1. Design and implementation of a Tesla coil Christopher Rutherford
20 April 2012
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2. Introduction Scope and context Tesla coil theory and operation
Tesla coil design
Lumped circuit equations
JavaTC calculator
Measurements
Conclusions

3. Scope and Context Objectives Constraints Remarks
Learn about Tesla coils
Make system that works
Satisfy an interest
Constraints
Use easily available parts and equipment
Keep costs to a minimum
Carried out in spare time
Remarks
Wheeler’s and Medhurst’s formulas are empirical

4. TC Operation Resonant transformer Lumped vs distributed
Spectrum analysis
Spherical artefacts

5. Secondary envelope and coupling

6. Tesla coil Implementation

7. Tesla coil parameters Rotary spark gap NST Protection filter
Asynchronous
2 breaks per rotation 50Hz (3000 RPM)
Average power proportional to break rate
Synchronous
Counter rotating
2 * 8 contacts per disk
Breaks per second is 16 * rotational speed
<1000 bps
NST Protection filter
Remove any RF that could damage NST
Series MOV, series resistors, series capacitor

8. Tesla coil parameters Multiple miniature capacitors(MMC)
(20s, 3p) 1.5KV = 30KV
NST Power Supply
4 * (25mA * 10KV) = 1KVA
Coils (Wheelers formula, inches)
Primary, Spiral coil, 10cm to 24 cm
L = ( N*R )^2 / ( 8*R + 11*W ) =14uH
Secondary, helical coil, 5cm * 72cm
L = ( N*R )^2 / ( 9*R + 10*H ) =26mH

9. Tesla coil parameters Secondary capacitance Resonant frequencies
Self, Medhurst equation (cm) , est 1940s
C/d = (l/d) √(d/l ) pF/cm
( )*10 = 9.9 pF
Top load C = (25.4*R) / 9=12 pF
Total 24pF
Resonant frequencies
F = 1/( 2 * PI * ((L * C)^0.5)
Primary 14uH and 0.033uF gives 234Khz
Secondary 26mH and 22pF gives 210Khz
V secondary Vs= Vp(Cp / Cs)^0.5=387Kv

10. JavaTC coil design for comparison

11. JavaTC Ls=23.7mH Cs=16pF Lp=12.9mH Cp=0.033uF Fp=243Khz Fs=256Khz
Accounts for variations in inductance's and capacitance's due to the non-uniform current distribution  at F0

12. Measurements Experiments carried out 6 years ago, Email
Equipment in storage
“Measured F0 by connecting oscilloscope and signal generator to base of TC, fo=250KHz. These figures in JAVATC produce fo= 252.9KHz so I'm happy with the 1% error.”
Can also see on F0 photo of secondary envelope, which appears to show same resonant frequency

13. Measurements Email Primary capacitor later upgraded to 33nF
“Pri, C=10nF, RSG short circuited, one side of pulse cap to ground and sig gen and osc on other side I see a resonant frequency at 285KHz. 9 turns, pancake, start radius=10 cm and radius=26 cm cable dia= 0.5cm. These figures in JAVATC produce fo= KHz so I'm happy with the 1% error.”
Primary capacitor later upgraded to 33nF
Used “Have JAVATC tune Primary Coil” to compensate for new capacitor. Striped away insulation at 5.8 turns (can see on photo)

14. Measurements

15. Measurements

16. Measurements Assume 20us / div (could be wrong) F0=1/(20us/5)=250KHz
Total energy transfer time
20us/0.55=36uS (javatc 22.98)
Half cycle energy transfer
5*2 half cycles / 0.55 cycles =10 ( javatc 11.24)

17. Measurements Output power proportional to ark length ~50% losses
P=(L/1.7)^2 = (35/1.7)^2=423W
~50% losses
Primary spark
Resistance
Radiation

18. Conclusions Observation Performance Improvements
Tesla coils follow known principles
(To a certain extent)
Performance Improvements
Higher/Tight coupling (k=0.6)
Lower height to width ratio
Lower losses in secondary

19. Conclusions Future investigation More analysis at low power
Solid state
Tertiary coils
Higher break rate (power proportional to break rate)

20. Questions / Answers