We seem to have less electronics expertise than in the past, what hppened to Andrew Gabriel?
I have one of the cheap 2T 650W gensets based on the Yamaha 50cc capacitively excited generator handed me by a mate to fix, a NUTool .
It won't start because the HT is too weak to produce a spark and I currently put that down to loss of magnetism in the flywheel magnets.
These things are cheap low quality copies and generally not worth repairing but I also have my first machine saw from 1974 that exhibits the same problem. They are both capacitor discharge ignition.
Any suggestions for a simple battery powered circuit which could sense the trailing edge of the pulse to the coil circuit I and use that to cut the current running through the coil and produce a decent spark?
I'd guess you want a BIP 373. It's a Bosch IC in the shape of a TO220 which takes 5v in (usually from a microprocessor), and drives a coil direct. You can usually find them sold singly on Ebay (under MegaSquirt) or from an MS dealer online. May be tricky to source from the more common electronics suppliers.
Primary side V | ___ | ___| | | ___| | | ___| | ___ | ___| \ ___| | ___| | ___| | __| \_|
+-------------------------------------------- Time excitor charges C, ^ SCR dumps to coil | spark on falling edge
The excitor cannot raise the capacitor voltage too high, or the output of the ignition coil could rise to too-high a voltage and damage the insulation in the ignition coil. As drawn in the Youtube video, the circuit does not have any regulation and runs open loop. And relies on the ratio of excitor pulses to firing pulse, for correct operation. There are ways to fix this (add some regulation), but those ways would also add failure modes to the design.
Since the SCR drains the capacitor exactly (SCR only stops conducting at a low voltage on that falling edge), the capacitor starts from a known state after a firing. It can't "drift off" or anything. The charging stairstep starts at close to zero.
So then the question would be, what needs fixing in the circuit ? Which part has failed, and what do we need to fix ? If a semiconductor fails short (as diode-like devices tend to do), a failure to function could just be a semiconductor problem. It doesn't always have to be a magnetics failure.
It's the kind of circuit, you'd need to see an oscilloscope picture of it working, to know how to deal with failures later, as the picture would tell you what a "normal" peak voltage on the primary is. The ignition coil has some turns ratio. You know you need a certain voltage to make a sufficiently hot spark, and so on. But seeing the waveform, and comparing to the current waveform, hints at how big a "fix" is needed, and what's going on. If the thing remains "flat" as you try to start it, then something has failed short.
I've been thrown across the basement floor by a spark coil, so you don't have to tell me about safety or anything :-) When I came to, my first question was "how did I get over here?". That's how far the shock made me jump. Stay safe...
Thanks Paul, this system is a bit later than that I think, I would have to pull the flywheel to see, the magnets are on the inside of the flywheel and it rotates round the excitor coil, Difference is the exciter and sensor are all in one. I think the trailing edge current from the discharge is what should cause the spark but because the magnets are weak the current generated in the primary isn't high enough to cause the voltage in the secondary to creat the spark.
Hence I was looking for something to switch the current in the coil primary from a 12V battery off when it senses that trailing edge.
I was hoping someone would know of a simple circuit where the present live feed from the CD was taken to ground by a suitable resistor and the voltage across the resistor would cause a current to flow therough the base of a transistor which would switch the current through the coil off and reset.
To make a bipolar transistor work, you need to know whether the pulse is negative going or positive going. To use the pulse directly, would likely be ugly, and involve a lot of "luck" regarding the incoming pulse shape.
The pulse shape probably does not make an ideal "gate" for the job. The gating period might need to be longer than the SCR firing period. The transistor has to be held in the state we want, until the SCR is finished.
For example, a monostable might produce a pulse of a known length. And then the shape of the pulse signal is rendered slightly less important. And when using the monostable, we can set up the circuit so that the monostable delivers the right amplitude of signal to the transistor, to make it saturate when on, and go open when off.
You can see examples here of the idea. The 555 isn't exactly the friendliest chip, but the article shows how it takes a short input pulse and makes it longer. There are some other circuits that might have a bit fewer pathologies. (I think I still have some 4538 chips here.) You can see in one diagram, how they show the 555 used with an NPN transistor, and in the other case, a PNP transistor. The signal to the transistor base has to be slightly different for those.
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It's possible the 555 runs between 5V and 15V. I'd have to check the datasheet. This constrains how the circuit could be kept simple. If the voltage the excitor is working with, is well outside this range, we might need to come up with another idea.
That's why I mentioned having an oscilloscope picture of the thing when its running, to get some idea whether the project is relatively easy, or absurdly hard.
