The capacitor is also part of the LC circuit formed by the discharge capaci=
tor =

and the primary of the high voltage transformer. The LC circuit will produc=
e  =

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 lo=
w =

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 b=
ig 1uf
> 600V.. good for ESR but also for space. Also, easier to adjust capacity t=
o find
> the optimal one.
> =

> Hejd=E5!
> 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 agai=
n)? A
> >> SCR seems such a primitive device to me, I'd prefer something like a M=
OSFET
> >> 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 char=
ges
> >(ionizer).
> >
> >The capacitor type I have used are metalized paper (Evox/Rifa PME271M) w=
hich
> >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 SC=
R. 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 M=
OSFET).
> >
> >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 sl=
owed
> >down so I can more easily control the charge time and thereby also contr=
ol the
> >output power.
> >
> >To lower the esr when discharging you could use several capacitors in =

> >parallell.
> >
> >/Ruben
> >
> >=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
> >Ruben J=F6nsson
> >AB Liros Electronic
> >Box 9124, 200 39 Malm=F6, Sweden
> >TEL INT +46 40142078
> >FAX INT +46 40947388
> >ruben@pp.sbbs.se
> >=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
> >
> >-- =

> >http://www.piclist.com PIC/SX FAQ & list archive
> >View/change your membership options at
> >http://mailman.mit.edu/mailman/listinfo/piclist
> =

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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

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