The pulsed nature of the charging circuit assures that there is always time when no electricity is flowing, thus allowing the SCR to switch off.
Notes:
This circuit comes into its own when used in vapor and gas deflagration cannons (potato guns, for example). The hot spark will ignite troublesome mixtures that a piezoelectric grill igniter won't budge. It can also be used to replace agrill ignition system, and many others. There are many applications, including just looking at the long bright arc and listening to the sharp POP. Most of you probably already know what you're going to do with it.4uF is a good value for the main capacitor. Any more or any less brings the circuit farther out of resonance. It's a good idea to use nonpolars since there are polarity reversals and pulse discharges. I use 4 microwave oven capacitors. They are usually around 1uF apiece. Just connect them in parallel. There are plenty of wires with spade lugs on both ends in microwaves, so save them to connect the capacitors. Then the whole assembly can be brought together with a large hose clamp or 2 smaller ones screwed into b$ !FoTH$R. I ouE $Lejt$, 9ka n dse pm.P T 0oiNt rI d B 't TH% c+hL SCB !ND e6RHI.E p bd hd EtHEp Wh( dHE oa aL@M0S in ond iasc @ c@r IND !"rA.&emAL #A !2 . @% d6sD !n a daaIB "uCk%p.B r @F p E Ib i0HL cIl O $rkm #@R3 !rA SAh,I T lARGRT A,d Th bDAT dn re If yoU#B Going T(E sad6!d! 2/uD% (A 1Ur YO 0iCI t0 @ fW & BSube D@A k e0 `t L%!Sd I*E 4had )c.t BRNd Ut& I'h #"ho/L aND CDhlE auO rh ps !ND scb!QAR ! /M` PL`C!1 t fIn HM PaMAy er fad i Te Ylij$c C-l# rE a a, BT A E $ C(o4 fOR ATp@rY af dIThb4eb eart` #mdNDcd m` LAwd AliLs TA ,(BeDA `p)mArq 4!EAHlc Cafbe eT%r ).ANg T`e iNEB`al "A,% of thE iGFITh/n CIh isdifbacu,T Dgj&D WK0i abeq0 aT The Dw `tdpED ext`a vOLdc h2l0 rEA Ly .ti%BH On` poSq)Bidhty tO$e@ iq sincE Os4 m 4` L #Ohda a s`mE cmnndCto0$ D qfu Cah H,` oE P`ar ad a cAr, dIlD wI( %2% Nl COnnea tc 4H$ @epp afd N(p v()Ch APT$y !R,!a )4 cOn*aC0r pG Af ht !-dfebt1 ' 4( PM3Itare b!t$0Y P$ thE, Th +pHeR %BA fAL s thE aO mO pIa!"y$E F RX OnneC4HN B H4 MhbebtC dg the ,g`thte ar cH q3)2 cR/4n Ten At H th!t tdiiN L&<"P bb I g)R@ oq A hdE f 4ha JrhA" b T@H cI2CUat( er!R) TiO ob thd !1pLmB H C 2@QIT GldkUQ. Af A 4RaDIT +AD A!DII* A `LR EjRa8 aG@)hob c0$A AnD %m by, d( L CgIl$ Ha BAT HafA !Oj.eaT1 t/ `e 0T quL9 a*` tHe DA t rm NAh cOLbDB43 N P$#i h .paa sWi4c aLQa`e T` @isDri`1dIr t@!t Aq driven by the cam shaft. This switch connects the coil to the chassis ground when a spark is needed. The high voltage terminal is connected to the upper part of the distributor that is turning and directing the pulses to the right spark plug at the right time. In a high energy ignition system, there is a capacitor that is discharged into the primary electronically, just as in the ignition coil driver circuit above. This eliminates the high currents on the rotary switch or even eliminates the switch altogether by using another type of cam shaft position sensor. The HV rotary switch is retained, however, or in some cases, the distributor is eliminated completely by either firing all spark plugs simultaneously or by using several separate ignition coils.
If you notice, the primary and secondary are wired in series with respect to the HV output terminals. This is to get an extra few hundred volts into the spark (thanks Jochen). Also, the primary is connected in reverse polarity to the way it would be in a car. This has no effect on output, however. It was only to facilitate a direct connection of the ground to the rest of the circuit without having to go through the diode/SCR. It could be run the other way too, but I like the elegance of it.
For the charging circuit, as pictured above, a camera flash circuit is preferred since it allows you to move the circuit around and use it in places where an outlet is not available. These can be found in disposable cameras or broken ones. All that needs to be done is to remove the xenon tube and storage capacitor after determining the polarity. The ignition coil driver circuit is then connected in place of the storage capacitor.
If the circuit is gong to be used near an outlet and not moved very often, then a rectifier diode and 100 ohm current limiting resistor can be used instead. A bridge rectifier may also be used if a pulse rate of over 60 times a second is needed, up to 120 times a second.
In case you're wondering, the 400V Fast diode makes a very big difference in the arc produced. It makes the arc almost twice as hot and twice as loud. It also saves the SCR from potential failure so don't leave it out!
You can experiment with different sizes of the 3n3 cap and 10M resistor. Those were just parts I had lying around and they work. If the resistor is too S%adD( D CCP $hh @`@ Ch ` H% "D0pJl B 0@P"@$ q@A* b x$D B @H '$8 a"$ 2 `n< "!(Ea $ T E 3E R@ 2 ,A6`a "d4f$%. `` ME" AB! %x w!,@ dL BI $!"$@, !@$$ &IB 0 D `@CaDp D `!ra H A! C` dhD Q @ B% P$ %P d 3@ K@!(HH`dI" B @% @R G`4 @ ad ` Dd qi" IS `a@T` `H!l #B@ . @k$@` p(! G @!a @i6$ (m `b!P f(D0a ` A1 i1 @b"T%ad FD c`$dD $% 4 0TAh &@ T@d ``@ `A, p0 @ a! @ ` h@!A d 4P @d`a P`aF a d `Aj ! a PA! ` `gp rh"D h G q ! `@tA 4 h @ @"hD ! B cA )L @AJ s "`K $0)E (`D@a A @r ' PP@r!`G `$ ` 4`1 (U6 A$ Cj$n0< 0 ( !0d3 H,PAJ@K` "Bb (($. PE2 qP` @(" j C`4(BR> .4 @ ` 0"P C ``` tE$"6 $F, R D 0I@@4bp0 L( 0 B%RA@A N @"D`4`hAf`<@` 0"p2 @ 20`r2 A %0 ,@1 h 2E! g !(A!A40B0. 40 B< 4 A0 `i` E@ D!c$IH! dr 4ha hpL!d`q J0` 0``bP4I" "f$C , AE`N(o0I ! HgL pn ,I@ j$ !P!ra P d% vaP! "" r$ L !*&@ :b 0 "Eb D`2, ! FA @ 2% bdg , an$ D"B` P d - lt@d0 cb` AB pe6%B @5@#dB < < BB" @`h s"@i`0Hb b!A P!. 'I0h @)rB , h"D8 a`R! &$ a&Bh! T"88`b (Hb`"! ($&d E B@e$Q 4*!2 @ `@ d ` D@P!@ !B H(UHh)E` @dia H(b( 0 "2 " h` a IG aT$P $F (t 2!@d0" 2 6 A4T d. (` P`"LIb hPih . Ia$L,`Ha !` 0`D`4.@ f(D (BAL ,(@a,` h,0 p D