Author Topic: Transistor ignition  (Read 10762 times)

Offline Allen Smithee

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Re: Transistor ignition
« Reply #15 on: January 27, 2015, 08:15:59 AM »
Most commercial (car) ignitions retard the spark by more than 300 - 360 degrees so that the trigger point is safely in the normal operating range for fail-safe fallback modes

AS
Quidquid latine dictum sit altum sonatur

Offline Don1966

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Re: Transistor ignition
« Reply #16 on: January 27, 2015, 05:54:48 PM »
Paul I think your right about using the CCP method. Woodward control uses this on there governor controls.
Although the use of a stable burst freq is used to fill the off time capture between pulses. You could use the burst freq count to calculate the time. Not having any mechanical method of advance the delay action is the only way to do the advance and retarding. Keep at it I am real curious as to your outcome.

Don

Offline Roger B

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Re: Transistor ignition
« Reply #17 on: January 27, 2015, 06:51:34 PM »
These 'predictive' advance/retard systems can cause problems. A year or so back Jo and I were struggling trying to use a Rexel system designed for high speed two strokes on lower speed four strokes (my vertical and Jo's R&B). Amongst the many problems this ignition system expected the sensor to be on the crankshaft and set the advance in crankshaft degrees, we had the sensor on the camshaft so, I think, we had twice the advance at half the revs (or maybe the other way round  ::)  :headscratch: ).
Best regards

Roger

Offline PStechPaul

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Re: Transistor ignition
« Reply #18 on: January 27, 2015, 07:16:22 PM »
There would need to be a setting for the ratio of crank to cam, and perhaps number of cylinders. Probably best to mount the sensor on the crankshaft to get a direct reading. The other problem with the predictive delay used for advance is the case where the speed is changing significantly from one revolution to the next. It changes a lot more over 300 degrees than it does over 60 degrees.  ;)

But first I need to get something working, and then the enhancements can be added.  8)

Offline Don1966

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Re: Transistor ignition
« Reply #19 on: January 27, 2015, 07:35:03 PM »
Agree Paul, but I think a speed trigger would be the way to go. Adjustable set for a given speed so it could cover a set range of speed before being pressed into use. Or even multi trigger for small incremental advances for different speeds. Just thinking out loud here.  Guys like George B and Steve H could give more input as to what speed runs the best without advance or retard on these small engines.   8)

Don

Offline Roger B

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Re: Transistor ignition
« Reply #20 on: January 27, 2015, 07:48:49 PM »
My current, limited, experience suggests that variable advance/retard comes a long way after getting it to run and getting the carburation sorted.  Maybe if you are working at strictlybusiness's level (unbelievable stuff) the firing point is critical. I could move the ignition timing of my vertical engine 30░ with no noticeable effect  :headscratch:
Best regards

Roger

Offline Don1966

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Re: Transistor ignition
« Reply #21 on: January 27, 2015, 08:47:24 PM »
I thought some of our readers would appreciate this.

Here is a short video I made of back EMF or fly back voltage as some might call it. The demo I built years ago to teach a class. The LED is connected backwards so the battery will not light it. The magnetic field will collapse and generate and EMF keeping current flowing in the same direction with the source now being the inductor so the LED light. This is the voltage that will hit your transistor or whatever switching device you use to turn it on and off with. This voltage can reach very high voltages and distroy what's driving it. Precaution are taken to damping this whenever possible in electronic circuits notice how the LED light only when the switch is open.

Regards Don


Offline Admiral_dk

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Re: Transistor ignition
« Reply #22 on: January 27, 2015, 09:03:18 PM »
Ok Don

Perhaps my statement overdid it a bit, but almost all ignition coils I've come across that wasn't dual spark (a sparkplug in each end of the high tension coil) has the secondary connected to the primary internally and doing this at the switch end (and not at the power end) this adds the primary voltage to the secondary voltage (if wired correctly). I haven't seen an old style ignition coil like the one you showed in your reply, since my childhood - that's not to say the they can't be found .... and I do know that vehicles made in the USA are different to most made elsewhere in a lot of small details  ;)

Paul

I really meant it - you will not find a transistor or Mosfet as the switching element in modern ignition systems and for a good reason - there's no way they can compete with the reliability nor simplicity in those modern IGBT's I referred to earlier

Offline Don1966

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Re: Transistor ignition
« Reply #23 on: January 27, 2015, 09:41:37 PM »
Ok

Don

Offline Allen Smithee

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Re: Transistor ignition
« Reply #24 on: January 27, 2015, 10:37:48 PM »
Yes, but that circuit is still showing the coil as an isolating transformer, where all the ignition coils I've ever seen have been autotransformers with no isolation between primary and secondary (because the secondary uses the coils in the primary as well, with a considerable copper saving and and consequent reduction in size and copper losses).

AS
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Offline Admiral_dk

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Re: Transistor ignition
« Reply #25 on: January 27, 2015, 11:09:48 PM »
Sorry - I just realized that I should have added "among the brands I've seen" - the statement without could be interpreted as if I'm claiming to know all, and I'm well adware of my limitations  :old:  ;)

Allan put it nicely - all the ones I've seen has been autotransformers - except from the dual spark ones, on my previous four banger Jap bikes.
That did not prevent those who drew the wiring schematics from putting an transformer in it, even though that actual component on the vehicle was an autotransformer when you measured it.

