Connecting the Variac to the mains

Today I finished the wiring to the SRSG (synchronous rotating spark gap), added IEC female connectors to the two outputs from the variac and slapped o a 13amp plu onto the input to the variac. Although I'm not yet ready to test the Tesla coil charging circuitry I can now test the variac up to full voltage.

Here the variac is plugged into a 230v mains outlet. The SRSG 240v feed from the variac module is connected to the correct input on the Tesla coil base. The main Tesla coil feed (0v -110v) is left unconnected but tapped by the volt meter.

The variac dial was set to zero and the on/off switch thrown. The synchronous motor spun up immediately and the meter continued to read zero. Rotating the variac knob resulted in an increase of RMS AC voltage on the meter.

The reading were as expected and hoped. The printed scale around the variac knob isn't that accurate, but I didn't really expect it to be, it's really just a guide. Think I will mark on a couple of accurate markers.


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Checking the Variac with the Oscilloscope

Started on a little more of the wiring today. My task was to add a feed wire from the variac unit which would supply 240v to the SRSG feed transformer. This wire would just tap into the input feed after the on/off switch within the variac housing. I thought it would be a good idea to test the variac electrical connections before adding anymore wiring. The safest way to do this was to use the function generator on my oscilloscope to supply a low amplitude (about 5v) 50Hz signal across the input terminals of the variac and test the output with one of the channels on the oscilloscope.

Here's the setup I used. The function generator is plugged into the inputs of the variac and also channel 2 of the oscilloscope. The oscilloscope probe is connected across the output of the variac and feed into channel 1 on the oscilloscope. With both channels set to the same volts/div and the scope set to dual trace (shows both channel traces on same screen) it's very easy to see to compare input and output of the variac. Far safer than slapping a plug on the variac, plugging into the mains and hoping all is OK.

You can see from the video when the variac control knob is set to zero there is no output. As the knob is rotated the flat trace rises till it reaches the same amplitude as the output of the function generator. This is the required result and shows that the variac is functioning correctly. When in use with the tesla coil I will only need to use about half the range of the variac as the NST is rated at 110v. Tomorrow I will add the wiring for the SRSG. 

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