CDI Module f or small petrol motors

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1 Replacement CDI Mdule f r small petrl mtrs By JOHN CL RE If the CDI mdule in yur mtrbike, utbard, ride-n mwer r ther small petrl mtr fails, yu culd be in fr a shck. Depending n the brand r mdel, they can cst up t 200. Yu can build this ne fr less than 30 and it will d the same jb fr mst engines. R EDErS have been asking us fr years t design a drp-in CDI mdule fr mtrbikes, utbards and ther small petrl mtrs. Yu can understand why. It can be a real shck t visit yur lcal dealer and ind ut the price fr such a mdule. It is even harder t justify the prices charged when yu see the circuit cmpnents invlved in these units. These days, a great many small petrl engines use a capacitr discharge ignitin (CDI) mdule. The high-vltage capacitr is charged directly frm a generatr lcated n the lywheel. battery may still be included and 32 used t drive lights and ancillaries, but this is used independently f the ignitin. reat imprvement CDI is a great imprvement n the ld magnet ignitin systems. nt nly des the CDI deliver higher spark energy, but it als dispenses with the pints, which were inevitably subject t wear and required peridic cleaning, adjustment and replacement. The ne drawback is that CDI systems dn t last frever they can fail. While the failure can be within the lywheel generating cils r the ignitin cil, it is mst likely t be the CDI mdule itself and then yu will ind that the replacement can be very expensive. The CDI Mdule described here may be used t replace a failed factry unit fr an engine that incrprates a generatr and trigger cil t prvide the high-vltage and the iring pint. Mst f these CDI systems perate in a similar way, but there are variatins in design that use the ppsite plarity fr vltage generatin and are therefre unsuitable fr ur mdule. While sme tests can be perfrmed t check fr suitability, we cannt guarantee that the mdule will wrk fr

2 every engine. Even s, because this CDI Mdule uses cheap and readily available parts, it may be wrth a try if yu are unwilling t frk ut lts f hard cash fr a genuine replacement mdule. INITION ILL SWITCH S1 ENERTOR Hw CDI wrks The cnnectins required fr a typical CDI mdule are shwn in Fig.1. The generatr (magnet) cil prvides the high vltage t charge a capacitr (in the CDI mdule), while the trigger cil prvides the signal t dump the capacitr s high vltage charge int the ignitin cil. kill switch shunts the high-vltage supply frm the generatr t prevent ignitin. Fig.2 shws hw CDI wrks. It cmprises three main cmpnents: the ignitin cil, a capacitr (C1) and a silicn-cntrlled rectiier (SCr). The SCr behaves as a switch. It is nrmally a high impedance until a small trigger vltage is applied between its gate () and cathde (). It then cnducts and behaves like a dide. fter triggering, the SCr switches ff when the current thrugh it falls clse t zer. Initially, the SCr is ff and capacitr C1 is discharged. psitive vltage frm the generatr then charges C1 via dide and the primary winding f the ignitin cil. The current path is shwn in red as IC. Capacitr C1 is discharged when the SCr is subsequently triggered, allwing current t lw back thrugh the ignitin cil primary. This current path is shwn in green as ID. The fast discharge f C1 and resulting current thrugh the ignitin cil causes a high vltage t be develped acrss the secndary winding f the ignitin cil, t ire the spark plug(s). nce the spark plug is extinguished, the cllapsing ield f the ignitin cil develps a reverse current lw via dide t partially recharge capacitr C1. Typically, the generatr cil delivers abut 1 in charging the capacitr up t abut 350V. If C1 is 1mF, then it will charge in abut 350ms much quicker than the time between sparks, even in a high-revving engine. N RPM advance nte that the CDI Mdule des nt incrprate rpm advance and s it prvides a ixed timing frm the trigger cil which is cmmn with small engines. Sme engines d incrprate rpm advance using a special trigger cil and SPR PLU CDI MODULE TYPICL MODULE FOR CDI WITH EXTERNL CONNECTIONS SHOWN Fig.1: this shws hw a typical CDI mdule is cnnected. The generatr (magnet) cil prvides a high vltage t charge a capacitr in the CDI mdule, while the trigger cil prvides the timing signal t dump the capacitr s high vltage charge int the ignitin cil. INITION CHRE CURRENT ENERTOR C1 SCR IC SPR PLU CONDITIONIN DISCHRE CURRENT ID BSIC CDI OPERTION Fig.2: hw the CDI mdule wrks. Initially, the generatr cil charges C1 t a high vltage (via dide ). trigger pulse (frm the trigger cil) then turns n the SCR and this quickly discharges C1 by allwing current t lw back