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: Best capacitor transistor for CDI with very fast rise time
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Electron
Tue, 19 Jan 10 23:05:29 +0000

My dears,
I have a 2 stroke race engine and I'd like to make a CDI (Capacitive
Discharge Ignition) unit for it. As it is a 300cc 2 stroke enduro bike
mostly driven at low RPM in rocks, etc.. what I really need thus is a
fast voltage rise time to fire fouled plugs.
While I've been googling (literally) for days now, and searched the
PICList archives from 2004 on, I haven't found a simple answer to my
question:

What is the best type of capacitor to get a FAST rise time? I guess one
with low ESR.. yes, but how low? In the order of 0.005 to 0.02 Ohm?

What is the best type of transistor to trigger it (FAST rise time again)?
A SCR seems such a primitive device to me, I'd prefer something like a
MOSFET or a IGBP. Any pro's and con's of each valid solution?

Thanks a lot, it's always delighting to read you.. I only wish I could
contribute more often with something useful.

Mario

4 replies folowing
=?ISO-8859-1?Q?Ruben_J=F6nsson?=
Wed, 20 Jan 10 07:16:43 +0000
>
> My dears,
> I have a 2 stroke race engine and I'd like to make a CDI (Capacitive
> Discharge Ignition) unit for it. As it is a 300cc 2 stroke enduro bike
> mostly driven at low RPM in rocks, etc.. what I really need thus is a
> fast voltage rise time to fire fouled plugs.
> While I've been googling (literally) for days now, and searched the
> PICList archives from 2004 on, I haven't found a simple answer to my
> question:
>
> What is the best type of capacitor to get a FAST rise time? I guess one
> with low ESR.. yes, but how low? In the order of 0.005 to 0.02 Ohm?
>
> What is the best type of transistor to trigger it (FAST rise time again)?
> A SCR seems such a primitive device to me, I'd prefer something like a
> MOSFET or a IGBP. Any pro's and con's of each valid solution?
>
> Thanks a lot, it's always delighting to read you.. I only wish I could
> contribute more often with something useful.
>
> Mario
>


I have made a couople of CDI type systems. Not used for ignition though but to
generate high voltage/field for corona discharging of electrostatic charges
(ionizer).

The capacitor type I have used are metalized paper (Evox/Rifa PME271M) which
are chareged to around 450V through an IGBT transistor (high side switch) and
an inductor and then discharged through the high voltage coil with an SCR. I
have also used a pulse capacitor with polypropylene as dielectric (Evox/Rifa
PHE426) with very good results. This is used for lower voltages though (charged
to around 40V with a P type MOSFET and then discharged with an N type MOSFET).

The way I have done it, the charging does not have to be fast and it is charged
via the inductor for two reasons: 1. The final voltage over the capacitor
becomes roughly twice the power supply voltage and 2. The charging is slowed
down so I can more easily control the charge time and thereby also control the
output power.

To lower the esr when discharging you could use several capacitors in
parallell.

/Ruben

==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
==============================
dicsEE
Wed, 20 Jan 10 08:06:28 +0000
 
Electron
Wed, 20 Jan 10 08:06:28 +0000

Tack Ruben, it was very helpful.

How do I calculate rise time basing on capacitor ESR, coil resistance, and
some other parameter?

It's just a RLC system in the end, or does it hide some snare?

PS: great idea to parallel e.g. 10 0.1uF 600V caps instead of using one big
1uf 600V.. good for ESR but also for space. Also, easier to adjust capacity
to find the optimal one.

Hejdå!
Mario


At 08.16 2010.01.20, you wrote:

>>
>> My dears,
>> I have a 2 stroke race engine and I'd like to make a CDI (Capacitive
>> Discharge Ignition) unit for it. As it is a 300cc 2 stroke enduro bike
>> mostly driven at low RPM in rocks, etc.. what I really need thus is a
>> fast voltage rise time to fire fouled plugs.
>> While I've been googling (literally) for days now, and searched the
>> PICList archives from 2004 on, I haven't found a simple answer to my
>> question:
>>
>> What is the best type of capacitor to get a FAST rise time? I guess one
>> with low ESR.. yes, but how low? In the order of 0.005 to 0.02 Ohm?
>>
>> What is the best type of transistor to trigger it (FAST rise time again)?
>> A SCR seems such a primitive device to me, I'd prefer something like a
>> MOSFET or a IGBP. Any pro's and con's of each valid solution?
>>
>> Thanks a lot, it's always delighting to read you.. I only wish I could
>> contribute more often with something useful.
>>
>> Mario
>>
>
>I have made a couople of CDI type systems. Not used for ignition though but to
>generate high voltage/field for corona discharging of electrostatic charges
>(ionizer).
>
>The capacitor type I have used are metalized paper (Evox/Rifa PME271M) which
>are chareged to around 450V through an IGBT transistor (high side switch) and
>an inductor and then discharged through the high voltage coil with an SCR. I
>have also used a pulse capacitor with polypropylene as dielectric (Evox/Rifa
>PHE426) with very good results. This is used for lower voltages though
>(charged
>to around 40V with a P type MOSFET and then discharged with an N type MOSFET).
>
>The way I have done it, the charging does not have to be fast and it
>is charged
>via the inductor for two reasons: 1. The final voltage over the capacitor
>becomes roughly twice the power supply voltage and 2. The charging is slowed
>down so I can more easily control the charge time and thereby also control the
>output power.
>
>To lower the esr when discharging you could use several capacitors in
>parallell.
>
>/Ruben
>
>==============================
>Ruben Jönsson
>AB Liros Electronic
>Box 9124, 200 39 Malmö, Sweden
>TEL INT +46 40142078
>FAX INT +46 40947388
>ruben@pp.sbbs.se
>==============================
>
>--
>>>
=?ISO-8859-1?Q?Ruben_J=F6nsson?=
Wed, 20 Jan 10 11:33:47 +0000
The capacitor is also part of the LC circuit formed by the discharge capacitor
and the primary of the high voltage transformer. The LC circuit will produce
some ringing which in turn may give a longer duration for the spark.

