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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I have been looking at the 1.5KE series transils datasheet, taking for example the 1.5KE400A, the maximum peak surge current is 2.8 amps, and at this current the clamping voltage is 550 volts !!
What do you guys think they will do in a DRSSTC with 200amp currents in the tank circuit? I think nothing, I would say they can just blow from over-dissipation
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I use 1.5KExxx for all my clamping needs. They fail short circuit then (typically) bus capacitors explode them. They are very good.
Industrial motor drives use Varistor-type for ac line for higher energy rating.
For DC Bus a varistor or transorb rated for actual duty would be expensive, unreliable and large, If the DC Bus has low inductance and electrolytics and film caps there should be no 'spikes' on the bus, more like voltage steps of variable steepness. If the bus caps, transistors, diodes etc. are well over rated for voltage then the bus can be allowed to rise significantly, further output power will quickly return the bus to nominal voltage. In extreme cases a low value high power resistor is switched across the bus, usually by an igbt. So maybe it's better to build a low-impedance Bus/Bridge than find the best kind of TVS ?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I agree. Build a low-inductance layout and let the anti-parallel diodes and DC bus capacitors do the clamping.
I've only ever used TVS in my mini OLTC coil, which had a single switch with nothing else to limit the voltage if it turned off at the wrong time. I've seen people jokingly refer to this as "Class-TVS" operation.
My original mini OLTC controller did often turn off at the wrong time, but it was caused by EMI from ground arcs, and the ground arcs also discharged the energy from the system, not to mention that the arc couldn't happen until the energy had been transferred to the secondary. So there wasn't enough left to fry things.
For my bigger OLTC, I was aware that TVS and MOVs wouldn't be up to the job, so I made an active crowbar that measured peak collector voltage and shut the system down if it got too high.
Sulaiman is talking about motor drives that can pump their DC bus voltage up by regeneration when braking. This isn't normally possible in a Tesla coil, but I've seen it happen at a teslathon. Someone was running their coil, and a DRSSTC sitting nearby suddenly exploded, even though it wasn't even powered. It had charged its DC bus up to a destructive level by receiving energy from the other coil.
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Are Transils usable for a (DR)SSTC?
I think nothing, I would say they can just blow from over-dissipation 
