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Build a Jacob’s Ladder
display & amaze
your friends
Ever since scientific showmen like Tesla
and Edison were able to generate really high
voltages, the Jacob’s Ladder display has been
creating awe amongst laymen. In this article,
we show how you can build your own Jacob’s
Ladder using a low cost circuit.
By LEO SIMPSON & JOHN CLARKE
Virtually any high voltage power
supply which generates more than
about 10kV (DC or AC) can be used
to provide a Jacob’s Ladder display.
The display consists of two vertical
wires close spaced at the bottom and
splayed apart to increase the gap as
the spark rises.
It is the paradoxical nature of the
ladder discharge which intrigues
most people. Who would believe
that the spark would want to become
longer and travel upward, seeming-
ly defying gravity? And surely the
spark would take the shortest path
rather than extend itself as it travels
upward.
In reality, the spark discharge is taking the easiest route from one electrode
to the other. Initially, the discharge
does take the shortest path which is
at the bottom of the wires. But the
Jacob’s Ladder display works because
the continuous spark discharge gets
hot and heats up the air around it.
This heated ionised air rises, carrying
Fig.1: the circuit uses 555 timer IC1 to pulse transistors Q1 & Q2
on and off. Q2 in turn drives a standard automotive ignition coil
and this delivers high voltage pulses to the ladder.
JACOB'S
LADDER
L1
IGNITION
COIL
F1
10A
12V
the discharge with it until the gap is
too wide to maintain the spark. The
discharge then starts at the bottom
again and works its way back up and
the cycle continues.
Why is it called a Jacob’s Ladder?
We don’t know who first came up
with the name but it is an allusion to
the Bible story of Jacob: “He dreamed
that he saw a ladder standing on the
earth, with its top reaching into heaven; a stairway for the angels of God
to go up and come down” (Genesis,
XXVIII;12).
The high voltage supply is easy –either of the plasma bottle displays from
the August or November 1988 issues
of SILICON CHIP will do the trick but
there is a better approach – adapt the
low cost Electric Fence Controller
from the July 1995 issue of SILICON
CHIP. A kit for this design is available
from Dick Smith Electronics and from
Jaycar Electronics and only requires a
few simple modifications.
D1
1N4004
10
470
16VW
ZD4
16V
1W
1k
7
12k
4
IC1
555
6
2
B
E
C
E
B
C
8
Q1
BC327 E
3 2.2k B
Q2
MJ10012
100
C
C 5W
B
5
1
0.1
0.33
VIEWED FROM BELOW
JACOB'S LADDER EHT DRIVER
68 Silicon Chip
E
ZD1
75V
5W
ZD2
75V
5W
ZD3
75V
5W
HT
GND
PARTS LIST
1 PC board, code 11306951,
171 x 79mm
1 12V ignition coil (see text)
3 280 x 5mm cable ties
5 PC stakes
2 3AG PC mount fuse clips
1 10A 3AG fuse
2 5mm ID crimp eyelet terminals
1 TO-3 transistor insulating cap
2 3mm screws, nuts and star
washers
1 red battery clip
1 black battery clip
1 ignition coil EHT connector
1 2-way terminal block
1 2m length of twin red/black
automotive wire
1 60mm length of red heavy duty
hook-up wire
1 60mm length of blue heavy
duty hook-up wire
1 370mm length of 1.5mm
copper wire
1 40mm length of 0.8mm tinned
copper wire
Semiconductors
1 555 timer (IC1)
1 BC327 PNP transistor (Q1)
1 MJ10012 500V NPN
Darlington (Q2)
1 1N4004 1A diode (D1)
3 75V 5W zener diodes (ZD1ZD3)
1 16V 1W zener diode (ZD4)
Capacitors
1 470µF 16VW PC electrolytic
1 0.33µF MKT polyester
1 0.1µF MKT polyester
This photo is really a composite of two separate photographs which were
combined using a computer. It shows how the spark climbs the ladder formed
by the two vertical wires attached to the ignition coil. Note that the multiple
discharge paths shown here are a result of the ¼-second exposure time used
when taking the photo. In practice, fewer sparks are visible at any one time.
Actually, using the Electric Fence
Controller to generate the spark discharge provides a big advantage in that
the resulting Jacob’s Ladder is much
more spectacular.
Instead of having just one spark
discharge which climbs up the wires,
our version produces about 130 sparks
second, so you have a whole series of
sparks which appear to be climbing
up the wires, as shown in the accompanying photo.
