Groups > sci.electronics.design > #453681

Ignition circuit issue.

From	Sylvia Else <sylvia@not.at.this.address>
Newsgroups	sci.electronics.design
Subject	Ignition circuit issue.
Date	2017-03-26 15:36 +1100
Message-ID	<ejoupmFhi4bU3@mid.individual.net> (permalink)

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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.

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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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