[ARC5] "Quiet" ARC-5 Keying

[Dennis Monticelli]

Hi Bill,

I wouldn't hesitate to use HV transistors as switches.  For regulator
service they can work well (I use them for that successfully), but that is
the application that can suffer from SOA related breakdown long before the
heat build-up does it in.  The failure usually occurs because of a short or
output overcurrent event event such as during a routine startup when the
surge in current to fill an electrolytic could place the transistor in a
simultaneous high current and high voltage condition for tens of
milliseconds.  There are circuit tricks to get around these limitations.

Dennis AE6C

On Sat, Aug 27, 2011 at 9:24 AM, Bill Cromwell <wrcromwell at gmail.com> wrote:

> Hi Dennis,
>
> I have not yet applied "high voltage" switching transistors inside tube
> based rigs but I have used a couple inside keyers that had to handle
> grid block keying and another application that switches high current
> loads on and off. All the way on and all the way off. I presume that
> attention to details so as not to smoke that transistor always applies.
> I am planning to start using high voltage transistors inside rigs to
> turn loads on/off. I also plan to use them in voltage regulators for
> tube oscillators, etc. I don't think they would work well regulating the
> typical plate voltage to a pair of 6146s but - those don't need that
> kind of regulation <g>. The details are more stringent in the regulator
> just because the transistor does dissipate power as heat.
>
> I don't see a fuse as being there to protect a switching transistor. I
> see it there to prevent a fire if/when there is a short and a current
> overload. All of my stuff is appropriately fused. I refuse to connect
> power if it's not. Yes...sometimes when the fuse has self-sacrificed a
> transistor died trying to protect the fuse. Both are cheap.
>
> 73,
>
> Bill  KU8H
>
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