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Old 04-29-2010, 07:18 PM
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ozmatic ozmatic is offline
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Join Date: Jan 2009
Posts: 15
Hi Harvey,
Thanks for your reply, and you seem to have understood my rather confusing description of the circuit. To answer your queries :
Yes, pin 3 is the bottom trace in both pics. Ground is 3 divisions below the center line. I always use the scope probes on x10, and state the actual voltage seen. The time /division is 1ms in both pics. I measured the wave period as 6.25 divisions, giving a frequency of 160Hz.
The trigger selector is set to DC and both channels are DC coupled.
The top trace has ground set at 1 division above the centre line, and is at 50v/division.It shows the output of the ignition coil secondry, with a spark gap only [in one pic], and with a fast acting diode in series with the gap [in the other pic].
I have a pic of the primary /secondary, but both waveforms are exactly the same, except for a difference of exactly 100v. That makes this ignition coil a 100:1 step up autotransformer. I was surprised at how exactly the same the two traces are.
I will try to get the circuit diagram done this weekend with the values as precise as I can get.
This circuit is not vey practical - if I increase the frequency , the reed relay becomes erratic, and if I increase the power to the coil, the relay will burn out. But at this frequency, it gives me the best pictures I have ever got of the charge /discharge of an ignition coil.
I measured the frequncy of the ringing to the best of my ability, and was surprised to find that it was 16kHz - 100 times the frequency of the waveform. Looking at the time that the spark gap is firing, on charge and discharge - they look to be the same without the diode, but there seems to be a lot of energy in the ringing as well. With the diode, there is appreciably less energy in the discharge, so the diode mut be dissipating a lot.
When the relay contacts separate, there is a visible "shoot through" before the diode blocks the negative current, even though the diode is an MUR 140 with a 50ns reverse recovery time. Also , in this shoot through, there is a temporary reversal at about -12v. I dont understand this.
I have to find my camera tripod and get some better pics, as the ones posted are a bit fuzzy. I will post them and the circuit diagram over the weekend. If I can get to understand this circuit it will help me a lot.
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