thrugh the cil primary. magnetic cre design that advances the iring edge with increasing rpm. This is achieved by having a stepped r shaped cil cre that has a larger gap at its leading edge cmpared t the trailing edge see Fig.3. t lw speeds, the cil vltage required fr triggering is develped at the trailing edge f the magnet, but as the revs increase, the leading edge f the magnet is able t induce mre vltage in the cil and s iring ccurs earlier. This is shwn in Fig.4. ther designs use electrnic advance, but these require extra pwer fr the circuit and tend t be used nly with battery-pwered systems. Circuit details The simplest circuit arrangement fr the CDI Mdule is shwn in Fig.5. n extra features versin is shwn in Fig.6. Vltage frm the generatr cil charges capacitr C1 (and ) via dide and the ignitin cil primary. s previusly mentined, dide is there t cnduct the reverse current lw frm the ignitin cil after the capacitr has discharged. The tw in-series W resistrs acrss capacitr C1 are there t discharge the capacitr if the SCr des nt ire. This is a safety feature that prevents a nasty electric shck if yu happen t cnnect yurself acrss the capacitr. It takes abut tw secnds fr the capacitr t discharge t a safe value. Prvisin has been made n the PC bard fr tw discharge capacitrs, C1 and. This allws the use f either tw 0.47mF capacitrs r tw 1mF capacitrs. higher capacitance will prduce greater spark energy, prvided the generatr cil can charge 33

3 51W resistr limits the gate current t a safe value. W resistr ties the gate t grund t prevent false triggering, while the 100nF capacitr ilters nise and transients that may cause the SCr t trigger at the wrng time. kill switch cnnectin has als been prvided t shunt the generatr current t grund and stp the mtr. (FLYWHEEL) N MN ET S SMLL P LRER P Circuit reinements CORE DVNCE HED DESIN Fig.3: sme engines achieve RPM advance using a special trigger cil with a stepped magnetic cre that has a larger gap at its leading edge cmpared t the trailing edge. This advances the iring edge with increasing RPM. FIRIN POINT TDC (TOP DED CENTRE) 1.5V VOLTE TIME DVNCE 50ms T LOWER RPM FIRIN POINT TDC reprduced by arrangement with SILICn CHIP magazine V VOLTE TIME DVNCE 10ms B T HIHER RPM Fig.4: the effect f a stepped trigger cre design is shwn in these timing advance wavefrms. t lw speeds, the cil vltage required fr triggering is develped nly at the trailing edge f the magnet (wavefrm ). Hwever, at higher revs, the leading edge f the magnet induces a greater vltage int the cil and s iring ccurs earlier (wavefrm B). the capacitrs t the full vltage in the required time. The trigger cil prvides the necessary signal t trigger the SCr. When 34 the cil vltage ges psitive, it feeds current t the gate f the SCr via a 51W resistr and dide D3. D3 prevents reverse vltage n the gate, while the The simple circuit f Fig.5 wrks well, but additinal circuitry can imprve reliability and prvide fr mre cnsistent triggering. The extended circuit is shwn in Fig.6. First, dide D4 has been added acrss the generatr and this shunts negative excursins acrss the cil t less than 0.7V. Withut D4, the ande f dide can be subject t 350V frm the negative swings f the generatr. This means that dide culd have ver 700V acrss it if the capacitr is charged t 350V. While is rated at 1000V, D4 reduces the maximum likely vltage acrss it t arund 350V r s, and thereby reduces the pssibility f reverse breakdwn f the dide. Triggering in this versin f the circuit has als been imprved in tw ways. First, we have added a series 10mF capacitr t the gate f the SCr. This capacitr prevents false triggering due t any DC ffset frm the trigger cil that may be mre psitive than it shuld be because f remnant magnetism in the cil s cre. The W resistr acrss the capacitr is there t discharge the capacitr; it is high enugh in value t prevent it triggering the SCr n its wn. Dide D5 prevents the 10mF capacitr frm being charged with reverse plarity when the trigger cil utput swings negative. The secnd imprvement invlves the use f a negative temperature ceficient (ntc) thermistr acrss the gate f the SCr. This thermistr reduces its resistance with increasing temperature and is used t cmpensate fr the lwered triggering requirement f the SCr (fr bth vltage and current) at higher temperatures. Effectively, the ntc thermistr frms a vltage divider with the 51W resistr. t 25 C, the thermistr is 500W, and s it attenuates the signal frm the trigger cil t 91%. Hwever, at 100 C, the ntc thermistr resistance is arund 35W and the trigger signal is divided dwn t 41% f the

