Ramsey Electronics PG13 Instruction Manual page 15

transformer, or under full load. In the case of the transformer we will be
supplying, it is rated for 3 amps. We idle at 0.5 amps. This means the wall
transformer will actually put out much more voltage under this low load. In fact
after measuring this I found the rectified DC to be close to 19.2 VDC, which is
17% higher than we would expect. Using this 19.2VDC, we can now find what
our output of the secondary will be. Since we are operating push-pull, 19.2
volts will be across 1/2 of the primary at any given time, so we will say the
primary has 4 1/2 windings.
19.2 V / 4.5 windings = ? Secondary Volts / 4000 windings.
Rearranging we get:
? Secondary Volts = 19.2 V * 4000 w / 4.5 w
Or: 17,066 volts AC.
Now you're probably going to say: "Where's the 20kV you promised?" Well,
it's here. Due to switching transients of the transistors, there are some pulses
induced in the primary windings that increase output voltage slightly. These
pulses are called "ringing" which can also be described as overshoot.
Typically under varying load conditions the overshoot will change about 20%,
or in other words add about 3.5kV to our output.
17.066 KVAC + 3.5 KVAC = 20.56KVAC.
This "ringing" is actually what a true Tesla Coil relies on to achieve
amazingly high voltages. A good Tesla coil design utillizes the ring, and the
harmonics of it. The better it "rings", the higher the output.
This output value is difficult to measure due to the variety of loads we will run
into, but this is a no-load output voltage. As soon as we begin to load the
circuit, this will vary quite a bit, since there is no regulation on the output.
Now if we used a 16VAC transformer? We will just scale the output
according to percentage:
20.56 KVAC + 21% * 20.56 KVAC = 24.8 KVAC.
24.8 KVAC. The output transformer is only rated for 25 KVAC, and this is
coming too close for comfort for reliable use, and the difference isn't very
noticeable. Also, 15 VAC stresses our transistors a bit too much. You are free
to try it though, just don't run it continuously. Doing this also voids the
warranty since your transformer may arc over in some experiments, thereby
destroying it!
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PG13 15