Roll Your Own CDI

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Are You Going to Roll Your Own CDI?

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E

elevator

New Member
Jun 4, 2011
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calif
hi there
new to forum but i made a points condenser ignition for ht engine to see if it gave more power and to help idle. the idle was great but no real top end help. the funny thing was atstop signs it would reverse rotation and go in rev, battery lasted 2 hours
 
rohmell

rohmell

Active Member
Jun 2, 2010
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New York
Here is the CDI circuit that I have been using:
ROHMELL CDI schem.jpg

Tomorrow, I will take some pics of the completed assembly.
 
E

elevator

New Member
Jun 4, 2011
4
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calif
Ok great. thanks for the circuit posting, I never had a cdi go out on a ht but many field coils from water, they were never insulated very well. The main area of needed improvement on a ht ignition was the idle, the cdi stops working at low rpm. The advancer settings never realy helped much on a small two stroke because I experimented with many settings and even advanced it so far it started running backwards using a cam and battery ignition had fat blue spark and would idle perfect.

I ride both ht and whizzers and need a circuit like you cdi one with and advance circuit, around 1000 rpm it advances. The four stroke engine is differant and the advance is very important but not so much on 2 strokes. can you send me a schematic for a cdi with an advancer circuit.
Thanks
Jim
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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New York
Basically, I am working with the HTs, and their CDI circuits. With these CDIs, it is easy to retard the timing because you can delay the existing signal, which comes from the magnet so that it appears later. Not so easy to advance, because you are wanting a signal to occur sooner than it is available from the magneto. In order to solve this problem, you first have to manually reset the magnet timing to a super advanced state, and then have a wide retard range using the CDI to give you a relative range of suitable advance/retard settings.
Adding capacitance to the gate circuit of the SCR will cause a retard effect.
There is a schematic in this thread that uses such a method (for Kawasaki, I think)
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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New York
If there is interest, I may consider making more and selling 'em.

Here are some pics of the completed CDIs:













 
Last edited:
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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38
New York
Some have mentioned that it looks too complicated/confusing/daunting to build, so here it is, instead of a schematic, here is how the parts connect to each other:

 
skrew37

skrew37

New Member
Feb 23, 2011
129
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Pennsylvania
Ok, please excuse my continued ignorance, but the 2.0uF (capacitor?) in the above pic replaces the two yellow ones in the previous pics?
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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New York
skrew37 said:
Ok, please excuse my continued ignorance, but the 2.0uF (capacitor?) in the above pic replaces the two yellow ones in the previous pics?
Click to expand...
Yes.
The capacitors in my CDIs are 1 uF each, so two in parallel are needed to get the value of 2 uF.

I used a stock picture of a capacitor to simplify the drawing.

These capacitors are available in all different types; mylar, polystyrene, polypropylene, polyester, etc.

I built my units with whatever I had lying around in the warehouse.
 
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
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New York
The capacitor carries high current pulses over to the ignition coil. A non-metallized dielectric film and a carrier film metallized on both sides acting as electrode would be ideal for this type of application. Since there is metallization on both sides, the electrical conductivity is considerably improved and the contact surface between the electrodes and the film layer is doubled. This results in better contact and allows for high current and pulse loading capability.
Film/foil Polypropylene capacitors made from a single metallized plastic film and metal foil electrodes in series connection would be ideal. This construction features a high volume capacitance and at the same time high pulse loading capability.

The best, best, best capacitors were made by FCI, but unfortunately, the factory burned down around 1986.
The yellow capacitors in my units are made by Philips and are metallized polyester.
 
Last edited:
rohmell

rohmell

Active Member
Jun 2, 2010
1,531
6
38
New York
Also, using an SCR with a TO-48 case style in unnecessary, a TO-220 cased SCR would work fine in this application.
Again, I used whatever I had lying around in the warehouse.
 
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