If they kept the primary voltage low, they might be able to use cheaper components to switch the primary. But perhaps the turns ratio on the coil would have to be extended, and it would take many many additional feet of wire while making the transformer. If the primary drive was say, 100V, then the turns ratio can be a bit lower. If the primary was 100V to 1000V, maybe I could use an IGBT to switch the exciter replacement source.
You probably have a much better feel for this circuit than I do. I don't really know what to expect, and I also don't expect the components in the thing are well marked, so we can't get any hints from that. If you found a 1000V SCR, we might assume one thing. And if we found a 40V BJT, we might assume another. If the capacitor only had a 25V WVDC, then the
555 circuit is looking more feasible. Maybe our
555 won't be destroyed by kickback from the transformer :-)
When I made a 15kV supply once, using a couple 2N3055, a flyback and a voltage doubler, the kickback from the action on the primary, blew out the feedback loop in a Hewlett Packard 10 amp bench supply. They had used insulated gate MOSFETs (diff pair), with only about 40V gate rating, and I guess the switching transients got in there. The instrumentation guy invited me into his little repair room, to show me what I blew up, and how he was installing a new one (the transistor he was installing is easily damaged by ESD, and it has a shorting strap that stays around the four legs of it, until it's soldered into a circuit). I ran my high voltage supply off two 6V lantern batteries from then on.
The best part of my 15kV supply ? Never getting a shock from it.
Many many years ago my then boss bought his son a Benelli motorcycle with an electronic ignition module. It failed and even in those far off days a replacement cost over a hundred pounds. He brought it to me and we discovered it was cast in resin, making it impossible to investigate. Fortunately a trip to the local reference library provided a photocopied circuit diagram and I was able to etch a pcb and create a replacement for only a few pounds. I've not spent a lot of time looking at specs but eBay search for electronic ignition could be a useful starting point for you.
Transformers are designed to work with AC signals.
Applying DC to coils isn't always a safe thing to do. Depends on numbers, resistance of coil, and so on.
If you measured the resistance of the primary on the coil, and it was 2 ohms, you'd expect 12V/2ohms = 6 amps to flow, and 6 amps * 12V = 72 watts. Which is likely to be too much for the physical design of the coil, if left there for any period of time.
It's hard to control the "intermittent" connection of a battery well enough, to ensure safety or a lack of damage. You might use or need an "interruptor" ("joy buzzer"), as a means of exciting the coil.
Drivers for coils might involved the usage of capacitors, if you knew what voltage to use. Like what voltage was normal.
Another option, would be to place a 1V amplitude AC signal on the primary (small enough that the SCR might not interact), then measure the voltage on the secondary (which is unloaded). And from that, determine the functioning turns ratio. Since ignition coils can develop wind-to-wind shorts, the number of turns still functional in a coil, might not be the original value.
Much of these tests rely on your knowledge of what would be "normal" for the circuit, in response. I bet, given my drothers, I could make that coil throw a spark, but that's not going to make the device run properly exactly. Such a test would prove there wasn't a dead short or an open in the coil. Like if the secondary was shorted out completely, you wouldn't expect to see a spark or any signal come out.
And based on my experience of being thrown across the basement floor by a car ignition coil, remember your elfen/safety. In my case, I'd thrown a cloth over something (because the workbench was dirty), and I happened to lean on the cloth, and the damn red high tension wire just happened to be underneath there. I swore an oath that day, that I would never again subject myself to something like that, and so far, it worked. The oath worked. No more HV accidents. No more throwing shop rags where they don't belong. And I've worked on HV stuff since then, with zero incidents to report. It's funny how coming to on the basement floor and wondering how you got there, clears the mind.
I stuck a coil out of an MGB into it, it gave a v weak spark but the thing didn't fire even when spun up with a drill.. Just for interest I'll mark the fan at TDC and put a strobe on it. It's a strange beast in that it isn't piston ported and the carb is directly mounted on the crankcase, reed valve I expect.
Dwell is important. This is the time volts are applied to the coil to allow the flux to build up to the maximum, before they are interrupted and the spark occurs.
Yes I didn't expect the car coil to do anything so there was a bit of a bonus it sparked at all.
In the meanwhile I'm sitting back and taking stock. The owner says he barely used the genset at all so I'm thinking it's more likely a problem from having been laid up. Else it's one of those early problems that show up from a manufacturing fault, typically the failure rate over time is bimodal, a large number of faults in machines early in life then a fault free period then an increase as items fail at end of life.
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