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Re: Ignition circuit issue.

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On 26/03/2017 4:33 PM, John Larkin wrote:
> On Sun, 26 Mar 2017 15:36:06 +1100, Sylvia Else
> <sylvia@not.at.this.address> wrote:
>
>> I'm using this circuit to provide 250V pulses to an ignition coil.
>>
>> https://www.dropbox.com/s/1w33uss3eedem05/ignition2.pdf?dl=0
>>
>> The IC is a Linear Technology LT1243.
>>
>> 12V is provided on G$1 to the left. G$2 accepts a trigger pulse, and G$3
>> is ground.
>>
>> LTSpice says that this works, and indeed the real implementation does.
>> Mostly.
>>
>> The difference is that after the circuit has been triggered once, it
>> won't charge the capacitor again (above about 25 volts) for a varying
>> period that can be up to half a minute. It's as if the wrong level is
>> being used internally to compare with the feedback pin. In that regard,
>> I'll mention that Q3 is used to pull FB up so that the circuit stops
>> driving the FET, since otherwise the SCR never turns off. I've checked,
>> and the problem is not that Q3 remains on, nor is it that the SCR fails
>> to turn off. In any case, the 1.5uF capacitor C1 does get charged
>> somewhat, as I mentioned above.
>>
>> When the issue resolves, it does so rapidly, within a fraction of a second.
>>
>> Connecting an oscilloscope probe to the RT/CT pin of the IC restores
>> function, but only if the scope is set to DC coupling - not AC coupling
>> - and functioning continues only as long as the probe remains connected
>> - until the aforesaid varying period has elapsed. Touching the pin with
>> a finger has a similar effect.
>>
>> That is, connect probe; works. Disconnect; stops working. Reconnect;
>> works, etc. until eventually it starts working continuously.
>>
>> This cannot be capacitive loading - there's already a significant
>> capacitance between RT/CT and ground.
>>
>> The fact that the coupling has to be DC made me think that some small
>> current was flowing to ground through the scope, so I bridged C4 with a
>> 100K resistor. It didn't make any difference.
>>
>> I tried changing the IC (a pain, given that it's surface mounted), but
>> again it made no difference.
>>
>> Any other ideas about what might be happening here.
>>
>> Sylvia.
>
> Just possibly the SCR is staying on, shorts the inverter, which
> thermal limits, and recovers later? It's awfully complex.

I don't see any sign of that - the voltage across the coil is zero at 
this point, whereas the capacitor C1 has about 25V across it, dropping 
to zero each time the circuit is triggered, and then climbing back toe 
25V. The scope shows a few steps, as it gets pumped to there, but it 
doesn't continue upwards.

>
>
> Back when it wasn't so lethal to ride motorcycles on the street (and I
> wasn't too bright) I use to make my own CD ignitions.
>
> I liked this topology
>
> https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/CDI.jpg
>
> because it was easy to drive the SCR gate with a simple RC circuit
> from the points. A low value resistor from gate to ground ensures
> turnoff. You could elect to kill the boost converter during the gate
> pulse, too.
>
> To get a fat spark, you need a pretty wide SCR gate pulse, to let the
> whole mess ring a few cycles. I usually ran around 80 to 100 mJ per
> spark, which will fire a plug no matter how fouled. Barely needs
> gasoline.

Looks like I'm using a bit under 50mJ, which seems to be working, albeit 
with a plug in reasonable condition.

>
> Actually, my HV supplies were open-loop 2-transistor DC/DC converters
> with full bridge rectifiers. It sort of relaxed for a while after
> being shorted, then started oscillating again, which was nice.
>
>

My application is the engine of a generator. It doesn't have points, but 
has some magnet/coil arrangement that produces a pulse at the correct 
point in the cycle.

Certainly charging through the coil simplifies things, but I had/have no 
idea what the inductance of the coil might be. Indeed, it turned out 
that my coil had insulation failure in the HT side, and I had to source 
a new one, so even if I'd tried to measure the old one, the new one 
could have been different.

Finally got the engine running today, using this ignition circuit, and 
LPG, after many hours spent cursing a PIC32MX that I was temporarily 
using to generate the trigger pulse - got caught out by yet another PIC 
errata - input capture doesn't work when the processor is in the idle state.

Sylvia.

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Thread

Ignition circuit issue. Sylvia Else <sylvia@not.at.this.address> - 2017-03-26 15:36 +1100
  Re: Ignition circuit issue. John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-25 22:33 -0700
    Re: Ignition circuit issue. Sylvia Else <sylvia@not.at.this.address> - 2017-03-26 17:33 +1100
      Re: Ignition circuit issue. Larry Sheldon <lfsheldon@gmail.com> - 2017-03-27 00:54 -0500
        Re: Ignition circuit issue. Sylvia Else <sylvia@not.at.this.address> - 2017-03-27 19:34 +1100
      Re: Ignition circuit issue. dagmargoodboat@yahoo.com - 2017-03-27 07:16 -0700
        Re: Ignition circuit issue. makolber@yahoo.com - 2017-03-27 09:18 -0700
        Re: Ignition circuit issue. Sylvia Else <sylvia@not.at.this.address> - 2017-03-28 11:13 +1100
  Re: Ignition circuit issue. Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-03-27 16:25 -0700
    Re: Ignition circuit issue. Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-03-27 16:31 -0700
      Re: Ignition circuit issue. Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-03-27 16:36 -0700
      Re: Ignition circuit issue. Sylvia Else <sylvia@not.at.this.address> - 2017-03-28 15:19 +1100
        Re: Ignition circuit issue. Neon John <no@never.com> - 2017-03-31 14:41 -0400
          Re: Ignition circuit issue. Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-03-31 11:55 -0700
    Re: Ignition circuit issue. Sylvia Else <sylvia@not.at.this.address> - 2017-03-28 12:03 +1100
      Re: Ignition circuit issue. piglet <erichpwagner@hotmail.com> - 2017-03-28 08:39 +0100
        Re: Ignition circuit issue. Sylvia Else <sylvia@not.at.this.address> - 2017-03-28 20:54 +1100

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