Make this DC CDI Circuit for Motorcycles

The circuit presented here is for a DC-CDI which are used in motorcycles. A DC-CDI is the one in which the high voltage (200-400VDC) is converted from 12V supply voltage.

Researched and Submitted by: Abu-Hafss

Studying the circuit, we see that it has two parts i.e. the CDI unit, enclosed in the pink box and the remaining circuit on the left is high voltage converter.

The working of the CDI may be found in this article.

The circuit on left is a high voltage converter based on a blocking oscillator. The components Q1, C3, D3, R1, R2, R3 and transformer T1 forms the blocking oscillator.

L1 is the primary coil and L2 is the feedback coil. C1, C2 and D1 are DC voltage smoothing components.

How it Works

When the circuit is powered on, R3 provides forward bais to the base of Q1. This turns on Q1 and current starts flowing thru the primary coil L1 of the transformer.

This induces voltage in the secondary or the feedback coil L2.

The red (phase) dots in the transformer symbol indicates that the phase of the voltage induced in L2 (and L3) is shifted 180°.

Which means when the bottom side of L1 is going negative, the bottom side of L2 will be going positive.

The positive voltage of the L2 is fed back to the base of Q1 thru R1, D1, R2 and C3. This causes the Q1 to conduct more hence, more current flows thru L1 and ultimately more voltage is induced into L2.

This causes L1 to saturate very rapidly which means no more changes in magnetic flux and hence no more voltage is induced into L2.

Now, C3 starts discharging through R3 and finally Q1 is switched off. This stops the current flow in L1 and hence the voltage across L1 comes to zero.

The transistor is now said to be "blocked". As C3 gradually loses its stored charge, the voltage on the base of Q1 begins reverting to a forward-bias condition by means of R3 thus switching on Q1, and hence the cycle is repeated.

This switching of Q1 is very fast such that the circuit oscillate at quite high frequency. The primary coil L1 and secondary L3 forms a step-up transformer and thus a fairly high alternating voltage (more than 500V) is induced in L3.

To convert it to DC a fast recovery diode D2 is deployed.

The zeners, R5 and C4 forms the regulator network. The sum of the values of the zeners should be equal to required high voltage to charge the CDI's main capacitor (C6).

Or alternatively a single TVS diode with desired breakdown voltage may be used.

When the output at the anode of D2 reaches the breakdown voltage (sum of zener values), the base of Q2 receives the forward bais and hence Q2 switches on.

This action steals the forward bais of Q1 thus stopping the oscillator temporarily.

When the output is dropped below the breakdown voltage, Q2 switches off and hence the oscillation resumes. This action is repeated very rapidly that the output is maintained slightly below the breakdown voltage.

The positive trigger pulse at point (D) in the CDI unit is also fed to the base of Q2. This is important to pause the oscillation because SCR U1 demands the current across its MT1/MT2 to be zero to be able to self-disconnecting.

Moreover, this increases power economy as all power supplied during discharging is wasted otherwise.

A special request from Mr. Rama Diaz to have multi CDI sections sharing a common HV converter circuit. Some parts of his request is quoted below:

Ok most engines these days don't have distributors anymore, they have a coil for each spark plug or in many cases have a dual post coil that fires 2 spark plugs at the same time, this is called "wasted spark" since only one of the two sparks is actually getting used each ignition event the other one just fires into the empty cylinder at the end of the exhaust stroke, so in this configuration a 2 channel CDi will run a 4cyl and 3 channel for 6cyl and 2 x 2 channel for v8 etc...

Almost all 4 stroke engines have 2 cylinders that are paired so only 1 coil (connected to 2 spark plugs) will fire at a time the other one/s will fire at the alternate ignition events driven by a separate trigger signal, Yes aftermarket ECU's have up to 8 completely separate ignition trigger signals....

yes we could just have 2 or 3 totally separate units but i would like to have everything contained in one unit if possible, and im thinking there would be some way to share some of the circuitry...

...so im thinking you could have one heavier current step-up section to provide the ~400v then have two (or 3) separate CDI coil driver sections with a separate trigger signal for each one to drive the coils independently....possible??

That way i could use 2 (or 3) dual post coils attached to 4 (or 6) spark plugs and have then all fire at the correct time in wasted spark configuration 🙂

This is exactly the way we often do it now inductively using simple transistor based ignitors but the spark strength is often not strong enough for turbo and high performance applications.

CIRCUIT DESIGN:

The entire circuit shown above can be used. The CDI unit enclosed in pink box can be used to drive one dual post ignition coil. For 4- cylinder engine, 2 CDI units; for 6-cyl, 3 CDI units can be used. When using multi CDI units, the diode D5 (encircled in blue) has to be introduced to isolate the C6 of each section.

TRANSFORMER SPECIFICATIONS:

Since the frequency of the oscillation is fairly (more than 150kHz), ferrite core transformers are used. A tiny 13mm EE core transformer can perfectly do the job but, handling such a small component might not be easy. A little bigger may be selected. Enameled copper wire 0.33 - 0.38mm for the primary (L1) and 0.20 - 0.25mm for the secondary L2 & L3.

The picture shows the bobbin's top view.

For primary winding, start from pin no. 6, wind 22 neat turns in the direction shown and end at pin no. 4.

Cover this winding with a transformer tape and then start the secondary winding. Starting from pin no. 1, wind 140 turns (in the same direction as that for primary) and make a tap at pin no. 2 and then continue another 27 turns and end at pin no. 3.

Cover the winding with tape and then assemble the 2 EEs. It is advisable to make an air gap between the 2 EEs. For this a tiny paper packing may be used. Finally use the tape to keep the 2 EEs united.