Variac literally trembles when powering my tesla coil - Page 1
Forum section: electronics / projects

author: davy peleman
author_rank: Regular Contributor
timestamp: January 09, 2018, 07:44:34 pm
content: i noticed that my variac gets very nasty transient spikes (i suppose they are) when powering up the tesla coil, sometimes i feel the trembling of its chassis. it only occurs on lower voltages (40- 50 volts) and not always. i did noticed the coupling between the coils and tuning of the lc tank circuit plays a big role. But while experimenting i already blown quite some fuses and mosfets i use irfp460's and its always a pain to replace them. my circuit board is etched and the traces are to small i think, so to much impedance for this high power circuit. i run the circuit on a max of 120 volts otherwise the mosfet blows. it's a single fet desing without schmitt trigger so maybe i demand to much of the mosfets gate. the gate is protected by double zeners (2 watts) and i use an additional fast high voltage diode between drain and source.Can somebody explain what is happening and if i'm killing my variac with this usage? Also how can i remedy this phenomenon? should i use a line filter? so many questions...

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 09, 2018, 07:55:44 pm
content: Schematic? Photo's?

author: amyk
author_rank: Super Contributor
timestamp: January 10, 2018, 02:40:25 am
content: By "trembling" do you mean loud mains buzzing and vibration? That could be caused by core saturation, suggesting that you're drawing an asymmetric current and creating a DC offset.

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 10, 2018, 06:54:48 am
content: As long as it's not getting too hot (enough to damage the enamal) you should be fine. Solid state stuff will blow up long before a variac will. A line filter would be a good idea, if it survives (I would suggest using the highest rated one you can find).

author: oldway
author_rank: Super Contributor
timestamp: January 10, 2018, 11:48:04 am
content: What's the power of your variac ?You need a fuse or a circuit breaker of the nominal currentin the output connectionto protect your variac eficiently.Not using such protection is the best way to "kill" your variac.A fuse in the input connection will not protect your variac against output short circuit, nor over-current.

author: davy peleman
author_rank: Regular Contributor
timestamp: January 10, 2018, 03:06:59 pm
content: it's vibrating sometimes but not getting hot, it's power is 1500va. i will protect it like suggested with an extra fuse on the output. but it really is a bad design so i'll fix that first.i'll make a half bridge with schmitt triggered antenna and gate drive transformer. because now the mosfet gate doens't have enough dead time i think. it's feedback goes directly (has clamping diodes) into the driver and that's not to good. never blown any drivers or timers though.just wanted to know if i could fry the variac and why it's vibrating. i think i got the answer, must be the current draw and saturation. thanks alot. also i would use the line filter of an old crt tv or make my own if it needs to be more robust.Now could big electrolitics on the input remedy that assymetrical current draw?

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2018, 06:27:27 pm
content: Is it driving a highly non-linear load, such as a bridge rectifier and smoothing capacitor? Non-sinusoidal currents can cause vibration in transformers. They can also cause greater copper losses and thus heating, than the equivalent RMS sinusoidal current, but as mentioned above, if it's not overheating, it isn't a problem.

author: davy peleman
author_rank: Regular Contributor
timestamp: January 10, 2018, 07:55:46 pm
content: ok with a better design i should be fine. thanks

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 11, 2018, 01:14:22 am
content: QuoteUm...you want to put high current through a small appliance line filter? If it has any sort of choke in it, it will be instantly vaporized.You need a common mode choke rated to probably the same as the variac (1500VA), or at least an amount above the actual current draw.

author: davy peleman
author_rank: Regular Contributor
timestamp: January 12, 2018, 08:29:05 pm
content: Thank you very much. Oef. Yes i'm a newbie. Glad you warned me there.