Capacitor Discharge Ignition - Noble Associates

1 Capacitor Discharge IgnitionELECTRONIC Ignition SYSTEM WILL PROLONG LIFE OF POINTS AND PLUGS ON OLDERCARS AND INSURE EASIER COLD STARTSBY BILL Noble , SANTA MONICA, CALIFORNIAW hile we all recognize the desirability of a 100%stock vehicle, an electronic Ignition can greatly ease theproblems of starting an older vehicle as well as prolongingthe life of the points and plugs (which are sometimes hard toget). The Capacitor Discharge Ignition system described inthis article can be installed on any vehicle with a 6 Vcoil/breaker point Ignition system; it cannot be used with amagneto. The Ignition system should be mounted in arelatively cool location away from the battery - under thedash is ideal.

2 It should be installed in such a way that when itis removed, the car is again stock. This allows using theignition system for regular driving while removing it forclub events or judging where authenticity is capacitive Discharge (CD) Ignition consists of threemain elements: an oscillator and transformer for generatinghigh voltage, a Capacitor for storing the energy, and a siliconcontrolled rectifier (SCR) for discharging the energy into thecoil. Because the Ignition energy is generated electronically,the points will last almost forever (the rubbing blockeventually wears out), and cold starting is much DescriptionFigure 1 shows the schematic for a negative groundignition. Q1, Q2, and the transformer form a free runningoscillator which generates a high voltage at the transformersecondary.

3 This voltage is rectified by the diode bridge (D1-D4) and applied to the energy storage Capacitor . The lowside of the Capacitor is grounded through the coil primarywinding. R3 provides a leakage path to safely Discharge thecapacitor when power is Q3, R5, R6, and the uf Capacitor are a one shotcircuit to generate short trigger pulses for the SCR. Aconventional SCR trigger circuit with a coupling capacitorwill not work due to the low voltage available when crankingthe starter motor. When the points first open, the gate of theSCR is raised to 6V through R5, because Q3 is off. When Capacitor charges above V, Q3 turns on, thusremoving the SCR gate , D5, D6, and the Capacitor provide transientdamping to protect the SCR.

4 The SCR itself is just a siliconswitch which grounds one side of the energy storagecapacitor when it is allows the Capacitor to Discharge through the coilprimary. When fully discharged, current through the SCRchanges direction due to the "flywheel" effect of the coilinductance. This turns the SCR off, and allows theremaining energy in the coil to be restored to the 2 shows the changes needed to Figure 1 tobuild a positive ground system. This involves PNPtransistors in the oscillator and a simple inverter to drive theSCR trigger circuit. Six volt power for the inverter is tappedoff the oscillator at the transformer the large components (T1, Q1, Q2, SCR) toheavy aluminum for heat sinking and structural support.

5 Theremainder of the circuit is built on inch perforatedglass/epoxy board. The particular physical layout chosen isnot critical and should be selected to simplify the particularinstallation. If the circuit is to be mounted under the hood, orin any other unprotected area, be sure to enclose thecomponents to prevent problems with moisture. If radiointerference is encountered once the circuit is built, standardsuppression techniques such as a bypass capacitorfrom the input to ground will prove project is obviously not one for someone who hasnever soldered before, however, there are few parts, layout isnot critical, and checkout is simple, so anyone who can readthe schematic should have no Connect the circuit to 6V power, and verify that100-300 VDC is present at the output of the bridge.

6 Thesystem should put out a high pitched whine as soon as poweris connected. If nothing happens, disconnect the diodebridge and try again. This will isolate the problem to theoscillator or When power is present at the bridge output, connectthe circuit to a spare Ignition coil, and ground the pointslead. Apply power. When the lead is opened, there should bea real solid spark from the coil.(Text continued on page 10)OCTOBER 1982 - PAGE 8If not, verify that there is + (or -) 6V at the pointlead when the lead is not grounded. If there is, andthere is no spark, remove the SCR, apply power andmomentarily short R3. If this does not cause a spark,the energy storage Capacitor is connected improperlyor is defective.

7 If there is a spark, the problem is in theSCR or trigger Put it in the car and start it up. If you installed abypass switch (see below), test the switch at this The PartsAll parts values in this circuit are non critical ,solong as somewhat close (plus or minus 20%) to therecommended value, and so long as voltage andcurrent ratings are adequate. Since the oscillator outputhas significant high voltage spikes in it, all componentsshould be able to stand 2 to 3 times the normal transformer may be any filament transformerwith a 6V center tapped secondary and a 115 voltprimary, so long as the current rating is adequate. Aminimum 6V current is 5 amps, 10 amp rating is you live in an area where there are electronic surplusstores, look for a 400Hz ;A transformer.

8 Since theseparts won't work on house current (60Hz) they areusually very cheap. Also, being designed, for higherfrequency, they have less iron and. thus are smallerand Be sure to check the point gap every 50,000miles or so, since the rubbing block will eventuallywear ] Oil the point pivot and distributor camwhenever you change oil in the Check the spark plug wires. They are oftenbad on old While the circuit will work on 12V with nochanges, I recommend that you buy a DELTA MK10 Bfor 12V negative ground applications. They areavailable as - kits, or used at swap meets, and are bothgood and 3 shows how to connect a 4 PDT switch sothat the Ignition can be easily switched between CDand regular Ignition .

9 I recommend this switch becausemany tune-up procedures require the normal Ignition ,and a quick-change capability is often handy. If theswitch has a center "Off" position, this provides anextra level of theft protection by turning the wholeignition system off.