October 2013 21
QUESTIONS and ANSWERS
Tesla Coil Project
QDo you have a schematic for building a traditional Tesla coil and a solid-state Tesla coil using a TV
flyback transformer? I would like to
build both of them.
I have always enjoyed your Q&A
— Brian J. Miller
ABy traditional, I think you mean a coil you can build yourself. Homemade Tesla coils typically involve a
paper or plastic tube, two or three
inches in diameter, and fairly long. I
wanted to simulate the coil in
LTspice, so I used a formula from the
ITT handbook for solenoid coils that
is said to be 1% accurate:
L = N2*(R2/( 9*R+ 10*C))
L is in microhenries
R is the coil radius
N is the number of turns
C is the length of the coil
(L was already used)
The inductance of 1,000 turns on
a two inch diameter form is 7,827
microhenries ( 7. 8 mH). Using that as
the secondary, I adjusted the primary
inductance to get 20 KV output, then
calculated the primary turns. I set the
coupling factor at 0.7, thinking that
with an air core and different lengths
of coils the coupling would not be
very good. That was a guess.
Since the voltage depends on the
rate of change of current, it is
essential to have a fast switching
transistor driving the primary. In
Figure 4, when the transistor turns on
the current builds up linearly until the
transistor turns off (see Figure 5).
When the transistor turns off, the
current continues to flow, charging
the stray capacitance and driving the
voltage to a very high value.
The transistor used here (IXBP
5N160 G) has a voltage rating of
1,600 volts; you do not want to
exceed that. ICs and transistors run
on smoke, you know; if the smoke
gets out, it won’t work.
The capacitor C1 and resistor R2
function to limit the peak voltage.
The capacitor is charged through D1
by the flyback and is discharged by
R2. At some point, the charge and
discharge energy becomes equal and
the charge on the capacitor
stabilizes. In this case, the stable
voltage is 1,200 volts. The power in
the resistor is 150 watts, and the
diode needs to be rated 2,000 volts
(you could series three 600V diodes
from the same batch).
Peak current in the diode is 18
amps, but the duty factor is 1/20 so
a one amp diode should work. Use a
high speed diode; a 1N4000 type
won’t turn off fast enough.
The saturation voltage of the
5N160 is about six volts and the
peak current is 18 amps, but with the
duty factor of 3.5/10, the average is
3. 15 amps which makes the power
dissipation of the transistor 20 watts.
With a good heatsink, the TO-220AB
package should handle that or the
TO-247 package could be used.
It appears that the fall time of the
current is 0.4 microseconds,
therefore the voltage should be:
E = L*dI/d T = 15 µH*18A/0.4 µS
I can’t explain why the voltage
on C1 is approximately double that.
You may find when you build it that
the output is half what was expected
and the C1, R2 protection is not
needed (so much for simulation!).
The circuit using the TV flyback
transformer will be the same. You will
have to experiment with the R2 and
C1 values so as not to blow up the
The circuit of Figure 6 is a pulse
generator to drive the 5N160; it is
intended to be a guide. I have not
built any of it. Q1, Q2, and Q3
are an op-amp with positive
feedback through R5 to charge and
There is negative feedback
through R7 to provide hysteresis.
When the voltage on C1 is higher
than the voltage on Q2 base, the
output goes low and discharges C1.
When the voltage on Q1 base is
lower than the voltage on Q2 base,
the output goes high and the cycle is
QMy 2013 KIA Optima has a backup camera and only works when I place it in reverse. I would like to be
■ FIGURE 4.
■ FIGURE 5.