marc426:
A couple of questions for you CDI specialists out there:
- Is the circuit pictured correct to charge a cap that's almost 50times bigger than originaly intended? I'm also trying to find out the recharge time to check the max frequency but my knowledge stops there...
The size of cap that can be charged is only limited by the leakage current of the cap,
more capacitance takes longer to charge.
- The 22nF and 27R on the high voltage side are there to discharge the cap when powered down, right?
They cannot do that as there is no DC path. They look to be a snubber circuit to reduce
EMI, or to protect the thyrister from dV/dt, but not sure.
- On all the CDI schematics that I've seen, I've sometimes seen the 1N4007 in // with the power cap. Sometimes not and sometimes the other way around... What it's purpose to begin with?
Can't really comment on a circuit I haven't seen - there are various topologies possible.
- Last but not least : I was of course thinking of controlling the thyristor directly from the arduino... Is that safe in any way?
I was, of course, thinking of using an optocoupler but I've heard some bad things about it's rise/fall speed and that in HV ignition switching, this speed was of a prime importance...
Opto couplers are fast enough surely! I wouldn't want to control this circuit directly from sensitive logic circuitry, the severe dV/dt and dI/dt values of the discharge are
enough to induce surprising large voltages around the thyristor and high-current
circuitry - optocoupling is one the way to go, otherwise you'll have to consider protection/
filtering circuitry in the gate driver.