páginas desderadio electronics august 1991 2

8/18/2019 Páginas DesdeRadio Electronics August 1991 2

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RALPH HU BSCHER

A G E

I

o L T O IL RI

oltage doublers re an easy n inexpensive way toexperiment with high voltage

J : L ~ ~ t ~ . _ . _ ~ t _ _ _ _ _ _ _ n ~ IIF Y OU VE E EN L OO KIN G F OR A W Y

to generate high voltage, yo u veundoubtedly run across th e voltag e doubler . Voltage doublingusing diode -capacitor combinations is a common practice . However, whole banks of doublers ,called cascades , can also be usedfor producing extremely high DCvoltages from moderate to highAC voltages . Such high DC voltages ma y be needed for TV sets,lasers, air purifiers, industrialsmoke- s t ack d u s t r emove r s ,negative -ion genera tors, and , ofcourse, fo r exper im en t ing , onwhich we ll concentrate here.

Half wave doublerFigure 1 shows a half-wave volt

age doub ler; we ll assume that Cland C2 ar e in itially discharged .D u ri n g t he first half-cycle shownin a, th e upper input terminal ispositive and th e bottom negative,so Dl conducts and Cl charges toabou t 170 volts peak. Diode D2can t conduct , s ince it s back -biased, so C2 discharges throughR v In th e second half-cycle bt he a na ly si s is s im il ar, e xc ep tt h t D2 c o n d u ct s n d C2charges .

Th e circuit is really a t rans formerless voltage amplifier. WhileTl can provide isolation, as wella s i nc re a se th e AC voltage initially going into th e doubler , th eamplification due to th e doublingaction would occur without it .When th e polarity reverses, bothth e input voltage and th e chargeacross Cl ar e i n s er ie s l ike twobat te r ies , producing a bo ut 3 40volts peak. One problem, though ,is that a half-wave doubler can tbe u se d w it h a load that drawsm u ch c ur re n t.

Full wa ve do ub le rLet s se e ho w a full-wave voltage

do u bler is re lated to a nd b uiltfrom b oth positive an d negativehalf -wave rect if i er s. F igure 2- ashows a half-wave rectifier with apositive output, Fi g . 2 b showsth e same version with a negativeoutput, and Fig . 2- c s ho ws th etw o comb ined into a ful l-wavevoltage rectifier .

Th e full -wave vo ltage doublershown in Fi g . 3 has been redrawnfor greater clari ty; it has betterregulation than a half-wave version , and is easier to filter . T hec i rc u it p r od u ce s n ea rl y d o ub leth e peak AC voltage of 170 vol ts,or a bo u t 340 volts peak acrossRL • For th e first half-cycle a , D2

is cut of f and Dl conducts , sothat V l e q ua l s a pp ro xi m at e ly170 vo lt s DC. On the next ha lfcycle b , th e positive vo ltage isreplace d by a negative voltage, soD2 conducts and Dl is cut off. R Lgoes across Cl and C2 in series,effectively creating a doubledlevel of a bo ut 3 40 volts DC .

Warning Th is article deals w ith andinvolves subject matter and the use ofmaterials and substances that may behazardous to health and life . Do not at-

tempt to implement or usethe informationcontained herein un less you are experi-enced and skilled with respect to suchsubject ma tt e r materials and sub-stances . Neither the publisher no r the au-thor make any representat ion as for thecomp leteness or accuracy of the informa -tion conta ined here in and disclaim anyliability for damages or injuries whethercaused byor arising from the lackof com-pleteness inaccuracies of the informa -tion misrepresentations of the directionsmisapplicatio n of the information orother-wise .

Unlike th e h alf-wave voltagedoubler, th e full -wave version hastwo capacitors across R L ratherthan one . Whereas C l s hown inFig. 1 is cut off and unsuppliedfor half of every cycle, Cl and C2in Fig . 3 are supplied on alternatehalf cycles . When t he c ap ac it orcorresponding to th e diode that scut off discharges, it can only doso through th e capacitor b eingsuppl ied , sl ightly decreas ingboth i ts current and the maximum voltage it reaches .

Me s uring high voltage DVoltage measurements will be

possib le on ly to about the secondo r t hi rd stage of a cascaded voltage doub le r with mos t volt

meters . Beyond that, you ll needto us e either a h igh-vo lt age DCmeter or an externa l voltage divi der for use with a standardhi g h - i m p e d a n c e voltmeter 10megohms or more).