4 INITION ILL SWITCH S1 C1 ENERTOR D3 1N Parts List CDI Mdule SPR PLU SC R1 100n F 1 PC bard, cde 772, available frm the EPE PCB Service, size 64mm 45mm 1 ptting bx, mm 1 500W NTC thermistr (500W at 25 C) 1 M3 10mm screw 1 M3 nut -D3 Semicnductrs 1 500V SCR (SCR1) 3 1N V dides (,,D4) 1 1N V dide (D3 fr Basic Versin; D5 fr Extra Features Versin) CDI MODULE (BSIC VERSION) Fig.5: this is the circuit fr the Basic Versin. The kill switch is there t stp the mtr by shunting the generatr cil s utput t grund, while the W resistr n SCR1 s gate prevents false triggering due t nise. trigger cil value. This attenuatin in signal level attempts t match the SCr s reduced trigger level requirement at higher temperature. S as the temperature rises, the signal is increasingly attenuated and as a cnsequence, the SCr ires at the same trigger cil vltage ver a wide temperature range. Withut the thermistr (see Fig.6), the SCr wuld be subject t timing changes with temperature. Cnstructin small PC bard cded 772, measuring 64mm 45mm, caters fr bth versins f the circuit. This bard is Capacitrs 1 10mF 25V radial electrlytic 1 1mF 275V C r 280V C metallised plyprpylene; r mF 275V C r 280V C metallised plyprpylene; r 2 1mF 275V C r 280V C plyprpylene see text 1 100nF MT plyester 1 10nF MT plyester available frm the EPE PCB Service. The PCB can it int a plastic utility bx that measures 70mm 50mm 20mm, which allws the whle mdule t be subsequently ptted. Begin cnstructin by checking the PC bard fr the crrect hle sizes. The fur crner munting hles shuld be drilled t 3mm, as shuld the hle fr the SCr munting tab. That dne, check the PC bard fr breaks in the cpper tracks r fr shrts between tracks. Make any repairs t the bard befre assembly cmmences. The cmpnent layut fr the simple versin f the circuit is shwn in Fig.7, while Fig.8 shws the mre Resistrs (0.25W 1%) 2 W 1 51W 1 W Miscellaneus utmtive wire, crimp cnnectrs, neutral-cure silicne sealant. INITION ILL SWITCH S1 ENERTOR C1, : 2 x 470n F 275V C OR C1 : 1 x 1 F 275V C OR C1, : 2 x 1 F 275V C D4 51 D5 1N 4004 SC R1 SPR PLU C1 10 F 25V 100n F 10n F NTC1 * * 500 T 25 C -D4 CDI MODULE (EXTR FETURES VERSION) Fig.6: the Extra Features Versin includes dide D4 t shunt negative excursins acrss the generatr cil t less than 0.7V, and thus limit the vltage acrss dide t arund 350V. It als features an imprved trigger circuit, t ensure cnsistent iring f the SCR with variatins in temperature. 35

5 D3 100nF 'BSIC' CDI VERSION 10nF TO INITION 'EXTR FETURES' CDI VERSION Fig.8: the Extra Features versin is the ne that we recmmend yu build. Take care with the rientatin f the dides and the 10mF electrlytic capacitr. can use 230V C mains wire salvaged frm a mains extensin crd. The wiring fr the chassis cnnectin shuld als be rated at 7.5 r mre. By cntrast, the trigger lead des nt have t be heavy duty, but shuld have suitable insulatin fr autmtive use. Sheath the wires in sme lexible tubing t prevent pssible chafing f the wiring insulatin. Better still, yu may be able t use the existing wiring fr the riginal CDI mdule. If yu want the best spark pssible, yu can try adding a secnd 1mF capacitr in parallel with the irst. This have prvided fr tw capacitrs and als fr tw different lead spacing n the PC bard. The SCr is munted hrizntally with its leads bent dwn by 90 s that they pass thrugh their hles in the PC bard. Secure its tab using an M3 10mm screw and M3 nut befre sldering the leads. Wiring next, add the wiring frm the PC bard t the generatr cil, kill switch and t the ignitin cil. The leads must all be rated at 250V C and 7.5. utmtive wire shuld be suitable, r yu This cmpleted CDI mdule is the Extra Features versin. Yu may have t experiment with the number f discharge capacitrs t get the best spark see text. 10 FELUD O M ID C TO INITION Fig.7: fllw this parts layut diagram t build the Basic Versin f the CDI Mdule. It can be used fr nn-critical applicatins. NTC1 51 ELUD O M ID C SCR1 D5 D4 CHSSIS 100nF cmplex versin. The chice is yurs, but we recmmend the versin in Fig.8. In fact, the fllwing assembly prcedure assumes that yu are building the extra features versin. Start by installing the dides, taking care t rient each ne crrectly. The resistrs can then g in their values can be checked against the accmpanying table and with a digital multimeter. next, install the thermistr, the smaller capacitrs and the 10mF electrlytic, making sure it is riented crrectly. The discharge capacitr(s) can then be installed. s nted abve, we C1 51 ILL SWITCH C1 CHSSIS SCR1 ILL SWITCH ENERTOR ENERTOR The bard shuld be installed in a plastic case and ptted using neutral-cure silicne sealant t ensure reliability (ie, t prtect against vibratin, misture and dust). Table 1: Resistr Clur Cdes 36 N Value W W 51W 4-Band Cde (1%) brwn black green brwn brwn black red brwn green brwn black brwn 5-Band Cde (1%) brwn black black yellw brwn brwn black black brwn brwn green brwn black gld brwn

