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Build

VHV

Supply

10,000 VOLTS

FROM

COMMONLY

AVAILABLE COMPONENTS

BY

PAUL

H. FUGE

THERE

STILL

EXISTS

a

real

need for

very- high -voltage

power supplies even

in

this era of

low-

voltage solid

-state elec-

tronics- especially in the

area of

experi-

menting. A

casual look around the vari-

ous school science fairs

will

reveal

that

interest is

still high for such projects

as

air

ionizers,

Van de

Graaff

generators,

Tesla

coils and the like.

(One practical use

for

a

VHV

supply

was

given in "The Not

Altogether

Forgotten Electret" in

the

March POPULAR

ELECTRONICS.)

In most

cases, the VHV power supply

is required

to deliver currents on the

order of only

a

few microamperes.

So,

DI

400

PIV

VHV

OUTPUT

By driving

the

very-high- voltage power supply with

a

variable

-voltage transformer,

output

voltage can

be made to vary above

and

below 10,000 volts.

to

meet this requirement

with

maximum

economy, the

VHV

Supply described

here

consists of an

SCR, a

capacitor,

a com-

mon automobile spark coil, and a simple

triggering circuit. Operated from

any

117

-volt a.c.

house line,

the supply

pro-

duces

an output on

the

order of 10,000

volts

which will

jump a j." spark gap

and melt

an electrode

made of solder.

46

How It Works.

Referring to the sche-

matic

diagram, when

line

power is

ap-

plied

to the circuit,

DI

conducts only

when

it is forward

biased, allowing Cl

to

charge up. Then,

when

DI

becomes re-

verse biased, C2

charges up through R1.

At some point

during

the

charge cycle,

the

potential

across C2 reaches

and

ex-

ceeds

the breakover voltage of trigger

diode D2.

When this happens, D2

con-

ducts and

delivers a triggering pulse

to

the gate of

SCR1, turning it on.

The instant

SCR1

fires, it

forms

a

se-

ries

circuit with Cl

and the primary of

spark coil T1

across the power line. As

a

result, the charge on

Cl rapidly

dis-

charges through the low- resistance

T1

primary, inducing

a much higher

voltage

across the secondary.

Then when D1

again becomes forward

biased

on

the next

cycle of the applied

a.c., SCR1 cuts off,

and the charge -dis-

charge

cycle

repeats itself

until the a.c.

power is disconnected.

While the output of

the VHV

Supply

is

a.c.,

it

can easily be converted

to

d.c.

by

installing

a

high -voltage

TV (silicon)

rectifier

and

filter capacitor

across

the

high -voltage

secondary of Ti.

However,

if you

do this, be

careful

to limit

the val-

ue of Cl to a small figure

to

prevent

dam-

aging the

rectifier

by high- current

spikes

when Cl

discharges.

If

an a.c. output

is

required,

the value of

Cl can

be any-

where between

2

and

100

AF,

although

the

larger values

will draw more

current.

Construction. Parts location

and orien-

tation

are

left to your

discretion

when

assembling the VHV

Supply. However,

since

potentials

on the order of 10,000

volts are developed by

the supply,

fully

encapsulate all connections in

a silicone

potting compound after soldering.

Then,

for added protection,

mount

the entire

circuit

inside a perforated steel

or

alu-

minum cabinet.

When the supply

is fully

assembled,

you

can adjust the

setting

of

RI

for

max-

imum output power.

Then, if

desired,

the

optimum

setting

of the potentiometer

can

be

measured and a fixed

1/' -watt re-

sistor

substituted

for it in

the circuit.

Finally, if you wish to vary the

out-

put

voltage above

or below the

designed

10,000

-volt

level,

you

can use an

adjust-

able auto

-transformer

between

the a.c.

line and input of the supply.

POPULAR

ELECTRONICS

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