CDI Tester Circuit for Automobiles

The first design below shows a simple workbench setup for testing all types CDI circuits. It uses a capacitive discharge ignition circuit along with a CDI coil to implement the testing procedure.

Audio/Video Representation

The circuit does not require any separate oscillator, rather the 100Hz from the bridge rectifier itself works like a pulsed trigger for the CDI trigger input, for generating the sparks at 100Hz fast rate.

Purpose Of The Setup

We are making this whole setup so that we can test one CDI coil ignition circuit right on the table, without needing any bike engine or alternator. We want to see that spark properly. So we use some small transformers and one bridge rectifier that will give us the same kind of high voltage like bike alternator but with very low current so that it stays safe for testing.

Basic Working Idea

We know that in an actual motorcycle, that alternator coil gives high voltage AC around 100V to 250V whenever engine keeps rotating. That AC goes inside CDI circuit where it charges one capacitor. Then the SCR gets triggered from one pickup coil pulse and it suddenly discharges that capacitor into ignition coil, and that makes the high voltage spark.

So here in this test circuit, we are trying to make that same kind of AC using two small 12V-0-12V transformers which are connected back-to-back. This gives us isolated 220V low current AC that behaves like the alternator supply.

Transformer Connection

You can see two small transformers on the right side each rated 12V-0-12V 500mA.

Now we take the first transformer and feed 220V AC mains to its primary. So its secondary gives about 12V AC.

Then we take the second transformer and we connect its 12V winding with the 12V winding of the first transformer.

So now the primary of the second transformer starts giving again around 220V AC but this time with very low current because the power is going through two transformer windings.

So we get safe, isolated and low current 220V AC output which acts like the alternator AC for the CDI circuit.

Bridge Rectifier For Trigger Pulses

Now the important part — the bridge rectifier made from 1N4007 diodes....it is connected to the 12V AC secondary of the left transformer. So this bridge rectifier only converts the 12V AC into 12V pulsed DC.

Because 12V AC has 50Hz frequency, so after full wave rectification it becomes pulsed DC at 100Hz. That means we get two DC pulses for each AC cycle. So this pulsed DC is applied to the trigger input section of the CDI circuit.

Now every time this pulsed DC hits the SCR gate input, then the SCR turns ON for a moment and discharges the main capacitor C4 through the ignition coil. So the ignition coil keeps firing sparks one after another at the same 100Hz rate.

What Happens Inside CDI

Inside the CDI circuit, the capacitor C4 gets charged through the 220V low current AC from the transformer secondary. That high voltage keeps the capacitor ready all the time.
At the same time, the pulsed DC from the bridge rectifier keeps triggering the SCR gate at 100 times per second.

So each time the SCR gets pulse, then it fires, dumps the capacitor charge into the ignition coil and creates a spark at the plug. Since this happens 100 times per second, so you get a rapid series of sparks at around 100Hz rate.

Spark Observation

You will see that ignition coil output wire or spark plug will keep sparking continuously at high rate. The spark will look like a fast chain of sparks because the CDI circuit is getting triggered continuously by that 100Hz pulse from bridge rectifier.

Why This Setup Is Safe And Useful

We use two small transformers so that mains side stays isolated. The voltage on CDI power side becomes high but current remains very low so it stays safe for testing. Also the rectified 12V pulse acts like one artificial pickup coil, so you do not need real engine rotation to test the CDI.

So with this simple back-to-back transformer and rectifier setup, you can fully test CDI coil, SCR, capacitor and ignition coil spark function on table easily and safely.

Using an Adjustable 555 Standalone Oscillator for the Triggering

The next circuit presented here is another useful tester circuit with a 555 oscillator for testing CDIs for motorcycles and three-wheelers.

Designed and Written By: Abu-Hafss Basically there are 2 types of CDIs:

Technical Specifications

a) AC CDI, in which the HVAC (about 180V) is obtained from the source coil inside the magneto housing.

b) DC CDI, in which the HVAC is generated by circuitry inside the CDI, from 12VDC.

Then there are further 2 types in AC-CDI and CD-CDI which are usually used in performance bikes or heavy bikes.

They are equipped with microprocessor to provide advance ignition curve for better burning of fuel inside the cylinder head. The captioned tester circuit is designed to test the CDIs without microprocessors.

Studying both the circuits will reveal that the lower circuit is the portion of the first circuit. The first circuit is to test AC-CDIs.

Circuit Operation

The transformer T1 converts 220VAC from mains to 12VAC and then T2 re-converts 12VAC to 220VAC.

This set-up is to isolate the rest of the circuit from grid mains. This 220VAC is fed into the CDI's HV Input, replacing the HVAC which is obtained from bike's source coil.

The rectifier bridge converts 12VAC into 12VDC and C1 smoothes it. The SCR U2 is used to stop the 12VAC supply to T2, which will be discussed shortly.

The IC U1 is 555 timer configured as an astable multivibrator with duty cycle about 20% and frequency about 17Hz.

The generated pulse train will replace the triggering coil pulse at fixed rate of (17 x 60 =) 1020 RPM.

The output is fed into the CDI's Trigger Input and the ground is connected to the (-)ve rail. The output is also connected to the gate of the SCR via R4.

Whenever there is a positive pulse, the SCR will temporarily cut-off the 12VAC supply to T2. Hence, 220VAC supply to CDI will be paused. This is necessary to avoid a shorted path when the SCR inside the CDI is dumping the charge of the main capacitor.

The CDI's output is connected to one end of the primary of an ignition coil. The other end is connected to the (-)ve rail.

One end of the secondary of the ignition coil is connected to a spark gap and the other end is connected to the (-)ve rail.

The second end of the spark gap is also connected to the (-)ve rail. A strong spark across the gap will indicate that the CDI is good.

If the CDI under test is DC-CDI, the lower circuit will be used. The connections will be the same except the power supply will be 12VDC. The HVAC supply is built-in inside the CDI. A good CDI will fire strong spark across the gap.