A good voltage divider that canbe used for th e purpose of highvoltage measurements is theRCA SK3868/DIV-1, a high-voltag e DC divider; it s used in TV s toreduce th e final anode voltage goin g to th e CRT to th e level requtred for th e focus vol tage . consists of resistors Rl 20 0megohms) and R2 40 megohms)in series, as shown in Fig. 4.There are three leads, on e for th efree ends of each resistor, and th eother at their juncture . If yo u putboth a lO -megohm meter shownas ZM in Fig. 4) and a 2 .7megohm resistor R3) in parallelwith th e 40-megohm resistor R2), you ca n achieve almost ex - g

actly 100: 1 range multiplication , for a full -scale det1ection of 20 kilovolts DC .


2/4

C1 DISCHARGESSLIGHTLY. IFPREVIOUSLY

CHARGED

C2 DISCHARGESSLIGHTLY. IFPREVIOUSLY

CHARGED

It

-I

I

• . ~,

HOV . -. . - . - --

-01

a

02

t-

~) + 01 - - - ~ ~ t + I 170V::: C 1 I

• - t. . - .02 C2 I

I

< 4 - - - - .... - _ - -

1 r o ~ :- ~ l iL___ ~ 1 7 _ _ V ~~ ;

_ C1 + 02

::1Rl

120VAC 01 +C2

:a-- C1 02+

::f - -20VAC 340V

C: EAK

-

... .

-:

FIG. 1-HALF-WAVE VOL TAGE DOUB LER.During the first ha lf-cycle a , D1 conducts, D2cuts off, C1 cha rges to 170 vo ltspeak, and C2 discharges through R L• Forthe second half -cycle b , the inp ut po larity is reversed, and both the input a nd C1are in series, producing 340 vo lts peak .Now D1 cuts of f while D2 conducts , andthe current div ides between C2 and R L ;the cycle then repeats .

0 2V s

C1

C3

C5

T =

o

064 6V s

sing noise . (We'll s ho w n y ou h owto demonstrate t he w in d lateron .) The ci rcui t delivers 3.75kilovolts DC when powered from12 0 volts AC , or 7 .5 kilovo lt s DCwhen powered from 240 volts AC.

Th e output of a cascaded voltag e doubler should be t e rmin a t e d w i t h a t l e a s t 2

megohms, and only then be allowed to extend beyond a protect ive p las t i c case , for sa fe ty .Voltages as high as 5 megavoltsDC have b e e n g e ne r at ed us ing

FIG. 6 THIS VOLTAGE DOUB LER i s t hesame asthe one shown in Fig . 5, a lthoughit s draw n d ifferently. You s hould be ab leto recognize both vers ions.

C6

C5

C4

C3

C2

C1

WHITE R3 BLACKWIRE 2.7MEG WIRE

R2o ~ i 0

: v A C ~ l jt D 1~ 1 1 - 4 -- - - -...... tt

HV HVDC1V CRT FOCUS DIVIDER RED WIRE) RCA PART SK3868/D IV-1

R1200MEG

FIG.4 TO MEASURE HIGH VOLTAGESwith an ordinary 10-megohm meter, youcan use the RCA SK3868/DIV-1 high-voltage divider . The c ir cu it provides a 1 :100voltage division, allowing 20 kilovolts tobe measured on a 200 -volt scale .

stabilized fo r moderate -currentapplications; it's called either aCockcroft-Walton or Gre inachercascaded voltage doubler.

You c an u se a sewing needle asan emitter for the doubler shownin Fi g . 8 to g e n er a te c o ro n awind . That will sound like a hi s -

FIG. 5 THISCASCADED DOUBLER uses1N4007d iodes rated at 1 k ilovolt PIV, andcapacitors from 0 .068-0 .1fLF with a 400volt DC work ing voltage .

FIG. 3-FULL-WAVE VOL TAGE DOUBLER, redrawn fo r greater c larity. Fo r the fi rst halfcycle a , D2 is cut off and D1 conducts , produc ing about 1 70 vo lts DC across C1 . On t henext half-cycle b , D2conducts and D1 is cut off . The o utput vo ltage i s now a cross C1 a ndC2 in series, doubling the level to about 340 volts DC .

