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Ignition circuit problem

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  Ignition circuit problem Sylvia Else <sylvia@not.at.this.address> - 2017-03-26 15:34 +1100
    Re: Ignition circuit problem Sylvia Else <sylvia@not.at.this.address> - 2017-03-26 15:35 +1100
      Re: Ignition circuit problem Larry Sheldon <lfsheldon@gmail.com> - 2017-03-26 20:39 -0500
        Re: Ignition circuit problem Larry Sheldon <lfsheldon@gmail.com> - 2017-03-26 21:27 -0500
        Re: Ignition circuit problem Sylvia Else <sylvia@not.at.this.address> - 2017-03-27 14:01 +1100
          Re: Ignition circuit problem Larry Sheldon <lfsheldon@gmail.com> - 2017-03-27 00:32 -0500
    Re: Ignition circuit problem Rich <rich@example.invalid> - 2017-03-26 05:02 +0000

#13371 — Ignition circuit problem

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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#13372

On 26/03/2017 3:34 PM, Sylvia Else wrote:
> I'm using this circuit to provide 250V pulses to an ignition coil.
>

Oops - sorry - wrong group.

Sylvia.

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#13374

On 3/25/2017 23:35, Sylvia Else wrote:
> On 26/03/2017 3:34 PM, Sylvia Else wrote:
>> I'm using this circuit to provide 250V pulses to an ignition coil.
>>
>
> Oops - sorry - wrong group.
ACKKKK!  That is cruel!

What is the right group?  Interesting question.




-- 
quis custodiet ipsos custodes?
-- Juvenal

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#13375

On 3/26/2017 20:39, Larry Sheldon wrote:
> On 3/25/2017 23:35, Sylvia Else wrote:
>> On 26/03/2017 3:34 PM, Sylvia Else wrote:
>>> I'm using this circuit to provide 250V pulses to an ignition coil.
>>>
>>
>> Oops - sorry - wrong group.
> ACKKKK!  That is cruel!
>
> What is the right group?  Interesting question.

Many years ago[1] I built a breadboard[2] preamp[3] for some exotic 
phonograph cartridge (NOSEITR[4]) by wiring everything 
point-to-point[5], creating a ball of stuff around a tube socket for the 
6C4 tube.  After some tinkering, it worked like a charm!

So I carefully took it apart, piece by piece, and reassembled it in a 
proper aluminum box-chassis.

And that SOB[6] motor-boated!  And I never was able to figure out why.

(I eventually, I think, saved enough money to buy a DYNAKIT Preamp.)

1.  How long ago?  This is part of my high-school-age "High Fidelity 
Sound System" for my bedroom.  "HiFi" was an obscene term, there was no 
"Stereo" yet, although the trade shows had displays of "binaural" 
records and the magazines agonized over ever getting wire recorders to 
do Binaural.

2.  From a technique of using a wooden board for a chassis for 
experimental designs.  I didn't have a suitable piece of wood, and if I 
could have afforded to replace my mother's prize breadboard I could have 
afforded an aluminum box.

3.  Preamplifier--typically had one or more of these functions:  On the 
input side, operate at the impedance the input device required, provide 
tone adjustments, provide a connector type not in the local standard, 
and so forth.  On the output side, proved a signal at the 
system-standard level and impedance via a system-standard connector. 
The term frequently applied to a chassis containing several preamps 
connected to a common output.

4.  No Other Specification Extant In The Record(s)

5.  No part-leads cut or bent (much).

6.  Do I have to explain EVERY thing?
-- 
quis custodiet ipsos custodes?
-- Juvenal

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#13376

On 27/03/2017 12:39 PM, Larry Sheldon wrote:
> On 3/25/2017 23:35, Sylvia Else wrote:
>> On 26/03/2017 3:34 PM, Sylvia Else wrote:
>>> I'm using this circuit to provide 250V pulses to an ignition coil.
>>>
>>
>> Oops - sorry - wrong group.
> ACKKKK!  That is cruel!
>
> What is the right group?  Interesting question.
>
>
>
>

sci.electronics.design is where I intended to post (and have now). No 
explanation has been forthcoming. Which is a nuisance, since this is 
intended to the electronic ignition system for a generator I'm trying to 
restore to function for use during rolling blackouts that we seem at 
increasing risk of getting in the summer, here in third world Sydney, 
Australia.

If I can't understand the behaviour, then I can't trust it to work when 
the day comes, and I'll have to redesign/rebuild it.

Sylvia.

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#13377

On 3/26/2017 22:01, Sylvia Else wrote:
> On 27/03/2017 12:39 PM, Larry Sheldon wrote:
>> On 3/25/2017 23:35, Sylvia Else wrote:
>>> On 26/03/2017 3:34 PM, Sylvia Else wrote:
>>>> I'm using this circuit to provide 250V pulses to an ignition coil.
>>>>
>>>
>>> Oops - sorry - wrong group.
>> ACKKKK!  That is cruel!
>>
>> What is the right group?  Interesting question.

> sci.electronics.design is where I intended to post (and have now). No
> explanation has been forthcoming. Which is a nuisance, since this is
> intended to the electronic ignition system for a generator I'm trying to
> restore to function for use during rolling blackouts that we seem at
> increasing risk of getting in the summer, here in third world Sydney,
> Australia.
>
> If I can't understand the behaviour, then I can't trust it to work when
> the day comes, and I'll have to redesign/rebuild it.

I have no for-sure-useful information, but it did pique my interest.

I filter as lot of stuff and my provider filters a lot of stuff, so I 
went to Google Groups and into the swamp to see if I could find a link 
to you and I did see that somebody HAD replied to comp.misc with 
interesting (to me) questions.




-- 
quis custodiet ipsos custodes?
-- Juvenal

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#13373

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 

Unless I miss counted, I count eight "the capacitors" in the schematic. 
Which one, specifically, is "the capacitor"?  I can guess by the end of
this paragraph that "the capacitor" might be C1, but that's still
slightly unclear (i.e, "the capacitor" might also be C5, which is also
connected to the voltage pump that is the center portion of the diagram
connected off the output of the inductor.

> 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.

However, the change in capacitive loading from the scope probe on RT/CT
will change the RC delay constant of the R6/C4 pair.  In order to
charge "the capacitor" (C1???) to 250V from a 12V supply, you've built
a voltage pump.  For the pumping effect to work, the on time to off
time ratio of the transistor that "charges" the inductor (U$2) has to
be large enough to allow enough flux to build up in the inductor such
that when the "charging" transistor turns off, there's enough energy to
bump the voltage up to your design parameters.  Maybe your existing
10K/2.2e-9 RC constant is very close, but just not quite, right, and
the slight extra capacitance from the scope probe (or a finger, same
effect) changes the RC delay (and the corresponding frequency and
on/off timing of the IC) just enough to move from an "almost but not
quite" setting to a "works all the time" setting.

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