Type Half Ignition

Rev - Dec 06, 2017

By Jan Zumwalt (EAA #66327)

See Also

Ford TFI Electronic Ignition
For additional information on this topic see VW Type 1 & 2 ignition.

CDI - Capacitor Discharge Ignition


The ignition process in any vehicle is the heart beat of the entire system. Without it, nothing will work. To create a spark older engines used a set of mechanical contacts (called points) that wore out quickly and were unreliable.

Nowadays a more efficient, dependable, and long lasting electronic circuit has been developed. It is called a "Capacitor Discharge Ignition" system, or CDI.

It incorporate an electronic circuit that functions in the same way the old contact breaker worked, but in a solid-state electronic manner.Kohler V-Twin

Kohler V-Twin



A dependable CDI system for a Kohler V-Twin is simple and representative of other 2cy engines. The electronics for chinese scooters i.e CDI modules, pickups, and coils can be found by doing Internet searches such as http://www.aliexpress.com/ and using "cdi" for the search term.

These CDI modules cost $15-$30 dollars and have built in spark timing advance so do a good job for starting and running. Note that some of the scooters have CDI modules that run on 12 volt DC. Others run on the AC from the generator coils. The DC units are the ones that should be used.

The pickup sensors for these are small magnetic sensors that detect a piece of metal going past. You can use a bolt shaft as the metal to trigger a pickup. With 2cy, you would place one bolt shaft on the flywheel going past two pickups. Note that the metal must pass **VERY** close to the pickup for it to work reliably. Gap should be set about the thickness of a dollar bill. For the V-Twin the pickups are spaced 90 degrees.

Set the static timing at about TDC and let the CDI retard the spark as needed. The CDI units draw about 200 milliamp when operating so a small battery will run them a long time. The system just described is designed to fire each spark plug on every revolution. If wired well, this type of system is as reliable as the components.

Basic CDI Circuit



Looking at the circuit above, we see a simple configuration consisting of a few diodes, resistors, a SCR and a single high voltage capacitor. The input to the CDI unit is derived from two sources of the alternator.

One source is a low voltage around 12 volts while the other input is taken from the relatively high voltage tap of the alternator, generating around a 100V. The 100 volts input is suitably rectified by the diodes and converted to 100VDC. This voltage is stored inside the high voltage capacitor nearly instantaneously.

The low 12 voltage signal is applied to the triggering stage and used for triggering the SCR. The SCR responds to the half wave rectified voltage and switches the capacitors ON and OFF alternately.

Since the SCR is integrated to the ignition primary coil, the released energy from the capacitor is forcibly dumped in the primary winding of the coil. The primary coil generates a magnetic induction (build up of charge) inside the coil of low voltage high amperage current. The secondary coil converts this to low amperage high voltage current. The secondary coil may output 10,000-20,000 volts.

This output is appropriately arranged across two closely held metal conductors inside the spark plug. The voltage being very high in potential starts arcing across the points of the spark plug, generating the required ignition sparks for the ignition process.
Parts List for the CIRCUIT DIAGRAM
R4 = 56 Ohms
R5 = 100 Ohms
C4 = 1uF/250V
SCR = BT151 recommended
All Diodes = 1N4007
Coil = Standard two-wheeler ignition Coil

Enhanced CDI Circuit



The basic circuit used the vehicle or alternator working voltage. We can increase the CDI performance by using a higher working voltage.

In the enhanced circuit, the primary winding is fed with high current pulsating DC generated by a standard IC555 circuit via a power transistor. This pulsating voltage is stepped up to 200V and becomes the operating voltage.

Enhanced CDI Circuit

If the CDI needs to be triggered by the alternator, this circuit should be used.



Multi-spark CDI Circuit




This multi-spark CDI circuit is universally suited for all types of automobiles. The unit can be built at home and can provide greater speed and fuel efficiency.

The circuit is really discrete stages. Both stages incorporate a IC IR2155 mosfet driver with built in 50% duty cycle oscillator. The upper stage consisting of Q1, Q2 are configured for generating a 300V DC working voltage from the 12V DC battery supply.

The IC2 along with the connected mosfets Q6/Q7 form a push pull type pump circuit for alternately charging and discharging a high voltage capacitor across the connected ignition coil. IC1 is wired up for oscillating at about 22kHz by using the 33k resistor and the 102 capacitor across pin 2/3 and pin 3/ground respectively.

