[Prev][Next][Index][Thread]

Re: Solid-state TC - transformer design

Tesla List wrote:
> 
> >From haba-at-cc.hut.fiSat Oct 26 23:42:01 1996
> Date: Sat, 26 Oct 1996 21:34:42 +0300 (EET DST)
> From: "Harri \"Haba\" Suomalainen" <haba-at-cc.hut.fi>
> To: tesla-at-pupman-dot-com
> Subject: Re: Solid-state TC - transformer design
> 
> On Thu, 24 Oct 1996, Tesla List wrote:
> >    I'm going to use a 10 ohm 225w power resistor placed before the
> > filter capacitors in the circuit to limit the rate of charge to the
> > capacitors. In previous circuts I blown the bridge rectifier even though
> > it was a high current, high voltage type! The filter caps are two 4500uf
> 
> No wonder. Assume you turn the power on at the mains peak voltage
> moment. Then caps are at zero volts and the current is limited by
> only wiring, bridge etc. losses. Current draw will be huge!
> 
> In SMPS people usually limit the current by a NTC resistor. (Someone
> did call it a gizmo already I think). The idea is that it has high
> resistance at the startup. When current flows through it (ie. unit
> has been on for a while) it heats up and the resistance lowers to
> a much lower value. Therefore losses are limited.
> 
> NTC has one drawback: if you turn the power off and capasitors
> are discharged quickly. Then if you turn the power quickly on before
> the NTC has had time to cool down it will not limit current maybe.
> This is a thing everyone should keep in mind. In coiling it can
> quite often be arranged so that the operator will let it cool down
> before reapplying power.
> 
> Then there is always the good old solution: resistor in series. However,
> it will be very lossy. Therefore you should trigger a thyristor/triac,
> relay or something to bypass it after a while when caps are charged
> already and it is no longer needed.
> 
> Someone did mention power factor correction. It certainly depends on
> the type of apparatus you have. If you use small caps which will
> discharge a lot during mains cycle you might not even need one.
> In that case you will have input power of rectified sinuoidal and
> output power rect. sin. modulated. You will hear the 50/60/100/120Hz
> note on that one.
> 
> If you have large caps you probably want to keep the voltage about
> constant and hear only a small "hssss.." In that case you may or
> sometimes even must consider using chokes to improve power factor.
> You may also have some nice switch mode power factor correction
> circuits there. It will mean more to do. For a bigginner I suggest
> using small caps so you will not need to think about those things.
> 
> In a first prototype even half-wave rectifying may be nice: it means
> half of the power. It is however an advantage when you debug the
> circuit. It will mean half less heat problems! :)
> 
> > in parallel. I have also recently acquired a surplus 120vac variac rated
> > at 45amps to control the voltage. You can disregard the filter network
> 
> That is of course one possibility. Keep the voltage down at the startup
> and let it build up gradually. On a prototype that is usually the
> case anyway. Only a fool applies full voltage to a first test!
> 
> Someone mentioned also snubbing. It depends a lot on the circuit and
> used switching devices if it is needed at all or how much. With
> thyristors it is a must to limit di/dt and dv/dt. With bipolar
> transistors it is a must to keep the device within SOA. With mosfets
> that have a poor layout it is needed to prevent voltage spikes.
> 
> I'm right now working on one nice prototype which will not need snubbers
> at all. Keeping the layout good and taking use of the parasitic
> elements will do the job. Anyway, the all just wast power and I cannot
> afford it with the low power rating I have in my appartment! :)
> --
> "If the troubles are a bit less sever (say, someone merely hit your keyboard
>  with an axe .."                  - Linux System Administrators guide V1.0
> Harri.Suomalainen-at-hut.fi - PGP key available by fingering haba-at-alpha.hut.fi


   Has anyone tried using a counterpoise ground instead of grounding the
opposite side of the output transformer(the end not connected the base
of the secondary) to earth ground? I have Duane A. Bylunds book and
there is a section on counterpoise grounds.
   I would like to use this method instead so I can operate the coil
indoors. His book leaves a few questions unanswered. Like can the
opposite side of the output transformer be connected to ONLY the
counterpoise?