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Variac blowing fuses (another newbie questin)



Original poster: otmaskin5@xxxxxxx Thx for all the responses. Several of you asked for more info in order to diagnose. Here's the info.

- The variac is supplying power to the NST only

- Original fuse was 20A - don't know whether it was fast or slow blow. Replacement fuse was 15A SLOW BLOW. When it blew, the solder breached the fuse's glass cylinder.

- The fuse blows after I've dialed the variac up to full power from zero. It didn't blow immediately - rather, the coil ran for maybe 10-20 seconds. I believe the variac was up to full power. Both times, the fuse blew during strong continuous arc to grounded target. Hasn't happened when streamers were not striking anything. I always power up starting the variac at zero.

- The variac provides 0-130 volts out.  Current rating is 20 amp.

- Fuse appears to be located in the input circuit of the variac (not totally sure as I wasn't able to get the cover off far enough to thoroughly examine the wiring).

- I don't know whether the NST is a high power factor type or a normal. I'll check with the mfg to see if I can find out. In the mean time, it's a Franceformer, 15000 volt, 60 ma, 890 VA, model # 15060.

- My tesla coil has a static spark gap. 9 gaps made of 3/4" copper pipe 0.03" for each gap. So far, its wired to use only 4 gaps (0.12" total) and small fan for quenching.

- The value of my tank capacitor is 0.03 uF +10%, 35KV. I'ts the Maxwell cap you see on eBay quite often.

- The variac works (as indicated by the voltmeter) without blowing fuses @ no load. Also, I run my small coil (9/30 NST powered) using the variac haven't blown fuses.

Hope this helps in figuring it out.  Thanks.