# FW: Re: Tesla Coil Efficiency Test

```Original poster: "John H. Couture by way of Terry Fritz <twftesla-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>

Many coilers have tried to measure the efficiency of their coils in the past
with varying results. In my Tesla Coil Construction Guide book I show a test
method that I used for a small TC that appeared to work for TC efficiency.
However, I am now having second thoughts about the test results and wonder

The test concept is simple. You do not need to know the details for the
Tesla coil. However, it should be in tune or the efficiency will be very
low. The TC can be treated as a black box with only the input and output
information that is known. The efficiency is found by the equation
Efficiency = Energy out / Energy in
The input energy is found by the setting of the variac. The output energy is
found by the intensity of an incandescent lamp inductively coupled to the
secondary coil. The incandescent lamp has the advantage that it integrates
the pulse output and gives an RMS output. The non linearity of the lamp is
not a factor because only one brightness value is used.

Energy = Watt seconds.

For the test -

Input energy - Neon Sign Transformer
Pri = 120 volts/60 Hz
Sec = 7500 volts  30 ma  225 watts
Variac output was connected to NST Pri and set at 45 volts.
Input = 225(45/120)^2 = 32 watts

Ouput energy - Auto Lamp #1141 - 12 volt - 18 watt at normal brightness
The lamp was inductively coupled to the TC secondary.
The variac output was increased to 45 volts when the lamp was at
normal brightness (18 watts) as shown by a light meter.

The Tesla coil efficiency was -

Efficiency = Energy out/Energy in
= 18/32 = .56 or 56 %
If we assume the inductive coupling was only .7 the efficiency of the TC
would be greater
Efficiency = 56%/.7 = 80%

This seems reasonable because small Tesla coils have higher efficiencies
than large coils.
Note that this test is different than the secondary current test shown in
the same book where the lamp is connected in a different manner to the
secondary.

I was interested in what other coilers thought of this test and what they
have found for the efficiencies of their coils.

John Couture

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