+

C1

. . . . . . . . ~ f _ +

B01

1 iNAC , VA' C1

12w8ill - . -J . . . . . . . - ---¢25VOC

FIG. 2 - TWO HALF-WAVE RECTIFIERS,one with a positive output a and onenegative b , combine to make a full-wavevoltage doubler c .

a

- 25VOC

Cascaded voltage doublersFigures 5 8 show four addi

tional voltage doublers. Th e on eshown in Fig . 5 is the m oststraightforward . I f yo u build it,use IN4007 diodes with peak inv e rs e v ol ta ge (PIV) ra t ings of 1k i l o v o l t f o r 0 1 - 0 6 a n d0 .068 .1 J lF c a p ac i to r s w i t h

g: working voltages of 400 volts DC .o Figure 5 is electrically identical tou iu:J th e on e in Fig. 6 , s o k ee p that in6 mind if you should come acrosso either format. F igure 7 shows anC? ex t ended ve r sion t ha t s b e tt er

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FIG. 3 THIS ISTHE DOUBLER from Figure 12 , after being sealed in candle wax.For better protection, you can immerse itin pure paraffin oil.

pian o wi r e w or k s w ell) to a co ldwa ter pipe a s ea r th g round wi th agood electr ic a l connec t ion . Di scharge a ll c ap a citor s t wice, si n cet he y gene rally either hold ch ar ge ,o r te n d t o r echar ge f ro m o th erca p aci tors . Don t u se an AC li n eg r o u n d or c ha ss is g r o u n d instead of an earth ground ed wat erpi p e, or you m ay blow a fus e ordama ge parts .

Fi gure 11 shows a switch forhi gh - voltage D C th a t you ca n u se

with an y of th e ca sca de d volt ge-doubl ers s h own h ere ; s ta n da rdswit ch es m ay pres ent a sho ckha zard . Also, u se an electro magn et i c interf er en ce EMIl line filt erlik e th e on e se e n a t lef t in t hephot o to k eep gh volt ge DCou t o f hou s e w ir ing , a n d to p reve n t s h o ck from s t at ic ch ar ge.Th e E MI filt er is from C o rc o rnC or p . 1600 Winch ester R oad ,

FIG. 12-A TYPICAL CASCADED voltagedoubler . Note the wide spacings betweenthe d iodes, the long connecting wires,and the smooth solder joints.

FIG. 11-HOMEMADE SWITCH fo r highvoltage DC . The line filter at left is a safetymeasure to keep high-voltage DC out ofthe house wiring .

DOWEL

FIG. 1Q-A HIGH-VOLTAGE GROUNDINGwand is used to discharge capacitors . A 2foot wooden dowel is at tached to a st if fwire ti p . The metal tip must be connectedto an earth ground , such as a cold-waterpipe.

FIG. 9-HIGH-VOLTAGE CAPACITORS . A0.25 floF 7 .5-kilovolt capacitor is o n theleft; a100pF,15-kilovolt capacitor is i n themiddle; and 0.0005 floF 5-kilovolt capacitoris on the r igh t.

s ho u ld a ll h ave in finit e r e si stance .

To pro pe r ly d is char ge ca p ac it ors, b ui l d a d ischa rg ing w andlike th e o n e s ho w n in Fi g . 10 . Us ea 2-foot woode n o r pl a s t i c)d owel, a nd co n ne c t a st i ff wire ti p

6Vs

C8

C7

C

t

T

IG. 7 THE COCKCROFT-WALTON , orre inac her cascaded voltage doubler ,a s improved performance for moderateurrent applications .

ascad ed vo ltage doubl er s , esecially w he n op era ti n g in a p resurized a tmosp he re . Th e bi ggestdvan tage to u s in g volt age doul ers is th at th ey u s e in expe ns i veow-volt a ge par ts . O th erwi s e , ifll t he part s ha d to b e of th e hi gholtag e var i ety,y ou wo ul d h ave tos e ex pe nsive a n d r ath e r la rge

apacitor s li k e t h e on e s ho w n ini g . 9 .

If you have probl em s with theirc u it in Fi g . 8 or an y o therig h -volt a ge ci r cuit), yo u mu s tischar ge e very ca p ac it or we llell y ou ho w in a minut e) b efor eou ch e ck fo r m al fun c t ion s .

When ex aminin g t he circu it forroblems , closely ch ec k the soler conn ections, a n d th en th e d id e directions and continuity .he IN400Ts should h ave a reistanc e of 1.1K wh en forwardiased a n d be op en when rever s eiased , while t he cap a ci tor s

:I>Coc f

l

01 - 026: 1N4 7 1kVPIVC1- C26 : .068- 1 ~ 400VDC

C26

SEWING NEEDLE1 U J AS EMITTER

ELECT ROD E~ 2 0 0MEG FOR CORONAWIND

3.75 .5kVDC

IG. THIS 25-STAGE VOLTAGE DOUBLER will generate corona wind. It delivers 3.75ilovolts DCwhen powered from 120 volts AC, or7 .5 kilovolts DCwhen powered from 240olts AC.

