CB77

DIY Electronic Ignition, Take Two

It’s been over a year since I threw together a proof-of-concept electronic ignition for my friend’s 1966 Honda CB77.  Not content with the original, overly generic design; I’ve been working on something a tad more bespoke.

BoardMountedOriginalPoints

The image on the left is, of course, the factory points setup for the CB77. On the right is the PCB I made to replace it, populated with two copies of my earlier design – albeit in somewhat different form.

Also, in addition to the Hall-effect triggered IGBTs, there are a number of components added for testing purposes, as well as not-yet implemented features that (testing has revealed) are in desperate need of revision. The light blue kynar is there just to break-out some test points.

Here are some more images, before and after population:
Front_UnpopulatedRear_Unpopulated
Front_PopulatedRear_Populated

As is my custom, after I laid out the major components I auto-routed the board – cleaning up traces as I saw fit (read: as much as I had patience for). There will likely be a lot more tidying before I consider it a finished product.

The primary components and functionality remain unchanged from my original design, with the Hall latches and IGBTs only translated to their surface-mount brethren. I went to great lengths to preserve reasonable isolation around anything subjected to high voltage DC, and even greater lengths to provide adequate heat-sinking for the IGBTs (via the ‘winged’ copper pours), all in a relatively aesthetic package (one that, ironically, is designed to tuck up under the points cover – completely unseen). Incidentally, I’m now quite professional at writing scripts for eagle in polar coordinates.

While the original points setup required only two wires, this design requires four. The two coil wires (just as in the points setup), with an added power and ground. Now, while the addition of the power wire was unavoidable, I fought a bit with whether or not to ground through the housing. In the end, I decided that one more wire was manageable, and would provide a more robust DC return, while also helping to preserve isolation (as grounding through the housing would require additional ground pours, encroaching on the IGBT heat-sinks).

For testing purposes, I’ve mounted this board to its parent housing, which I’ve in-turn mounted to a test fixture I cobbled together.

Test_Rig

I’m almost ashamed to say it, but a lot more effort went into building that than really shows. Regardless, it’s been invaluable in testing and evaluating the board’s functionality.

Triggering is managed by a pair of magnets (with opposite outward-facing poles) embedded in a wooden disc, mounted in the collet of a spindle motor. The spindle motor is powered by a 48V 600W supply, with variable RPM managed via PWM.

The results:
11490RPM

The blue trace is taken from the gate of the right-side IGBT, with the yellow taken from the ‘negative’ terminal of the corresponding coil (with respect to ground). 191.5Hz works out to 11,490 RPM. Plenty.

And then there’s the other end of the spectrum:
525RPM

This is roughly 525 RPM, and it’s where the limitations of my benchtop supply become painfully apparent. It’s a 0-15V 3A supply, here set to 14.6V. Current draw was sufficient to load it down to around 10 Volts – with the loading of the second coil causing some stepping in the blue waveform. Despite all this, and the pitiful 30V counter-EMF on the coils, the plugs kept firing away happily, with sharp blue sparks.

Unfortunately, despite this hugely successful outcome, there’s still a lot more testing left to do. With an appropriate trigger wheel, I’m extremely confident I could bolt this into any CB77, do some minor tuning, and be on my way. But for how long? That’s another question entirely.

While I feel I’ve provided adequate heat-sinking, and while the IGBTs remained at room temperature throughout all stages of testing; it is an air cooled motor with almost zero airflow under the points cover. Without some real-world testing and abuse, there’s really no telling what conditions will be faced. The good news is that there’s some precedent, with the max operating temp of the Hall latches and IGBTs exceeding that of the FR4 PCB, of which many other ignition conversion PCBs are made.

As for what’s next, I need to revise or eliminate the aforementioned non-functional features, further clean up the board routing, have a proper trigger wheel machined to mount on the original points cam, and do some real-world testing (volunteers?).

Once the design is finished and tested, I will be releasing everything I have – completely open source (as is everything I’ll ever post to this blog). If anyone is interested in the eagle files prior to that, just hit me up and I’ll happily share (but don’t expect a lot of sense, cleanliness, or support).