Offline PStechPaul

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Re: Transistor ignition
« Reply #26 on: January 27, 2015, 11:41:48 PM »
Don's circuit is almost identical to mine, except for the duty-cycle-limiting capacitor, and the isolated secondary. The bodies of the plugs are connected to the low side of the secondary and through the engine to ground. This could also be a junction between primary and secondary, and it could be just as well connected to 12V or ground. This makes little difference when the current stops and the collapsing magnetic field causes the high flyback voltage (not quite a back EMF, which is usually used to describe motors and transformers). No matter what the interrupting device is, the voltage will be limited only by the properties of the resonant LC circuit, or by the breakdown voltage of a TVS, zener, or the device.

With simple mechanical points, the voltage will be limited by the air gap, which is typically about 20 to 50 thousandths of an inch, and the general "rule of thumb" of 10,000 volts per inch means that they will arc at 200 to 500 volts. But a problem with mechanical points is that they open slowly, and will start arcing even when only 0.001" apart, which could be as low as 10 volts. Once an arc has formed, it becomes a plasma consisting of vaporized metal from the points, and this lowers the breakdown voltage of the air between the points. While this arcing is happening, energy is lost, and the output voltage of the coil is low because it is determined by the turns ratio of about 350, so it may start at only 3500 volts at the moment the points open. The plasma reduces the voltage that would otherwise be created according to the widening gap, but if there is enough energy, eventually there will be enough voltage to produce a sufficient arc across the spark plug gap (typically 0.030 to 0.060) to ignite the air-fuel mixture. This environment may change the 10,000 volts/inch approximation, and I think the ignition system typically relies on 30,000 to 100,000 volts for operation. This translates to a primary voltage of 100 to 300 volts.

Once the spark plug fires, the voltage will likely drop because of the continued reduction of energy in the coil, but (I think) it doesn't much matter once ignition has started. However, some old ignition systems (like the Model T) used a continuous high voltage AC that provided timing via the distributor. Modern ignition systems are mostly capacitive discharge which require electronics to sense flywheel position and time the spark.

One problem with the mechanical points used in simple model engines is the fact that the slow opening may actually retard the spark significantly from the position at which they first open. I think this is why some engines need as much as 40-60 degrees of advance, even (or especially) at low speeds. It does not seem logical for the spark to occur that early in the compression cycle, so it must be that the actual spark of sufficient voltage must occur a considerable time after, and close to TDC.

The LC resonant circuit may also affect the point at which maximum voltage occurs. When the points open, the current in the primary of the coil goes through the capacitor (condenser), and the voltage will start at zero. As it charges up, it will reach a point where it holds the coil's energy as a voltage, and then the current flow reverses and an exponential decaying sine wave will occur. But if the spark plug fires, it will take out much of the energy and the waveform will be shorter and lower in amplitude. For my example of a 50 mH coil and 0.1 uF capacitor, the resonant frequency is about 2.55 kHz and the first peak should occur at the quarter cycle point, or 100 uSec after point opening. At 600 RPM, this translates to 360*0.1/100 or 0.36 degrees, so it may be insignificant.

BTW, I was not familiar with the specialized ignition IGBTs you mentioned. It seems they have their own clamping circuit as well as a current sensing lead, which may be quite useful. The data sheet for the FGB3040CS:
http://www.mouser.com/ds/2/149/FGB3040CS_F085-347381.pdf
« Last Edit: January 27, 2015, 11:56:58 PM by PStechPaul »

Offline Don1966

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Re: Transistor ignition
« Reply #27 on: January 27, 2015, 11:57:18 PM »
Paul I believe the guys are missing the point here. I am not worried about how the coil is built. You don't drive a coil by the ground terminal with a transistor you drive the positive side when it has a common connection. The high voltage is created when the coil collapses this is when the switch is off. The path for the high voltage ground is now going through the source not a good idea. This voltage can distroy the control circuits as well as the driving circuit.

Don

Offline Admiral_dk

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Re: Transistor ignition
« Reply #28 on: January 28, 2015, 07:22:41 PM »
Ah now I get your objections Don and I do understand them  ;)

Again I can only refer to the systems I worked on and none of them had the power to the coil come from the box .... The ignition coil is connected to the ignition switch (and from there to fuse and battery). The "midpoint" on the "autotransformer" is connected to the switch (old fashioned point style). In a few cases I've seen the other end of the switch having a separate ground to the rest of the box, going directly to the battery - most has one (or more parallel) ground wires going to the chassis ground. And I'll give you, that a problem with those ground wires from the box is a disaster waiting to happen if or when our old "friend" Murphy "pays a visit"  :zap:  :Mad:  :cussing: .... on the other hand - I've never had one of them go bad yet - knock on wood etc.

The only systems I've seen where the primary of the coil is connected to ground have all been CDI systems and all of those had the output for the box connected to the midpoint again (non dual spark).

Yes Paul - they are quite clever aren't they, those devices  ;)

Best wishes
Per

Offline GordonL

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Re: Transistor ignition
« Reply #29 on: January 29, 2015, 01:00:49 PM »
As usual it does not take long for the discussion on things electronic to get far beyond my understanding. I guess I should just stick with the machining of engines and not try to move beyond my ability.

It does show that we have some pretty impressive talent here and on other similar forums.

At this point in my life I am content to just enjoy my hobby and my shop and not try to learn a whole new career at 75.