Re: Dump the RSG!

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: Dump the RSG!
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 5 Nov 1997 16:58:26 -0600
Approved: tesla-at-stic-dot-net

From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Wednesday, November 05, 1997 4:37 PM
To: 	Tesla List
Subject: 	Re: Dump the RSG!

Hello Terry,

> From:   terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
> Sent:   Tuesday, November 04, 1997 10:10 PM
> To:     Tesla List
> Subject:    Re: Dump the RSG!
> 
> >
> >From:  Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
> >Sent:  Tuesday, November 04, 1997 2:41 PM
> >To:    Tesla List
> >Subject:   Re: Dump the RSG!
> >
> >Greg, all,
> >             I've been sitting on some Westcode devices (SCRs and 
> >diodes) for some time now while agonizing over exactly how to use 
> >them without destroying them. The kind of ratings I am talking about 
> >(from memory) are 2.5kV holdoff/device (fwd and rev) with current 
> >ratings around 2kA or so. di/dt should not be a problem with a 
> >sufficiently low frequency coil. The agonizing bit comes from the 
> >implications of stacking them. If one turns on fractionally before 
> >the other (say two stacked), what happens to the sluggard? Bit of a 
> >worry I think?
> 
> I know the answer to that all too well!
> 
>         If they recieve too high of voltage at these currents the junctions
> will breakdown and the device will go to a permanant short.  The die will
> simply arc and shatter.  They will not handle multiple hits and survive.
> The first over voltage and it is all over.  No room for error.  Worse yet,
> when one part shorts the rest now see a higher voltage and you risk loosing
> the whole stack.  Its sort of sad.  They don't make much noise or anything.
> It just stops working and when you go and test them with a multimeter they
> are all just dead. :-(

I don't plan on using them at anything like the continuous current 
rating, even peak current will be well below this. di/dt and 
operating frequency are inherent limitations here.  I certainly 
wouldn't allow output discharges to connect with them.

>         You really need a voltage protection device across the 
> SCRs.  We use
> very fast, high power, and very high frequency transorbs that may be an
> option but ours are custom and very expensive.  There are off the self
> devices that may work and be cheap (we buy the bare die and package them
> ourselves).  The bad part is that you may need some trial and error
> experimentaion to get it right.  During development of a RF power supply
> design, we went through $250,000 worth of parts in one month trying to
> getting it right.  In the end it worked out great but it's not for the faint
> of heart or wallet.  

Needless to say I would not try to run these at full power to begin 
with. A gentle ramp up in power to check operational parameters would 
be mandatory. 

> If you really want to play this game, let me know and I'll give you some
> tips on things not to do! 

I'm planning to do it alright. The devices are completely useless for 
anything other than doorstops if not passing some current. As I said, 
I haven't been in a hurry to rush in before having a very careful 
think about it. One doesn't throw a rare opportunity away.

Thanks,
Malcolm