This produces alternate switching of its output mosfetsQ1/Q2 connected across pins 5/7. The switching performs a push pull reaction over the connected transformer and saturates the coil in each direction, which results in pumping of the entire 12V DC across the two half windings of the transformer.

This action results in a stepped up induction across the secondary winding of the transformer giving rise to the required 300V AC switched at the 22kHz rate. The mosfets have spike protection by using 60V zener diodes. External 10 ohm gate resistors ensures a relatively exponential charge and discharge of the mosfet, thereby reducing noise.

A couple metalized capacitors rated at 10uF are installed to decouple DC from T1 so that Tr1 receives the 12V switching optimally across its winding. The stepped up voltage at the output of TR1 is rectified by the 4 fast recovery type diodes configured as a bridge rectifier. Ripples are further filtered by the metalized high voltage capacitor rated at 1uF/275V.

Even with the above protection the IC1 stage has no ability of controlling the output voltage in response to the rising and falling 12V DC input variations from changing engine RPM. To tackle this, an innovative transformer output voltage correction feature is Incorporated using a voltage feedback circuitry involving ZD1 thru ZD4 along with Q3 and a few passive components.

The four 75V zeners start conducting as soon as the voltage begins drifting above the 300V mark, which in turn results in the conduction of Q3. This action from Q3 results in dragging pin1 voltage of IC1 from 12V to gradually 6V. Pin1 being the shut down pinout of the IC1 alerts the IC to trigger its internal under voltage cut-off feature resulting in an instantaneous shut down of its output pulses which in turn switches off the mosfets for that particular instant.

The mosfets being switched OFF means no output voltage and Q3 unable to conduct which again restores the circuit to its original functional mode, and the operations repeat and rotate keeping the output voltage quite stabilized at the specified 300V volt mark.

Another clever enhancement technique employed here is the use of three 33k resistors feedback loop from the output of TR1 to the IC1 supply pinout. This loop ensures that the circuit stays functional even when the vehicle is not running at optimal speeds or the supply voltage dropping considerably below the required 12V level. During such situations, the discussed 33kx3 feedback loop keeps the voltage level to IC1 well above 12V ensuring optimal response even under conditions with steep voltage falls.

The 300V from TR1 is also applied to IC2 which is specifically configured as a high side mosfet driver, because here its output is not connected with a center tap transformer rather a single coil which needs a full drive across its winding in forward reverse method during each alternate pulse from IC2.

Thanks to the IC IR2155 which has all the necessary features built in and effectively starts working as a high side driver with the help of just a few external passive parts C1, C6, D7. The conduction of Q6/Q7 pumps the 300V volts from TR1 inside the connected ignition coil primary via the 1uF/275V capacitor.

The calculated configuration of various components across pin2 and pin3 of IC2 constitutes the intended multi sparks across the connected coil due to the interactions between these components. More precisely, the parts form a timer design with the help of the 180k resistor at pin2 along with the 0.0047uF capacitor across pin3 of IC2. The 10k resistor and the 0.0047uF capacitor between pin3 restricts over current while it's being triggered by the MMV circuit.

The output from Q5 facilitates a low voltage output for integrating a tachometer in order to provide valid readings on the meter rather than connecting directly to the spark plug.

If the multi spark feature is not useful or for some reason inappropriate, it can be disabled by eliminating C3, D10, D11, the couple of 180k resistors, and the 33k and 13k resistors. You must also substitute the 33k resistor with a 180k resistor and install a jumper in place of D10.

The above mods will force IC2 to generate just a single 0.5ms pulses as soon as Q7 is triggered. The ignition coil now fires only in one direction while Q7 is ON and once in the opposite direction when Q6 is ON.

The MOV is a safty feature. It MOV neutralizes high voltage transients if the ignition coil is left open. The 680k resistors across C2 provide a safe discharge path for C2 when the coil is disconnected from the circuit. This safeguards the circuit and the user from nasty high voltage discharge from C2.

TR1 Winding



TR1 Winding Details

Start from pin7 (left hand side) using 0.25mm enameled copper wire as shown in the diagram and end at pin8(left hand side) with 360 turns. This completes the secondary winding.

For the primary side wind in a bifilar (wind both together) manner, starting at pin2 and pin4 (right hand side) and ending after 13 turns at pin11 and pin9 respectively (left hand side) using 0.63mm wire.

The bobbin used is for suiting N27 Ferrite Core

L1 is 12 turns of 1mm wire on a Neosid Ringcore 17-732-22 Article Source