TCBA Volume 15 - Issue 1
Page 8 of 18
Do Air Core Chokes Really Protect Transformers?
Herman W. Flynn, 6 Nassau Ct., Somerville, NJ 08876
I have a question regarding the efficiency of air core solenoids as R.F. chokes in protecting neon sign transformers used to power Tesla coils. I did the following experiment with the set-up shown at the right.
A. Vacuum tube TC at 700 kc; B. 8"X8" aluminum plate; C. R.F. milliameter; D. coil under test.
The aluminum plate was placed so that the reading on the meter “C: was taken directly from the plate (no coil). The reading was 80 ma. When a 60 turn air core (1"diameter) choke was placed at letter “D”, the reading was identical. The choke showed no effect.
When 30 turns of insulated wire were wound on a ferritte core (Radio Shack 273-104) and again connected to “D”, the current dropped to 20 ma.
The experiment was continued with a 44-turn ferrite core and the current dropped to 8 ma.
With both ferrite coils in series, the current was 5 ma. When measured with an LCR meter, the inductance of the four coil arrangements is shown below. And the inductive reactances (at 700 kc) appear at the right of the inductance readings.
Inductance
| Air core coil | 0.021 mh |
|---|---|
| 30 turn ferrite coil | 1.93 mh |
| 40 turn ferrite coil | 3.52 mh |
| Ferrite coils in series | 5.50 mh |
Inductive Reactance*
- 92 ohms
- 8480 ohms
- 15,500 ohms
- 24,200 ohms
* (neglecting resistance and XC, these values are also the impedances)
Summarization
| COIL | R.F. CURRENT | L | X1 or Z |
|---|---|---|---|
| Direct | 80 ma | ---------- | ---------- |
| Air core | 80 ma | 0.021 mh | 92 ohms |
| 30 Ferrite | 20 ma | 1.93 mh | 8480 ohms |
| 44 Ferrite | 8 ma | 3.52 mh | 15,500 ohms |
| Both | 5 ma | 5.50 mh | 24,200 ohms |
The results appear to show that a 60 turn air-core coil has no effect on R.F. current as compared with iron-core coils. My question: How does an air wound coil protect a transformer from R.F. voltages?
(Editor's note: Mr. Flynn's experiment poses a question that should be addressed. He has invited TCBA readers to respond to his question. Any comment coming to TCBA will be printed in the forthcoming newsletter. I'll start it off by stating that R.F. chokes do respond to specific frequencies. That is, they will respond to a certain range of frequencies and not to others. What would be the result if an air core R.F. choke with an inductance equal to the 30 turn ferrite choke was applied? Or, the same question might be posed when applying a 60-turn R.F. choke to a different range of frequencies? )