The result is noisy and smelly, and
all those sparks look quite nasty and
dangerous – as indeed they are.
How it works
The Jacob’s Ladder is based on an
automotive ignition coil. These can be
purchased new from automotive retail
ers but will be cheaper if purchased
secondhand from motor wreckers.
Select one which requires a ballast
resistor.
The circuit comprises a 555 timer
IC, two transistors, the ignition coil
Resistors (0.25W 1%)
1 12kΩ
1 100Ω 5W
1 2.2kΩ
1 10Ω
1 1kΩ
and several resistors, capacitors and
diodes – see Fig.1. The revised circuit
pulls a lot more current than the Electric Fence Controller and generates
lots of fat, juicy sparks instead of the
deliberately restricted high voltage
transients of the original circuit.
IC1 is a 555 timer used to produce
the short pulses. Note that we used
a standard 555 timer here since it is
more rugged than the CMOS (7555)
version and less likely to be damaged
by any high voltage transients which
September 1995 69
The ignition coil is secured to the PC board using plastic cable ties, while a
plastic cap is fitted to Darlington transistor Q2 to prevent unexpected shocks
during testing. Note that you don’t have to buy a new coil – a secondhand coil
obtained from a wrecker’s yard will do the job quite nicely.
may be present on the PC board.
IC1 is connected to oscillate at about
133Hz, as determined by the 0.33µF
capacitor at pin 6 and the associated
12kΩ and 1kΩ resistors. The two resistors set the duty cycle of the pulse
train delivered by pin 3 at essentially
14:13; ie, close to a square wave.
When pin 3 is high, transistor Q1
is held off and no base current flows
in Q2. When pin 3 goes low, Q1 is
switched on due to the base current
flow through the 2.2kΩ resistor and
Q1 switches on Q2 via its 100Ω base
resistor. The coil now begins to charge
via fuse F1. The instant pin 3 goes high
again, Q2 switches off and the coil
develops a high voltage and generates
a spark across the gap.
Q2 is an MJ10012 Darlington power
transistor, specifically designed as
a coil driver in automotive ignition
systems. It has a 500V collector-emitter rating so it can withstand the high
voltages developed across the coil’s
primary winding.
Depending on the spark gap, the
coil’s peak primary voltage will only be
about 200V or so, but if the gap is very
large or the coil is operated without
any EHT output lead, the secondary
voltage can be excessive and there can
be a flashover inside the coil. Not only
can this damage the coil but it can also
produce a very high primary voltage
points. This done, solder in all the
low profile components such as the
IC, diodes and resistors. Table 1 lists
the resistor colour codes but it is also
a good idea to check the resistor values using a digital multimeter before
soldering them in position.
Now solder in the capacitors, taking
care to ensure that the 470µF electrolytic is oriented as shown. Take care
to ensure that the semiconductors
are correctly oriented as well. In par
ticular, note that D1 (1N4004) faces
in the opposite direction to the three
zener diodes (ZD1-ZD3). Note that ZD4
is mounted under the PC board across
the 470µF capacitor. Pin 1 of the IC is
adjacent to a notch in one end of the
plastic body.
Transistor Q1 should be pushed
down onto the board as far as it will
easily go before soldering its leads.
Q2 is secured directly to the board
(ie, no insulating washer) using 3mm
machine screws and nuts.
As well as securing Q2 in place,
these mounting screws and nuts also
connect Q2’s collector (ie, the case)
to a track on the PC board. To ensure
reliable connections, use star washers
under the screw heads and solder the
nuts to their surrounding copper pads.
This done, fit an insulating cap to Q2
– this will prevent any nasty shocks
during the testing procedure.
The 100Ω 5W wirewound resistor
is mounted about 6mm above the
PC board, to avoid any possibility of
charring – it does get hot.
which may damage Q2.
Accordingly, three 75V 5W zener
diodes, ZD1 to ZD3, are connected in
series across Q2 to limit the primary
voltage developed by the coil to about
225V, well within the transistor’s rating of 500V.
Power supply
Power for IC1 is provided by the battery via fuse F1, the 10Ω resistor and
diode D1. A 470µF capacitor filters the
supply to provide reliable triggering
for the timer. Transient protection is
provided with ZD4, a 16V zener diode.
A 0.1µF capacitor at pin 5 filters the
trigger point voltage to ensure that the
timer does not false trigger.
Diode D1 offers reverse polarity protection for IC1, while the fuse protects
the battery from supplying excessive
current should a fault occur.