6 Warning This CDI Mdule is nt intended fr use as a replacement fr CDI units that generate their wn high vltage frm an inverter requiring a 12V battery supply. T replace ne f these units, yu culd adapt ne f ur previus designs, such as the Prgrammable Ignitin System that appeared in EPE frm September, Octber, Nvember may imprve the fatness (intensity) f the spark. In sme cases thugh, a 1mF capacitance will give the best spark because 2mF may lad the generatr cil t much and lwer the charge vltage. nce the bard is cmplete, run the external cnnectins and test the CDI fr crrect peratin. djust the ignitin timing accrding t the manufacturer s instructins. Ptting the circuit s previusly indicated, we used a ptting bx (Jaycar Cat. HB-5204) t huse the CDI unit. Ptting allws the cmpnents t be prtected frm vibratin, water and dust. Yu must use a neutral-cure silicne sealant fr this jb. D nt use an acid-cure silicne, as this will crrde the wires and cpper pattern n the PC bard. nte that the capacitr(s) will prtrude a little frm the tp f the ptting bx. The bx can be munted n the engine frame using suitable brackets. It shuld be placed away frm the exhaust side f the engine. Make sure that any munting screws fr the bx d nt penetrate and make cntact with the circuit. Testing the generatr cil Smetimes the generatr cil can fail due t either a shrted turn r a brken wire. Yu can test fr a break in the cil by measuring its resistance ie, between its utput and grund. If the cil is, its resistance will prbably be less than 200W. shrted turn is nt easily checked except using a special shrted turns tester. Hwever, yu can get sme idea if the cil is delivering suficient vltage by measuring it with a multimeter set t read C vlts up t 300V. The vltage is measured when the engine is turned ver. Take care if making this measurement, since the generated vltage can give yu an electric shck. DO NOT tuch any f the wiring when turning the mtr ver. nte that the vltage measured acrss the generatr cil will nt be anywhere near the vltage that it develps when running. That s because the multimeter des nt respnd well t the lw-frequency vltage luctuatins that ccur when kicking the engine ver. In additin, mst multimeters d nt respnd t the peak f the wavefrm but t the average f a sinewave. In practice, yu shuld get a reading f abut 50V C frm the cil. nther way f testing the cil vltage is t cnnect the CDI mdule and measure the DC vltage between the cathde f dide and the chassis while kicking the mtr ver. The reading shuld at least get t 200V DC if yu can kick the mtr ver fast enugh. lternatively, if an scillscpe is available, the vltage wavefrm can be measured with the prbe set t 10:1. ne pint we have nt mentined is the plarity f the vltage. The capacitr needs t charge t a psitive vltage befre the trigger signal ccurs. If the vltage frm the generatr cil is negative befre triggering ccurs, it will mean that the CDI mdule described here is nt suitable fr replacing the mdule in yur engine. Yu can check the plarity using a multimeter set t DC vlts it s just a matter f checking the vltage n SCr1 s ande () ges psitive befre the SCr is triggered and negative after the trigger. Trigger cil testing The trigger cil can be tested in the same way as the generatr cil (ie, measure the vltage between dides D3 r D5 s cathde and chassis as the mtr is kicked ver). This vltage will be quite small cmpared t that frm the generatr cil, and nly ccurs ver a shrt prtin f each engine revlutin. Typically, yu might measure a trigger vltage f less than 1V using a multimeter set t read C vlts. The trigger cil vltage can als be bserved n an scillscpe. f curse, the real test is when it is used with the CDI mdule itself, as it must be able t trigger the SCr. EPE 120 ex VT The MIC rugged PICmicr micrcntrller is an industrial grade cntrller which can be used t cntrl a wide range f different electrnic systems: fr mains switching, mtr cntrl, sensr mnitring and much mre. Free Flwcde 3 with every MIC Rugged PIC 18F4455 Prgrammable frm USB dwnlad sftware prvided Cmpatible with third party PICmicr, BSIC and C cmpilers Includes free Flwcde 3 graphical prgramming sftware (n upgrade rights) 8 digital r analgue inputs 4 relay utputs mtr utputs with speed cntrl - 12V, 500m 4 line 16 character display Cntrl keypad Fully netwrkable via built in CN bus Lab View and Visual Basic cmpatible - DLL supplied In-Circuit Debug ver USB with Flwit 37