The ESR is so low compared to other impedances in the circuir during the
discharge so I don't think it will matter too much for the timing. Use a low
ESR to keep temperature rise to a minimum. This may be a problem depending on
how often you need to produce a spark.

/Ruben

>
> Tack Ruben, it was very helpful.
>
> How do I calculate rise time basing on capacitor ESR, coil resistance, and
> some other parameter?
>
> It's just a RLC system in the end, or does it hide some snare?
>
> PS: great idea to parallel e.g. 10 0.1uF 600V caps instead of using one big 1uf
> 600V.. good for ESR but also for space. Also, easier to adjust capacity to find
> the optimal one.
>
> Hejdå!
> Mario
>
>
> At 08.16 2010.01.20, you wrote:
> >>
> >> My dears,
> >> I have a 2 stroke race engine and I'd like to make a CDI (Capacitive
> >> Discharge Ignition) unit for it. As it is a 300cc 2 stroke enduro bike
> >> mostly driven at low RPM in rocks, etc.. what I really need thus is a
> >> fast voltage rise time to fire fouled plugs.
> >> While I've been googling (literally) for days now, and searched the
> >> PICList archives from 2004 on, I haven't found a simple answer to my
> >> question:
> >>
> >> What is the best type of capacitor to get a FAST rise time? I guess one
> >> with low ESR.. yes, but how low? In the order of 0.005 to 0.02 Ohm?
> >>
> >> What is the best type of transistor to trigger it (FAST rise time again)? A
> >> SCR seems such a primitive device to me, I'd prefer something like a MOSFET
> >> or a IGBP. Any pro's and con's of each valid solution?
> >>
> >> Thanks a lot, it's always delighting to read you.. I only wish I could
> >> contribute more often with something useful.
> >>
> >> Mario
> >>
> >
> >I have made a couople of CDI type systems. Not used for ignition though but to
> >generate high voltage/field for corona discharging of electrostatic charges
> >(ionizer).
> >
> >The capacitor type I have used are metalized paper (Evox/Rifa PME271M) which
> >are chareged to around 450V through an IGBT transistor (high side switch) and
> >an inductor and then discharged through the high voltage coil with an SCR. I
> >have also used a pulse capacitor with polypropylene as dielectric (Evox/Rifa
> >PHE426) with very good results. This is used for lower voltages though (charged
> > to around 40V with a P type MOSFET and then discharged with an N type MOSFET).
> >
> >The way I have done it, the charging does not have to be fast and it
> >is charged
> >via the inductor for two reasons: 1. The final voltage over the capacitor
> >becomes roughly twice the power supply voltage and 2. The charging is slowed
> >down so I can more easily control the charge time and thereby also control the
> >output power.
> >
> >To lower the esr when discharging you could use several capacitors in
> >parallell.
> >
> >/Ruben
> >
> >==============================
> >Ruben Jönsson
> >AB Liros Electronic
> >Box 9124, 200 39 Malmö, Sweden
> >TEL INT +46 40142078
> >FAX INT +46 40947388
> >ruben@pp.sbbs.se
> >==============================
> >
> >--
> >> >> >>
> --
> > > >


==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
==============================
Electron
Wed, 20 Jan 10 20:07:56 +0000
At 12.33 2010.01.20, you wrote:
>The capacitor is also part of the LC circuit formed by the discharge capacitor
>and the primary of the high voltage transformer. The LC circuit will produce
>some ringing which in turn may give a longer duration for the spark.


And the ESR will directly affect the Q of the system, thus the spark duration..


>The ESR is so low compared to other impedances in the circuir during the
>discharge so I don't think it will matter too much for the timing. Use a low
>ESR to keep temperature rise to a minimum. This may be a problem depending on
>how often you need to produce a spark.


Racing, high RPM 2 stroke engine.. and anyway I'd like to play with multispark,
so.. ;)

Cheers,
Mario