In my quest to set my coils so they all fire at the same time, having dealt with timer inaccuracies (they run on DC), I designed this instrument to observe coil firing points. I had been thinking of using the current measuring method and when I discovered a Hall effect current converter IC about 6 months ago (ACS712-TELC-20A)the idea of this instrument was born. While one could sample coil current with a low value resistor, I didn't like that idea as any resistance would cause the coil magnetic field to build up more slowly.
Somewhat coincidentally, Mike Kossor's design appeared in this forum while I was still creating the theoretical design, and although the principles have some things in common, mine is a CRO only device. It is thus not a marketable item to the general Model T owner. Nevertheless, I will describe it for the technical minded here.
Basically, the unit powers the coil at 6, 9 , or 12V at a rate equivalent to 2000rpm. The internal timebase that switches the coil is also available to drive the horizontal axis of the CRO in X-Y mode so that there are no CRO triggering issues; the CRO timebase not being used at all.
The ACS712-TELC-20A provides 100mV output for every 1A that flows through its internal current shunt. Thus, setting the CRO to 100mV/DIV shows coil current in amps per division on the vertical scale.
The coil is provided with supply for 6mS every 60mS. 6mS was chosen because it shows one complete fire. 60mS between fires is equivalent to running the coil in an engine at 2000rpm.
As it turned out, the tester certainly shows where the coil fires, but it also shows multiple firing on maladjusted coils. So, I can use this to set coils for magneto powered cars.
The tester also is useable as a simple 'buzz box' unit with 6 volts applied continuously to do the initial current setting, with the CRO then used to observe firing points.
Here we see the tester in dual trace mode with the 'electronic timer' on the top trace, and coil current on the lower. Each horizontal division is 1mS.
Next we can see the difference between 6 and 12V.
On 12V the coil fires in 2.2mS.
For 6V it takes 3.5mS to fire. So we see why users of 6V ignition systems need more advance.
Here is what multiple sparking looks like. For this demo I slipped a paper clip over the upper point cushion spring to upset the correct adjustment.
And finally, inside the unit. It was just made out of junk box parts I had lying around except of course for the Hall effect IC.
Fantastic! I've always used my scope to set up coils. I just wish I had a nifty little setup like yours there!
Looks like a electrician's nightmare.
Hello John, thanks for sharing this the ACS712-TELC-20A looks just the job and I have put one on order as I have been looking to do similar with my scope. Out of interest what did you use for the 6ms / 60ms timing, looks like you used discreet components but I'm thinking a 556 IC would do the job here. Also cant see a load (spark gap) anywhere?
I used a cross coupled multivibrator for the timing using two germanium transistors. This drives a BD139 emitter follower which in turn drives the coil switching transistor, 2N3055. A neon bulb/diode circuit protects the 2N3055, when it switches it off, from the high voltage from the coil's collapsing magnetic field.
Initially, I went down the path of using a 555 and even a UJT for the timebase, but had problems with the energy radiating from the coil causing spurious triggering. Even with the coil several metres away it was problematic. I figured that a simple low gain and low bandwidth circuit would solve the problem, hence the germanium audio transistors (type AC128, although I'm sure types such as 0C72, 0C74, etc would work just as well). This solved the problem.
The spark gap is just to left of the coil; not visible because of the angle...here's another view:
A few thoughts on this approach:
1.I have found that a rising voltage works better for testing with an o'scope.
2. I use a digital scope. It greatly simplifies trigger and capture issues.
3. I use a hall-effect current probe too, but you can use the power wire for a current dropping resistor to measure current with a voltage probe which wouldn't effect the operation.
JohnH, I understand the flyback diode, but what does the neon bulb do?
The neon simply protects the 2N3055 if is exposed to more than about 70V when it switches off. I could have used a zener but a neon would be more reliable - it doesn't go permanently short circuit if there's a brief overload. Or I could have just used transistor with 400V Vce rating.
The neon does actually flicker from time to time showing it's doing its job.
Thanks for the response John. Built the 555 time base before I read your response! Can confirm your correct though, it is badly affected by noise from coil. I will look in to your idea of cross coupled multivibrator though not quite so familiar with this circuit.