Construction
The circuit is constructed on a PC
board coded 11306951 and measuring
171 x 79mm. This board, together
with the ignition coil mounted on it,
fits neatly inside a 230mm length of
90mm plastic stormwater pipe (available from plumbing supply outlets).
Fig.2 shows the assembly details for
the PC board.
Begin the assembly by installing
PC stakes at the five external wiring
TABLE 1: RESISTOR COLOUR CODES
❏
No.
Value
4-Band Code (1%)
5-Band Code (1%)
❏
1
12kΩ
brown red orange brown
brown red black red brown
❏
1
2.2kΩ
red red red brown
red red black brown brown
❏
1
1kΩ
brown black red brown
brown black black brown brown
❏
1
10Ω
brown black black brown
brown black black gold brown
70 Silicon Chip
▲
JACOB'S
LADDER
Fig.2 (left): install the parts on the PC board as shown in this
wiring diagram, making sure that all polarised parts are correctly
oriented. The EHT connection to the coil is made using a brass
EHT ignition coil connector.
Warning!
TERMINAL
BLOCK
This Jacob’s Ladder display uses very high
voltage which can give a nasty shock. Do not
put your fingers near the display or coil while
ever the power is applied.
Fig.3 (below): check your PC board for defects by
comparing it against this full-size etching pattern
before installing any of the parts.
CABLE TIE
IGNITION
COIL
CABLE TIE
CABLE TIE
10
100
5W
Q1
IC1
555
F1
D1
1k
12k
12V
BATTERY
POSITIVE
Q2
2.2k
0.1
1
0.33
ZD1-ZD3
470uF
ZD4
12V
BATTERY
NEGATIVE
September 1995 71
The fuse clips can now be installed.
Note that these each have a little lug at
one end to retain the fuse after it has
been installed. These lugs must go to
the outside ends, otherwise you will
not be able to fit the fuse.
The ignition coil is secured to the
PC board using three cable ties (see
photo), after which the leads can be
run to its primary terminals. These
leads should be terminated using
5mm eyelet connectors to allow for
easy connection to the coil. Don’t
just crimp the connectors to these
leads – solder them as well to ensure
long-term reliability.
Finally, complete the construction
by fitting the twinlead battery cable
(red to positive, black to negative). The
free ends of this cable are fitted with
large (30A) battery clips
the battery leads and carefully slide
the assembly into its 90mm tubular
plastic housing. This done, feed the
battery cable through the hole in its
end cap, secure it using a cordgrip
grommet and reconnect the leads to
the PC board.
The board assembly will be held
in position when the end caps are
fitted and, generally, this should be
sufficient. However, if you wish the
board to be held even more securely,
wrap a small amount of foam rubber
around the top of the coil so that the
assembly is a tight fit within the tube.
We made our Jacob’s Ladder spark
gap with a 220mm length and a 150mm
length of 1.5mm copper wire. The
shorter length was soldered to an
ignition connector which plugs into
the coil EHT, while the longer wire
was soldered to the GND terminal.
We used a 2-way terminal block to
separate the wires at the base of the
ladder. If enamelled copper wire is
used, scrape the insulation away along
the inside edges to allow the spark to
travel freely.
Testing
Before you apply power, you must
provide a temporary spark gap for
the ignition coil, otherwise it could
be damaged, as noted above. The gap
can be made quite simply with a paper
clip. Extend the paper clip so that you
have a hooked section at each end.
Fit one hook into the EHT socket on
the coil and make sure that it cannot
fall out easily. This done, bend the
other end of the clip so that it is close
to (less than 5mm) but not touching
the negative primary connection of
the coil.
Now for the smoke test. Immediately, there should be a continuous spark
across the temporary spark gap. Do not
attempt to touch any part of the coil
while power is applied because it can
give you a very nasty shock!
If everything works OK, disconnect
M
FRO
NEW N CHIP
O
SILIC
No kinks
A large coffee jar placed over the
ladder will prevent high-voltage
shocks, although it does tend to
diminish the spectacle of the display.
Note that the two wires should be as
straight as possible without any kinks.
Any slight kinks will mean that the
sparks will not progress smoothly up
the ladder but will tend to “stick” at
the kinks. So keep the wires as straight
as possible and splay them apart very
slightly so that the gap at the top is no
more than about 20mm.
You can also place a large coffee
jar over the complete assembly for
safety’s sake (see photo) although this
does tend to diminish the spectacle of
SC
the display.
20 Electronic
Projects For Cars
On sale now at selected newsagents
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