Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > sci.electronics.design > #453845

Re: Ignition circuit issue.

Show all headers | View raw

On 3/26/2017 01:33, Sylvia Else wrote:
> 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.

I just sortakinda fell into this thread and I do not have the 
credentials to be jumping in, but I do have to say that that that 
"magnet/coil arrangement"  is called a "magneto" on the backyard and 
shade-tree mechanic circuits I used to hand on, and it puts out a 
shocking pulse tall enough to fire a plug on a good day.

> 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.
>
>
>
>
>
>
>
>


-- 
quis custodiet ipsos custodes?
-- Juvenal

Back to sci.electronics.design | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

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

csiph-web