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FIG. 4 CORONA WIND DEMONSTRATION. The f lame will deflect toward the groundplate . Note the black ground wire attached near the screw threads.

7

• Another experiment y ou c ou ldtr y involves holding a fluorescenttube near th e emitter . Th e tubewill g low, bu t be careful not totouch th e terminals on t he e nd s ,or you ll ge t a shock.• Lines of force of a n elect rostat ic field can b e d em onstrated by placing th e electrodes th e high-vo ltage DC o u tp u t a ndground) in a tray covered withc a s t o r oil c o n t a i n i n g somef ar in a. T he farina will produceth e pattern of the electric fieldli nes; s imi l a r to iron f i l ingsshaken lightly on a piece of paperin th e presence of a bar magnet.• If yo u place two round doork no bs o n insulated stands madefrom plastic cups filled with candle wax, a nd th en charge them,then a plast ic ball suspendedfrom a str ing will be drawn to andt ou ch t he positive electrode, andfall back to center when thespheres are d ischarged s ee Fig .16 -a . A plastic ba ll coated withconduc ti ve l acque r swings toward th e positive electrode l ike a

pendulum; when th e ball andd oorkn ob touch , th e ball becomes positively charged, so theyrepel on e another. It then swingstoward th e negative s i de , absorbs e lec t rons, becomes negatively charged, and is repelledback to th e positive . Th e processrepeats indefinitely as long as th ehigh-voltage DC is present, an d itwill continue to operate for sometime after it s shut off . Th e chargeexchange is slow, and there ll bearcing at th e positive electrode.• A grounded metal ball alternates between both electrodes,like th e conducting plastic ball.However, th e arcs are smaller du eto i ts greater weight, and shouldbe observed at both ends, butmore o n the positive side .• A light cotton ball shod bedrawn to th e positive electrodeand hang there by i tself, assho wn in Fig . 6 b . It s then repel led 0 .5-inch toward th e negative electrode , and the processshould repeat indefinitely. R E

good for corona generat ion andelectron e m is s io n . T h e sharpened ends will have a smal l radius of curvature a tight curve orbend), giving rise to a highly distorted electric field a t it s surface .Th e high electric field is whattends to ionize ai r molecules inth e Vicinity .

When you bui ld a cascadedvoltage doubler , yo u ca n encaset he c ir cu it i n p ur e paraffin oil orcandle wax to reduce th e chancesof ge tt i ng shocked . It will alsominimize corona loss, so thehigh -voltage DC arrives where it sneeded. Figures 12 and 13 show atypical ladder-type vo ltage doubler before and after being sealedin wax.

ExperimentsT he re a re many expe r imen ts

tha t c an p ro du ce observable

effects du e to the high-voltage DCproduced by voltage doublers .• With a high-voltage emit te rpointed at a ground plate used toattract ions), with a burning ca ndle placed in between them seeFi g . 14), you ll see the candleflame deflect toward th e metalplate.• You ca n make a rotor for an io nmotor, using a light pivot madefrom a rivet w it h t hi n, stiff wire like piano wire) a tt ac he d, a ss ho wn i n Fig . 15. Th e rotor mustbe balanced on to p of th e sewingneedle emit ter much as in acompass) used for the doublershown in Fig. 8 . We ran a similarconstruction project in RadioElec t ron ic s , February, 1991.)When powered up , th e rotor wills pi n a n d a h i ss ing sound will be

. heard . Both ends of th e wire ar ebent at opposite right angles, soth e emit ted electrons propel th ewire in a circle. You shou ldsharpen b oth e nd s of th e rotorwire to provide a sharp surface

SPHERIC LELECTRODE

RIVET

IRE I

SOLDERW: ·ONTO SIDE

FIG. 1 5 THIS ION MOTORROTOR whenplaced on top of the sew ing needle emit-ter will spin in a circle .

aSPHERIC L SPHERIC LELECTRODE ELECTRODE

~ ~ ~ ~ ~LIGH TCOTTON LL

FIG. 1 6 TWO CHARGED DOOR KNOBScause a suspended lacquer coated plas-tic ball to behave like a pendulum 8). Al i gh t co tton ball w il l b ou nc e b ac k andforth without being suspended on a string b .

L i b e r t y v i l l e IL 6 4 8Z 312-680-7400) , m o de l 2 06 1,

rated at 20 amps, 25 0 volts, ando 50-400 Hz . Th e high-voltage DCswitch in Fig . 11 also uses an old

6 100-amp fuse box , s ho wn o n t hes right; it ma y look like an antique,i but it will prevent any shocks.4

páginas desderadio electronics august 1991 2

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