make circuit adjustments without stopping the engine

The major difference in the Wipac ignition is the integral condenser in the coil. Shorting a charged condenser will be very hard on the transistor.

[Charles uk]

Where did the 100V figure for the primary output come from?

Was this measured or guessed?

[Top Cat]

Well Charles I have only come to this site/thread recently and the first bit seems a bit disjointed....Daryl should answer but I think the 100v has come from choosing the highest voltage rated transistor at an economical price ....Condensers for cars are marked usually 300v .I always design for 300v for my two cyl motors hence the MOSFETS but thats difficult on a seagull I am discovering.

As for the discharge of the condenser in a Wipac being hard on the transistor ....I think not...when the transistor is ON the condenser is discharged ..it charges when the transistor turns OFF (spark) and by the time the transistor goes on again it has disipated its energy into the primary. As for a test circuit I think just spinning the flywheel with a drill and using a large enough spark gap would simulate the worst conditions...

So much fun from a £30 boot sale buy ......problems to solve ,brain overheating ,,,, .

[electrosys]

Where did the 100V figure for the primary output come from?

Was this measured or guessed?

Well - there are several patents from well-respected companies where 100-150v is being suggested as typical of the primary back-EMF voltage, so I think it's fair to say that someone, somewhere, has actually measured this. One firm (forget which, offhand) quotes 300v - but that seems a bit excessive.

It's difficult to be precise about this voltage, because the amount of inductive energy stored in the primary winding (and thus the size of it's back-EMF spike, and the size of the spark at the plug) will vary according to the speed of the engine. Indeed, at higher RPMs, it is this increase in voltage developed across the primary coil *before* the semi-conductor is switched into non-conduction (being directly responsible for the size of the back-EMF spike) which is used in many circuits to control advance-retard timing: the logic being that at higher speeds, the threshold voltage (which is usually around 4v) used to switch-off the power semiconductor is reached earlier in the cycle, and therefore switch-off also occurs earlier in the cycle, which results in the production of an ignition spark advanced in time.

It is this circa. 4v threshold voltage (when the inductive energy in the primary is considered to have reached it's maximum) which renders MosFETs unusable in this circuit, as most will not even begin to conduct until 4v is reached - by which time the circuit would switch 'em off - thus they would never get to convert the induced voltage into stored energy in the winding.

Bipolar transistors conduct at a much lower voltage, and thus the primary coil is allowed time to develop it's store of induced energy before the semiconductor equivalent of the 'points opening', occurs.

[Top Cat]

designed for the job ,,with the mosfet we struggle for voltage and with this we struggle for base current...

BU941/BU941P BU941PFI HIGH VOLTAGE IGNITION COIL DRIVER NPN POWER DARLINGTON

http://uk.rs-online.com/web/p/darlingto ... r/4191144/

[electrosys]

"with the mosfet we struggle for voltage and with this we struggle for base current..."
That just about sums up the situation perfectly. Although I would say with an hFE of 300 (min), the BU941 is about the best device suggested thus far.

The only bummer (which is no-one's fault) is that at one-off prices of around a fiver (+ VAT and delivery), it is debatable whether making these modules is really worthwhile, when one-off commercial modules can be sourced for as little as £7 (delivered). And maybe slightly cheaper in bulk.

[Daryl]

The cost of the switching transistor was my main concern, as was the avalability of some types to me. I found 140V pp across Villiers coils using a cro. But suspect the Wico coil may be higher. I apreciate the exta thaughts & input from you guys as it has been a great help bouncing diferent ideas around. (not beaten yet just a bit short of money at the moment). Ian one of my testbeds is an old 40+ seagull power head with no piston or barrel, setup in a vice & can be driven with an electric drill (Villiers coil). No good Seagulls were harmed in this as it had a faulty crankshaft.

The reason i went down this path is the modules were $30 here & now almost imposible to get here. Freght costs from the US are very high compared to here.

[Collector Inspector]

Note.





This electronic Fires two pots at once........................figure that EMF etc then.

B

[Oyster 49]

Those Wipac FW magnetos seem to be fitted to loads of different motors! singles, twins with alternate firing 2 coils, twins with single coils both firing at once. All seem to have the same backplate!

[Charles uk]

Here's how Wipac did it for Seagull, 1 primary 2 secondary coils.

On the Wipac common base plate

Singles, 1 primary, 1 secondary coil.

Twins with single coils both firing at once, 1 primary 2 secondary coils.

Twins with alternate firing, flywheel with 2 sets of magnets , both plugs firing every 180 degrees.

Don't quote me on this, I'm only suggesting that it should work this way.

[Top Cat]

I will order some parts up and try this on my WSPC...as I dont have a carb I can spin it with a drill and use it as a test bench ..if it works ok with that 500v transistor I dont think it will blow ..will also upgrade the 337 as its only rated 45v...may be a week or two ..lots on and its cold in the garage .....

[Charles uk]

If I remember right 40tpi posted the primary output voltages over a year ago of these 2 ignitions & it was something like 130v @ 2000 revs.

As for the timing, if your planning on making it variable, between 0 & 1000 revs the books say it should be between 12 & 15 degrees BTDC for starting, then a very steep rise up to circa 23 BTDC, then at 3500 start to retard slowly to about 18 degrees BTDC @ 7500 revs at which point the rod will be thinking of throwing it's toys out of the pram.

When you can buy an identical Seagull with a missing transom bracket for less than £20 I can't quite see the logic for replacing an ignition system that has worked perfectly for 30 years for a R & D prototype,call me old fasioned if you like but a couple of miles is a long way to row!

[Ian Malcolm]

I think a pessimist would tie back the existing points arm so it clears the cam and the points are open and fit the Atom style module in the spare space opposite the HT connector so you can revert to traditional ignition simply by removing the nut and cover and snipping the wire and cable tie. Beats rowing!